Journal of Experimental Botany - recent issues

Emerging complexity: jasmonate-induced volatiles affect parasitoid choice

Wasternack, C., Hause, B. — 2009-06-08

Rhizosphere manipulations to maximize 'crop per drop' during deficit irrigation

Dodd, I. C. — 2009-06-08

Symbolism of plants: examples from European-Mediterranean culture presented with biology and history of art: JULY: Lotus

Kandeler, R., Ullrich, W. R. — 2009-06-08

Natural selection and the evolutionary ecology of the arbuscular mycorrhizal fungi (Phylum Glomeromycota)

Helgason, T., Fitter, A. H. — 2009-06-08

Darwin's model of evolution by natural selection was based on his observations of change in discrete organisms in which individuals are easy to define. Many of the most abundant functional groups in ecosystems, such as fungi and bacteria, do not fit this paradigm. In this review, we seek to understand how the elegant logic of Darwinian natural selection can be applied to distributed clonal organisms. The arbuscular mycorrhizal (AM) fungi are one such group. Globally, they are ubiquitous in terrestrial ecosystems, are locally distributed among many host plant species, and are significant drivers of nutrient cycling in ecosystems. The AM fungi are intractable to study, as the few taxa that can be cultured cannot be grown in the absence of plant roots. Research has focused on the plant–fungus interface, and thus on the symbiotic phenotype. A model is discussed for the interchange of materials at the interface that throws the emphasis of research onto the behaviour of the individual organisms and removes the need to test for phenomena such as selectivity, co-evolution, and cheating. The AM fungi are distributed organisms with an extensive external mycelium that is likely to be under strong environmental selection. AM fungi show sufficient phenotypic variation and fitness differentials for selection to occur, and developments in genetic analyses suggest that a better understanding of heritability in these organisms is not far away. It is argued that direct selection on fungal traits related to their survival and performance in the soil independent of the host is likely to be the major driver of differentiation in the AM fungi, and the evidence for direct fungal responses to soil conditions such as pH, hypoxia, and temperature is reviewed.

The evolution of the starch biosynthetic pathway in cereals and other grasses

Comparot-Moss, S., Denyer, K. — 2009-06-08

In most species, the precursor for starch synthesis, ADPglucose, is made exclusively in the plastids by the enzyme ADPglucose pyrophosphorylase (AGPase). However, in the endosperm of grasses, including the economically important cereals, ADPglucose is also made in the cytosol via a cytosolic form of AGPase. Cytosolic ADPglucose is imported into plastids for starch synthesis via an ADPglucose/ADP antiporter (ADPglucose transporter) in the plastid envelope. The genes encoding the two subunits of cytosolic AGPase and the ADPglucose transporter are unique to grasses. In this review, the evolutionary origins of this unique endosperm pathway of ADPglucose synthesis and its functional significance are discussed. It is proposed that the genes encoding the pathway originated from a whole-genome-duplication event in an early ancestor of the grasses.

Update on the genetic control of flowering in garden pea

2009-06-08

The garden pea has been a model for the genetics of flowering for several decades and numerous flowering loci have been identified, but until recently little was known about the molecular nature of these loci. This paper presents an update on recent work on the molecular genetics of flowering in pea, outlining progress in gene and mutant isolation, expression analyses, grafting and other physiological studies, and candidate gene assessment. Work so far has led to the identification of the LATE1 and DNE loci as orthologues of Arabidopsis GIGANTEA and ELF4, respectively, and candidate genes for several other loci are being evaluated. Expression analysis of an expanded FT-like gene family suggests a more complex role for this group of genes. These results provide the first insight into the circadian clock, photoperiod response mechanism, and mobile signals in pea, and identify both conserved and divergent features in comparison with Arabidopsis.

At the end of the day: a common molecular mechanism for photoperiod responses in plants?

Lagercrantz, U. — 2009-06-08

Photoperiod or daylength affects a diverse set of traits in plants, including flowering and tuberization in annuals, as well as growth cessation and bud set in perennials. During the last 10–15 years, great progress has been made in the understanding of molecular mechanisms controlling photoperiodic induction of flowering, in particular in the model species Arabidopsis thaliana. An obvious question is to what extent the molecular mechanisms revealed in A. thaliana are also shared by other species and other traits controlled by photoperiod. The purpose of this review is to summarize data on the molecular mechanisms of photoperiod control in plants with a focus of annual growth rhythm in perennial plants.

Evolution of petal identity

Irish, V. F. — 2009-06-08

Petals appear in many angiosperm taxa, yet when and how these attractive organs originated remains unclear. Phylogenetic reconstructions based on morphological data suggest that petals have evolved multiple times during the radiation of the angiosperms. Based on the diversity of petal morphologies, it is likely that the developmental programmes specifying petal identity are distinct in different lineages. On the other hand, molecular genetic analyses have suggested that the specification of petal identity in different lineages utilizes similar genetic pathways. Together, these observations indicate that the evolution of petals has relied on the repeated recruitment of a suite of interacting developmental control genes, albeit in different ways in different lineages. These observations suggest that this gene regulatory network represents a ‘deep homology’ in plant evolution. A major challenge is to understand how this ancestral developmental pathway has been redeployed in different angiosperm lineages, and how changes in the workings of this pathway have led to the myriad shapes, colours, and sizes of petals.

Climate change and the flowering time of annual crops

Craufurd, P. Q., Wheeler, T. R. — 2009-06-08

Crop production is inherently sensitive to variability in climate. Temperature is a major determinant of the rate of plant development and, under climate change, warmer temperatures that shorten development stages of determinate crops will most probably reduce the yield of a given variety. Earlier crop flowering and maturity have been observed and documented in recent decades, and these are often associated with warmer (spring) temperatures. However, farm management practices have also changed and the attribution of observed changes in phenology to climate change per se is difficult. Increases in atmospheric [CO₂] often advance the time of flowering by a few days, but measurements in FACE (free air CO₂ enrichment) field-based experiments suggest that elevated [CO₂] has little or no effect on the rate of development other than small advances in development associated with a warmer canopy temperature. The rate of development (inverse of the duration from sowing to flowering) is largely determined by responses to temperature and photoperiod, and the effects of temperature and of photoperiod at optimum and suboptimum temperatures can be quantified and predicted. However, responses to temperature, and more particularly photoperiod, at supraoptimal temperature are not well understood. Analysis of a comprehensive data set of time to tassel initiation in maize (Zea mays) with a wide range of photoperiods above and below the optimum suggests that photoperiod modulates the negative effects of temperature above the optimum. A simulation analysis of the effects of prescribed increases in temperature (0–6 °C in +1 °C steps) and temperature variability (0% and +50%) on days to tassel initiation showed that tassel initiation occurs later, and variability was increased, as the temperature exceeds the optimum in models both with and without photoperiod sensitivity. However, the inclusion of photoperiod sensitivity above the optimum temperature resulted in a higher apparent optimum temperature and less variability in the time of tassel initiation. Given the importance of changes in plant development for crop yield under climate change, the effects of photoperiod and temperature on development rates above the optimum temperature clearly merit further research, and some of the knowledge gaps are identified herein.

Molecular population genetics and agronomic alleles in seed banks: searching for a needle in a haystack?

Prada, D. — 2009-06-08

Seed banking has been the single most significant reaction of the research community to the alarming rates of plant genetic erosion occurring in the wild. One enduring challenge for a wiser utilization of the resources enclosed in seed banks, however, has been the estimation of their genetic potentials for agriculture's benefit. Key to detecting in landraces and/or wild relatives of modern crops any allelic variant lost during domestication and crop improvement is the use of molecular information to determine structure, evolution, and function of the genes harbouring these alleles. This paper reviews some of the theoretical and statistical issues surrounding the use of molecular population genetics tools for the detection of agronomical valuable alleles in seed banks. Emphasis is made on the technical limitations imposed by seed banking that may lessen the success of integrated and multi-disciplinary molecular approaches. The influence that population stratification and linkage disequilibrium exert on specific experimental designs for a better understanding of the evolutionary history of potential agronomic-related genes is also examined.

Differentially expressed membrane transporters in rice roots may contribute to cultivar dependent salt tolerance

Senadheera, P., Singh, R. K., Maathuis, F. J. M. — 2009-06-08

Salinity tolerance in rice, like in other glycophytes, is a function of cellular ion homeostasis. The large divergence in ion homeostasis between the salt-tolerant FL478 and salt-sensitive IR29 rice varieties can be exploited to understand mechanisms of salinity tolerance. Physiological studies indicate that FL478 shows a lower Na⁺ influx, a reduced Na⁺ translocation to the shoot, and maintains a lower Na⁺:K⁺ ratio. To understand the basis of these differences, a comparative investigation of transcript regulation in roots of the two cultivars was undertaken. This analysis revealed that genes encoding aquaporins, a silicon transporter, and N transporters are induced in both cultivars. However, transcripts for cation transport proteins including OsCHX11, OsCNGC1, OsCAX, and OsTPC1 showed differential regulation between the cultivars. The encoded proteins are likely to participate in reducing Na⁺ influx, lowering the tissue Na⁺:K⁺ ratio and limiting the apoplastic bypass flow in roots of FL478 and are therefore important new targets to improve salt tolerance in rice.

The effects of dwarfing genes on seedling root growth of wheat

Wojciechowski, T., Gooding, M.J., Ramsay, L., Gregory, P.J. — 2009-06-08

Most modern wheat cultivars contain major dwarfing genes, but their effects on root growth are unclear. Near-isogenic lines (NILs) containing Rht-B1b, Rht-D1b, Rht-B1c, Rht8c, Rht-D1c, and Rht12 were used to characterize the effects of semi-dwarfing and dwarfing alleles on root growth of ‘Mercia’ and ‘Maris Widgeon’ wheat cultivars. Wheat seedlings were grown in gel chambers, soil-filled columns, and in the field. Roots were extracted and length and dry mass measured. No significant differences in root length were found between semi-dwarfing lines and the control lines in any experiment, nor was there a significant difference between the root lengths of the two cultivars grown in the field. Total root length of the dwarf lines (Rht-B1c, Rht-D1c, and Rht12) was significantly different from that of the control although the effect was dependent on the experimental methodology; in gel chambers root length of dwarfing lines was increased by ~40% while in both soil media it was decreased (by 24–33%). Root dry mass was 22–30% of the total dry mass in the soil-filled column and field experiments. Root length increased proportionally with grain mass, which varied between NILs, so grain mass was a covariate for the analysis of variance. Although total root length was altered by dwarf lines, root architecture (average root diameter, lateral root:total root ratio) was not affected by reduced height alleles. A direct effect of dwarfing alleles on root growth during seedling establishment, rather than a secondary partitioning effect, was suggested by the present experiments.

Jasmonic acid-induced volatiles of Brassica oleracea attract parasitoids: effects of time and dose, and comparison with induction by herbivores

2009-06-08

Caterpillar feeding induces direct and indirect defences in brassicaceous plants. This study focused on the role of the octadecanoid pathway in induced indirect defence in Brassica oleracea. The effect of induction by exogenous application of jasmonic acid (JA) on the responses of Brussels sprouts plants and on host-location behaviour of associated parasitoid wasps was studied. Feeding by the biting–chewing herbivores Pieris rapae and Plutella xylostella resulted in significantly increased endogenous levels of JA, a central component in the octadecanoid signalling pathway that mediates induced plant defence. The levels of the intermediate 12-oxophyto-dienoic acid (OPDA) were significantly induced only after P. rapae feeding. Three species of parasitoid wasps, Cotesia glomerata, C. rubecula, and Diadegma semiclausum, differing in host range and host specificity, were tested for their behavioural responses to volatiles from herbivore-induced, JA-induced, and non-induced plants. All three species were attracted to volatiles from JA-induced plants compared with control plants; however, they preferred volatiles from herbivore-induced plants over volatiles from JA-induced plants. Attraction of C. glomerata depended on both timing and dose of JA application. JA-induced plants produced larger quantities of volatiles than herbivore-induced and control plants, indicating that not only quantity, but also quality of the volatile blend is important in the host-location behaviour of the wasps.

Comparing induction at an early and late step in signal transduction mediating indirect defence in Brassica oleracea

Bruinsma, M., Pang, B., Mumm, R., van Loon, J. J. A., Dicke, M. — 2009-06-08

The induction of plant defences involves a sequence of steps along a signal transduction pathway, varying in time course. In this study, the effects of induction of an early and a later step in plant defence signal transduction on plant volatile emission and parasitoid attraction are compared. Ion channel-forming peptides represent a class of inducers that induce an early step in signal transduction. Alamethicin (ALA) is an ion channel-forming peptide mixture from the fungus Trichoderma viride that can induce volatile emission and increase endogenous levels of jasmonic acid (JA) and salicylic acid in plants. ALA was used to induce an early step in the defence response in Brussels sprouts plants, Brassica oleracea var. gemmifera, and to study the effect on volatile emission and on the behavioural response of parasitoids to volatile emission. The parasitoid Cotesia glomerata was attracted to ALA-treated plants in a dose-dependent manner. JA, produced through the octadecanoid pathway, activates a later step in induced plant defence signal transduction, and JA also induces volatiles that are attractive to parasitoids. Treatment with ALA and JA resulted in distinct volatile blends, and both blends differed from the volatile blends emitted by control plants. Even though JA treatment of Brussels sprouts plants resulted in higher levels of volatile emission, ALA-treated plants were as attractive to C. glomerata as JA-treated plants. This demonstrates that on a molar basis, ALA is a 20 times more potent inducer of indirect plant defence than JA, although this hormone has more commonly been used as a chemical inducer of plant defence.

MUM ENHANCERS are important for seed coat mucilage production and mucilage secretory cell differentiation in Arabidopsis thaliana

Arsovski, A. A., Villota, M. M., Rowland, O., Subramaniam, R., Western, T. L. — 2009-06-08

Pollination triggers not only embryo development but also the differentiation of the ovule integuments to form a specialized seed coat. The mucilage secretory cells of the Arabidopsis thaliana seed coat undergo a complex differentiation process in which cell growth is followed by the synthesis and secretion of pectinaceous mucilage. A number of genes have been identified affecting mucilage secretory cell differentiation, including MUCILAGE-MODIFIED4 (MUM4). mum4 mutants produce a reduced amount of mucilage and cloning of MUM4 revealed that it encodes a UDP-L-rhamnose synthase that is developmentally up-regulated to provide rhamnose for mucilage pectin synthesis. To identify additional genes acting in mucilage synthesis and secretion, a screen for enhancers of the mum4 phenotype was performed. Eight mum enhancers (men) have been identified, two of which result from defects in known mucilage secretory cell genes (MUM2 and MYB61). Our results show that, in a mum4 background, mutations in MEN1, MEN4, and MEN5 lead to further reductions in mucilage compared to mum4 single mutants, suggesting that they are involved in mucilage synthesis or secretion. Conversely, mutations in MEN2 and MEN6 appear to affect mucilage release rather than quantity. With the exception of men4, whose single mutant exhibits reduced mucilage, none of these genes have a single mutant phenotype, suggesting that they would not have been identified outside the compromised mum4 background.

Characterization of a canola C2 domain gene that interacts with PG, an effector of the necrotrophic fungus Sclerotinia sclerotiorum

Wang, X., Li, Q., Niu, X., Chen, H., Xu, L., Qi, C. — 2009-06-08

Sspg1d, one of endopolygalacturonases, is an important fungal effector secreted by the necrotrophic fungus Sclerotinia sclerotiorum during early infection. Using sspg1d as bait, a small C2 domain protein (designated as IPG-1) was identified by yeast two-hybrid screening of a canola cDNA library. Deletion analysis confirmed that the C-terminus of IPG-1 is responsible for its interaction with sspg1d in the yeast two-hybrid assay. The sspg1d/IPG-1 interaction was further confirmed in plant cells by a biomolecular fluorescence complementation (BiFC) assay. A transient expression assay showed that the IPG-1–GFP fusion protein was targeted to the plasma membrane and nucleus in onion epidermal cells. Following treatment with a Ca²⁺ ionophore, it was distributed throughout the cytosol. Real-time PCR assay demonstrated that IPG-1 was highly induced by Sclerotinia sclerotiorum in canola leaves and stems. Southern blot analysis indicated the presence of about five homologues of IPG-1 in the canola genome. Two additional members of the IPG-1gene family were isolated by RT-PCR. Their sequence similarity with IPG-1 is as high as 95%. However, they did not interact with sspg1d in the yeast two-hybrid assay. Possible roles of IPG-1 and its association with sspg1d in the defence signalling pathway were discussed.

Induction of PtoCDKB and PtoCYCB transcription by temperature during cambium reactivation in Populus tomentosa Carr.

Li, W.-F., Ding, Q., Chen, J.-J., Cui, K.-M., He, X.-Q. — 2009-06-08

Cell cycle progression requires interaction between cyclin-dependent kinase B (CDKB) and cyclin B (CYCB). The seasonal expression patterns of the CDKB and CYCB homologues from Populus tomentosa Carr. were investigated, and effects of temperature and exogenous indole-3-acetic acid (IAA) on their expression were further studied in water culture experiments. Based on the differential responses of dormant cambium cells to exogenous IAA, four stages of cambium dormancy were confirmed for P. tomentosa: quiescence 1 (Q1), rest, quiescence 2-1 (Q2-1), and quiescence 2-2 (Q2-2). PtoCDKB and PtoCYCB transcripts were strongly expressed in the active phases, weakly in Q1, and almost undetectable from rest until late Q2-2. Climatic data analysis showed a correlation between daily air temperature and PtoCDKB and PtoCYCB expression patterns. Water culture experiments with temperature treatment further showed that a low temperature (4 °C) kept PtoCDKB and PtoCYCB transcripts at undetectable levels, while a warm temperature (25 °C) induced their expression in the cambium region. Meanwhile, water culture experiments with exogenous IAA treatment showed that induction of PtoCDKB and PtoCYCB transcription was independent of exogenous IAA. The results suggest that, in deciduous hardwood P. tomentosa growing in a temperate zone, the temperature in early spring is a vital environmental factor for cambium reactivation. The increasing temperature in early spring may induce CDKB and CYCB homologue transcription in the cambium region, which is necessary for cambium cell division.

Pectin methylesterase NaPME1 contributes to the emission of methanol during insect herbivory and to the elicitation of defence responses in Nicotiana attenuata

Korner, E., von Dahl, C. C., Bonaventure, G., Baldwin, I. T. — 2009-06-08

Pectin methylesterases (PMEs) catalyse the demethylation of pectin within plant cell walls, releasing methanol (MeOH) in the process. Thus far, PMEs have been found to be involved in diverse processes such as plant growth and development and defence responses against pathogens. Herbivore attack increases PME expression and activity and MeOH emissions in several plant species. To gain further insights into the role of PMEs in defence responses against herbivores, the expression of a Manduca sexta oral secretion (OS)-inducible PME in Nicotiana attenuata (NaPME1) was silenced by RNA interference (RNAi)-mediated gene silencing. Silenced lines (ir-pme) showed 50% reduced PME activity in leaves and 70% reduced MeOH emissions after OS elicitation compared with the wild type (WT), demonstrating that the herbivore-induced MeOH emissions originate from the demethylation of pectin by PME. In the initial phase of the OS-induced jasmonic acid (JA) burst (first 30 min), ir-pme lines produced WT levels of this hormone and of jasmonyl-isoleucine (JA-Ile); however, these levels were significantly reduced in the later phase (60–120 min) of the burst. Similarly, suppressed levels of the salicylic acid (SA) burst induced by OS elicitation were observed in ir-pme lines even though wounded ir-pme leaves contained slightly increased amounts of SA. This genotype also presented reduced levels of OS-induced trypsin proteinase inhibitor activity in leaves and consistently increased M. sexta larvae performance compared with WT plants. These latter responses could not be recovered by application of exogenous MeOH. Together, these results indicated that PME contributes, probably indirectly by affecting cell wall properties, to the induction of anti-herbivore responses.

The effect of sucrose and abscisic acid interaction on sucrose synthase and its relationship to grain filling of rice (Oryza sativa L.)

Tang, T., Xie, H., Wang, Y., Lu, B., Liang, J. — 2009-06-08

Rice grain filling is a process of conversion of sucrose into starch catalysed by a series of enzymes. Sucrose synthase (SUS) is considered as a key enzyme regulating this process. This study investigated the possible roles of sucrose and abscisic acid (ABA) in mediating the activity and expression of SUS protein of grains during grain filling in rice (Oryza sativa). Field-grown rice plants and detached cultured panicles were used as experimental materials. Several treatments, including spikelet thinning, leaf cutting, and applications of different concentrations of exogenous sucrose and ABA, were imposed during grain filling. A higher SUS activity was found in superior grains than in inferior grains in the earlier stage of grain filling, which was significantly and closely related to a higher grain filling rate and starch accumulation. An increase in sucrose concentration in grains as a result of different treatments increased both SUS activity and SUS protein expression in grains. An increase in ABA concentration gave similar results. Furthermore, effects of interactions between sucrose and ABA on the activity and expression of SUS protein in grains were also found. It was suggested that sucrose- and ABA-mediated rice grain filling is largely due to an increase in SUS activity and SUS protein expression.

Altered expression of cytosolic/nuclear HSC70-1 molecular chaperone affects development and abiotic stress tolerance in Arabidopsis thaliana

2009-06-08

Molecular chaperones of the heat shock cognate 70 kDa (HSC70) family are highly conserved in all living organisms and assist nascent protein folding in normal physiological conditions as well as in biotic and abiotic stress conditions. In the absence of specific inhibitors or viable knockout mutants, cytosolic/nuclear HSC70-1 overexpression (OE) and mutants in the HSC70 co-chaperone SGT1 (suppressor of G₂/M allele of skp1) were used as genetic tools to identify HSC70/SGT1 functions in Arabidopsis development and abiotic stress responses. HSC70-1 OE caused a reduction in root and shoot meristem activities, thus explaining the dwarfism of those plants. In addition, HSC70-1 OE did not impair auxin-dependent phenotypes, suggesting that SGT1 functions previously identified in auxin signalling are HSC70 independent. While responses to abiotic stimuli such as UV-C exposure, phosphate starvation, or seedling de-etiolation were not perturbed by HSC70-1 OE, it specifically conferred -ray hypersensitivity and tolerance to salt, cadmium (Cd), and arsenic (As). Cd and As perception was not perturbed, but plants overexpressing HSC70-1 accumulated less Cd, thus providing a possible molecular explanation for their tolerance phenotype. In summary, genetic evidence is provided for HSC70-1 involvement in a limited set of physiological processes, illustrating the essential and yet specific functions of this chaperone in development and abiotic stress responses in Arabidopsis.

Nitrogen affects cluster root formation and expression of putative peptide transporters

2009-06-08

Non-mycorrhizal Hakea actites (Proteaceae) grows in heathland where organic nitrogen (ON) dominates the soil nitrogen (N) pool. Hakea actites uses ON for growth, but the role of cluster roots in ON acquisition is unknown. The aim of the present study was to ascertain how N form and concentration affect cluster root formation and expression of peptide transporters. Hydroponically grown plants produced most biomass with low molecular weight ON>inorganic N>high molecular weight ON, while cluster roots were formed in the order no-N>ON>inorganic N. Intact dipeptide was transported into roots and metabolized, suggesting a role for the peptide transporter (PTR) for uptake and transport of peptides. HaPTR4, a member of subgroup II of the NRT1/PTR transporter family, which contains most characterized di- and tripeptide transporters in plants, facilitated transport of di- and tripeptides when expressed in yeast. No transport activity was demonstrated for HaPTR5 and HaPTR12, most similar to less well characterized transporters in subgroup III. The results provide further evidence that subgroup II of the NRT1/PTR family contains functional di- and tripeptide transporters. Green fluorescent protein fusion proteins of HaPTR4 and HaPTR12 localized to tonoplast, and plasma- and endomembranes, respectively, while HaPTR5 localized to vesicles of unknown identity. Grown in heathland or hydroponic culture with limiting N supply or starved of nutrients, HaPTR genes had the highest expression in cluster roots and non-cluster roots, and leaf expression increased upon re-supply of ON. It is concluded that formation of cluster roots and expression of PTR are regulated in response to N supply.

Root-to-shoot Cd translocation via the xylem is the major process determining shoot and grain cadmium accumulation in rice

Uraguchi, S., Mori, S., Kuramata, M., Kawasaki, A., Arao, T., Ishikawa, S. — 2009-06-08

Physiological properties involved in divergent cadmium (Cd) accumulation among rice genotypes were characterized using the indica cultivar ‘Habataki’ (high Cd in grains) and the japonica cultivar ‘Sasanishiki’ (low Cd in grains). Time-dependence and concentration-dependence of symplastic Cd absorption in roots were revealed not to be responsible for the different Cd accumulation between the two cultivars because root Cd uptake was not greater in the Cd-accumulating cultivar ‘Habataki’ compared with ‘Sasanishiki’. On the other hand, rapid and greater root-to-shoot Cd translocation was observed in ‘Habataki’, which could be mediated by higher abilities in xylem loading of Cd and transpiration rate as a driving force. To verify whether different abilities in xylem-mediated shoot-to-root translocation generally account for the genotypic variation in shoot Cd accumulation in rice, the world rice core collection, consisting of 69 accessions which covers the genetic diversity of almost 32 000 accessions of cultivated rice, was used. The results showed strong correlation between Cd levels in xylem sap and shoots and grains among the 69 rice accessions. Overall, the results presented in this study revealed that the root-to-shoot Cd translocation via the xylem is the major and common physiological process determining the Cd accumulation level in shoots and grains of rice plants.

Co-ordination of early and late ripening events in apples is regulated through differential sensitivities to ethylene

Johnston, J. W., Gunaseelan, K., Pidakala, P., Wang, M., Schaffer, R. J. — 2009-06-08

In this study, it is shown that anti-sense suppression of Malus domestica 1-AMINO-CYCLOPROPANE-CARBOXYLASE OXIDASE (MdACO1) resulted in fruit with an ethylene production sufficiently low to be able to assess ripening in the absence of ethylene. Exposure of these fruit to different concentrations of exogenous ethylene showed that flesh softening, volatile biosynthesis, and starch degradation, had differing ethylene sensitivity and dependency. Early ripening events such as the conversion of starch to sugars showed a low dependency for ethylene, but a high sensitivity to low concentrations of ethylene (0.01 µl l^–1). By contrast, later ripening events such as flesh softening and ester volatile production showed a high dependency for ethylene but were less sensitive to low concentrations (needing 0.1 µl l^–1 for a response). A sustained exposure to ethylene was required to maintain ripening, indicating that the role of ethylene may go beyond that of ripening initiation. These results suggest a conceptual model for the control of individual ripening characters in apple, based on both ethylene dependency and sensitivity.

Subcellular localization and biochemical comparison of cytosolic and secreted cytokinin dehydrogenase enzymes from maize

2009-06-08

Cytokinin dehydrogenase (CKX; EC 1.5.99.12) degrades cytokinin hormones in plants. There are several differently targeted isoforms of CKX in plant cells. While most CKX enzymes appear to be localized in the apoplast or vacuoles, there is generally only one CKX per plant genome that lacks a translocation signal and presumably functions in the cytosol. The only extensively characterized maize CKX is the apoplastic ZmCKX1; a maize gene encoding a non-secreted CKX has not previously been cloned or characterized. Thus, the aim of this work was to characterize the maize non-secreted CKX gene (ZmCKX10), elucidate the subcellular localization of ZmCKX10, and compare its biochemical properties with those of ZmCKX1. Expression profiling of ZmCKX1 and ZmCKX10 was performed in maize tissues to determine their transcript abundance and organ-specific expression. For determination of the subcellular localization, the CKX genes were fused with green fluorescent protein (GFP) and overexpressed in tomato hairy roots. Using confocal microscopy, the ZmCKX1–GFP signal was confirmed to be present in the apoplast, whereas ZmCKX10–GFP was detected in the cytosol. No interactions of ZmCKX1 with the plasma membrane were observed. While roots overexpressing ZmCKX1–GFP formed significantly more mass in comparison with the control, non-secreted CKX overexpression resulted in a small reduction in root mass accumulation. Biochemical characterization of ZmCKX10 was performed using recombinant protein produced in Pichia pastoris. In contrast to the preference for 2,6-dichlorophenolindophenol (DCPIP) as an electron acceptor and trans-zeatin, N⁶-(²-isopentenyl)adenine (iP) and N⁶-(²-isopentenyl)adenosine (iPR) as substrates for ZmCKX1, the non-secreted ZmCKX10 had a range of suitable electron acceptors, and the enzyme had a higher preference for cis-zeatin and cytokinin N-glucosides as substrates.

Complementary regulation of four Eucalyptus CBF genes under various cold conditions

2009-06-08

CBF transcription factors play central roles in the control of freezing tolerance in plants. The isolation of two additional CBF genes, EguCBF1c and EguCBF1d, from E. gunnii, one of the cold-hardiest Eucalyptus species, is described. While the EguCBF1D protein sequence is very similar to the previously characterized EguCBF1A and EguCBF1B sequences, EguCBF1C is more distinctive, in particular in the AP2-DBD (AP2-DNA binding domain). The expression analysis of the four genes by RT-qPCR reveals that none of them is specific to one stress but they are all preferentially induced by cold, except for the EguCBF1c gene which is more responsive to salt. The calculation of the transcript copy number enables the quantification of constitutive CBF gene expression. This basal level, significant for the four genes, greatly influences the final EguCBF1 transcript level in the cold. A cold shock at 4 °C, as well as a progressive freezing which mimics a natural frost episode, trigger a fast and strong response of the EguCBF1 genes, while growth at acclimating temperatures results in a lower but more durable induction. The differential expression of the four EguCBF1 genes under these cold regimes suggests that there is a complementary regulation. The high accumulation of the CBF transcript, observed in response to the different types of cold conditions, might be a key for the winter survival of this evergreen broad-leaved tree.

13C and 15N allocations of two alpine species from early and late snowmelt locations reflect their different growth strategies

Baptist, F., Tcherkez, G., Aubert, S., Pontailler, J.-Y., Choler, P., Nogues, S. — 2009-06-08

Intense efforts are currently devoted to disentangling the relationships between plant carbon (C) allocation patterns and soil nitrogen (N) availability because of their consequences for growth and more generally for C sequestration. In cold ecosystems, only a few studies have addressed whole-plant C and/or N allocation along natural elevational or topographical gradients. ¹²C/¹³C and ¹⁴N/¹⁵N isotope techniques have been used to elucidate C and N partitioning in two alpine graminoids characterized by contrasted nutrient economies: a slow-growing species, Kobresia myosuroides (KM), and a fast-growing species, Carex foetida (CF), located in early and late snowmelt habitats, respectively, within the alpine tundra (French Alps). CF allocated higher labelling-related ¹³C content belowground and produced more root biomass. Furthermore, assimilates transferred to the roots were preferentially used for growth rather than respiration and tended to favour N reduction in this compartment. Accordingly, this species had higher ¹⁵N uptake efficiency than KM and a higher translocation of reduced ¹⁵N to aboveground organs. These results suggest that at the whole-plant level, there is a compromise between N acquisition/reduction and C allocation patterns for optimized growth.

Overexpression of PwTUA1, a pollen-specific tubulin gene, increases pollen tube elongation by altering the distribution of {alpha}-tubulin and promoting vesicle transport

Yu, Y., Li, Y., Li, L., Lin, J., Zheng, C., Zhang, L. — 2009-06-08

Tubulin genes are intimately associated with cell division and cell elongation, which are central to plant secondary cell wall development. However, their roles in pollen tube polar growth remain elusive. Here, a TUA1 gene from Picea wilsonii, which is specifically expressed in pollen, was isolated. Semi-quantitative RT-PCR analysis showed that the amount of PwTUA1 transcript varied at each stage of growth of the pollen tube and was induced by calcium ions and boron. Transient expression analysis in P. wilsonii pollen indicated that PwTUA1 improved pollen germination and pollen tube growth. The pollen of transgenic Arabidopsis overexpressing PwTUA1 also showed a higher percentage of germination and faster growth than wild-type plants not only in optimal germination medium, but also in medium supplemented with elevated levels of exogenous calcium ions or boron. Immunofluorescence and electron microscopy showed -tubulin to be enriched and more vesicles accumulated in the apex region in germinating transgenic Arabidopsis pollen compared with wild-type plants. These results demonstrate that PwTUA1 up-regulated by calcium ions and boron contributes to pollen tube elongation by altering the distribution of -tubulin and regulating the deposition of pollen cell wall components during the process of tube growth. The possible role of PwTUA1 in microtubule dynamics and organization was discussed.

Inefficient double-strand DNA break repair is associated with increased fasciation in Arabidopsis BRCA2 mutants

2009-06-08

BRCA2 is a breast tumour susceptibility factor with functions in maintaining genome stability through ensuring efficient double-strand DNA break (DSB) repair via homologous recombination. Although best known in vertebrates, fungi, and higher plants also possess BRCA2-like genes. To investigate the role of Arabidopsis BRCA2 genes in DNA repair in somatic cells, transposon insertion mutants of the AtBRCA2a and AtBRCA2b genes were identified and characterized. atbrca2a-1 and atbrca2b-1 mutant plants showed hypersensitivity to genotoxic stresses compared to wild-type plants. An atbrca2a-1/atbrca2b-1 double mutant showed an additive increase in sensitivity to genotoxic stresses compared to each single mutant. In addition, it was found that atbrca2 mutant plants displayed fasciation and abnormal phyllotaxy phenotypes with low incidence, and that the ratio of plants exhibiting these phenotypes is increased by -irradiation. Interestingly, these phenotypes were also induced by -irradiation in wild-type plants. Moreover, it was found that shoot apical meristems of the atbrca2a-1/atbrca2b-1 double mutant show altered cell cycle progression. These data suggest that inefficient DSB repair in the atbrca2a-1/atbrca2b-1 mutant leads to disorganization of the programmed cell cycle of apical meristems.

The expression of caffeic acid 3-O-methyltransferase in two wheat genotypes differing in lodging resistance

Ma, Q.-H. — 2009-06-08

Stem lodging-resistance is an important phenotype in crop production. In the present study, the expression of the wheat COMT gene (TaCM) was determined in basal second internodes of lodging-resistant (H4564) and lodging-susceptible (C6001) cultivars at stem elongation, heading, and milky endosperm corresponding to Zadoks stages Z37, Z60, and Z75, respectively. The TaCM protein levels were analysed by protein gel blot and COMT enzyme activity was determined during the same stem developmental stages. TaCM mRNA levels were higher in H4546 from elongation to the milky stages and in C6001 the TaCM mRNA levels decreased markedly at the heading and milky stages. The TaCM protein levels and COMT activity were also higher in H4564 than that in C6001 at the heading and milky stages. These results corresponded to a higher lignin content measured by the Klason method and stem strength and a lower lodging index in H4564 than in C6001 at the heading and milky stages. Therefore, the TaCM mRNA levels, protein levels, and enzyme activity in developing wheat stems were associated with stem strength and lodging index in these two wheat cultivars. Southern analysis in a different population suggested that a TaCM locus was located in the distal region of chromosome 3BL, which has less investigated by QTL for lodging-resistant phenotype.

Erratum

2009-06-08

Preface

Flexas, J., Loreto, F., Niinemets, U., Sharkey, T. — 2009-05-20

Estimating mesophyll conductance to CO2: methodology, potential errors, and recommendations

2009-05-20

The three most commonly used methods for estimating mesophyll conductance (g_m) are described. They are based on gas exchange measurements either (i) by themselves; (ii) in combination with chlorophyll fluorescence quenching analysis; or (iii) in combination with discrimination against ¹³CO₂. To obtain reliable estimates of g_m, the highest possible accuracy of gas exchange is required, particularly when using small leaf chambers. While there may be problems in achieving a high accuracy with leaf chambers that clamp onto a leaf with gaskets, guidelines are provided for making necessary corrections that increase reliability. All methods also rely on models for the calculation of g_m and are sensitive to variation in the values of the model parameters. The sensitivity to these factors and to measurement error is analysed and ways to obtain the most reliable g_m values are discussed. Small leaf areas can best be measured using one of the fluorescence methods. When larger leaf areas can be measured in larger chambers, the online isotopic methods are preferred. Using the large CO₂ draw-down provided by big chambers, and the isotopic method, is particularly important when measuring leaves with high g_m that have a small difference in [CO₂] between the substomatal cavity and the site of carboxylation in the chloroplast (C_i–C_c gradient). However, equipment for the fluorescence methods is more easily accessible. Carbon isotope discrimination can also be measured in recently synthesized carbohydrates, which has its advantages under field conditions when large number of samples must be processed. The curve-fitting method that uses gas exchange measurements only is not preferred and should only be used when no alternative is available. Since all methods have their weaknesses, the use of two methods for the estimation of g_m, which are as independent as possible, is recommended.

Resistances along the CO2 diffusion pathway inside leaves

Evans, J. R., Kaldenhoff, R., Genty, B., Terashima, I. — 2009-05-20

CO₂ faces a series of resistances while diffusing between the substomatal cavities and the sites of carboxylation within chloroplasts. The absence of techniques to measure the resistance of individual steps makes it difficult to define their relative importance. Resistance to diffusion through intercellular airspace differs between leaves, but is usually of minor importance. Leaves with high photosynthetic capacity per unit leaf area reduce mesophyll resistance by increasing the surface area of chloroplasts exposed to intercellular airspace per unit leaf area, S_c. Cell walls impose a significant resistance. Assuming an effective porosity of the cell wall of 0.1 or 0.05, then cell walls could account for 25% or 50% of the total mesophyll resistance, respectively. Since the fraction of apoplastic water that is unbound and available for unhindered CO₂ diffusion is unknown, it is possible that the effective porosity is <0.05. Effective porosity could also vary in response to changes in pH or cation concentration. Consequently, cell walls could account for >50% of the total resistance and a variable proportion. Most of the remaining resistance is imposed by one or more of the three membranes as mesophyll resistance can be altered by varying the expression of cooporins. The CO₂ permeability of vesicles prepared from chloroplast envelopes has been reduced by RNA interference (RNAi) expression of NtAQP1, but not those prepared from the plasma membrane. Carbonic anhydrase activity also influences mesophyll resistance. Mesophyll resistance is relatively insensitive to the manipulation of any step in the pathway because it represents only part of the total and may also be countered by pleiotropic compensatory changes. The parameters in greatest need of additional measurements are S_c, mesophyll cell wall thickness, and the permeabilities of the plasma membrane and chloroplast envelope.

Role of mesophyll diffusion conductance in constraining potential photosynthetic productivity in the field

Niinemets, U., Diaz-Espejo, A., Flexas, J., Galmes, J., Warren, C. R. — 2009-05-20

Limited mesophyll diffusion conductance to CO₂ (g_m) can significantly constrain plant photosynthesis, but the extent of g_m-limitation is still imperfectly known. As g_m scales positively with foliage photosynthetic capacity (A), the CO₂ drawdown from substomatal cavities (C_i) to chloroplasts (C_C, C_i–C_C=A/g_m) rather than g_m alone characterizes the mesophyll diffusion limitations of photosynthesis. The dependencies of g_m on A, foliage structure (leaf dry mass per unit area, M_A), and the resulting drawdowns across a dataset of 81 species of contrasting foliage structure and photosynthetic potentials measured under non-stressed conditions were analysed to describe the structure-driven potential photosynthetic limitations due to g_m. Further the effects of key environmental stress factors and leaf and plant developmental alterations on g_m and CO₂ drawdown were evaluated and the implications of varying g_m on foliage photosynthesis in the field were simulated. The meta-analysis demonstrated that g_m of non-stressed leaves was negatively correlated with M_A, and despite the positive relationship between g_m and A, the CO₂ drawdown was larger in leaves with more robust structure. The correlations were stronger with mass-based g_m and A, probably reflecting the circumstance that mesophyll diffusion is a complex three-dimensional process that scales better with mesophyll volume-weighted than with leaf area-weighted traits. The analysis of key environmental stress effects on g_m and CO₂ drawdowns demonstrated that the effect of individual stresses on CO₂ drawdowns varies depending on the stress effects on foliage structure and assimilation rates. Leaf diffusion limitations are larger in non-senescent older leaves and also in senescent leaves, again reflecting more robust leaf structure and/or non-co-ordinated alterations in leaf photosynthesis and g_m. According to simulation analyses, in plants with a larger part of the overall diffusion conductance from the ambient atmosphere to the chloroplasts in the mesophyll, photosynthesis is less sensitive to changes in stomatal conductance. Accordingly, in harsher environments that support vegetation with tougher long-living stress-tolerant leaves with lower g_m, reductions in stomatal conductance that are common during stress periods are expected to alter photosynthesis less than in species where a larger part of the total diffusion limitation is determined by stomata. While structural robustness improves plant performance under environmental stress, low g_m and inherently large CO₂ drawdown in robust leaves limits the photosynthesis of these plants more severely under favourable conditions when stomatal conductance is high. The differences in overall responsiveness to environmental modifications of plants with varying g_m need consideration in current large-scale ecosystem productivity models.

Importance of mesophyll diffusion conductance in estimation of plant photosynthesis in the field

Niinemets, U., Diaz-Espejo, A., Flexas, J., Galmes, J., Warren, C. R. — 2009-05-20

Mesophyll diffusion conductance to CO₂ (g_m) is an important leaf characteristic determining the drawdown of CO₂ from substomatal cavities (C_i) to chloroplasts (C_C). Finite g_m results in modifications in the shape of the net assimilation (A) versus C_i response curves, with the final outcome of reduced maximal carboxylase activity of Rubisco (V_cmax), and a greater ratio of the capacity for photosynthetic electron transport to V_cmax (J_max/V_cmax) and alterations in mitochondrial respiration rate (R_d) when estimated from A/C_i responses without considering g_m. The influence of different Farquhar et al. model parameterizations on daily photosynthesis under non-stressed (C_i kept constant throughout the day) and stressed conditions (mid-day reduction in C_i) was compared. The model was parameterized on the basis of A/C_C curves and A/C_i curves using both the conventional fitting procedure (V_cmax and R_d fitted separately to the linear part of the response curve and J_max to the saturating part) and a procedure that fitted all parameters simultaneously. The analyses demonstrated that A/C_i parameterizations overestimated daily assimilation by 6–8% for high g_m values, while they underestimated if by up to 70% for low g_m values. Qualitative differences between the A/C_i and A/C_C parameterizations were observed under stressed conditions, when underestimated V_cmax and overestimated R_d of A/C_i parameterizations resulted in excessive mid-day depression of photosynthesis. Comparison with measured diurnal assimilation rates in the Mediterranean sclerophyll species Quercus ilex under drought further supported this bias of A/C_i parameterizations. While A/C_i parameterization predicted negative carbon balance at mid-day, actual measurements and simulations with the A/C_C approach yielded positive carbon gain under these conditions. In addition, overall variation captured by the best A/C_i parameterization was poor compared with the A/C_C approach. This analysis strongly suggests that for correct parameterization of daily time-courses of photosynthesis under realistic field conditions, g_m must be included in photosynthesis models.

The impact of blue light on leaf mesophyll conductance

Loreto, F., Tsonev, T., Centritto, M. — 2009-05-20

Blue light has many direct and indirect effects on photosynthesis. The impact of blue light on mesophyll conductance (g_m), one of the main diffusive limitation to photosynthesis, was investigated in leaves of Nicotiana tabacum and Platanus orientalis, characterized by high and low g_m, respectively. Leaves were exposed to blue light fractions between 0% and 80% of incident light intensity (300 µmol photons m^–2 s^–1), the other fraction being supplied as red light. Leaves exposed to blue light showed reduced photosynthesis and unaltered stomatal conductance. The g_m, measured using the chlorophyll fluorescence-based method, was strongly reduced in both plant species. Such a reduction of g_m may not be real, as several assumptions used for the calculation of g_m by fluorescence may not hold under blue light. To assess possible artefacts, the electron transport rate measured by fluorescence (J_f) and by gas-exchange (J_c) were compared in leaves exposed to different fractions of blue light under non-photorespiratory conditions. The two values were only equal, a prerequisite for correct g_m measurements, when the illumination was totally provided as red light. Under increasing blue light levels an increasing discrepancy was observed, which suggests that J_f was not correctly calculated, and that such an error could also upset g_m measurements. Blue light was not found to change the absorbance of light by leaves, whereas it slightly decreased the distribution of light to PSII. To equate J_f and J_c under blue light, a further factor must be added to the J_f equation, which possibly accounted for the reduced efficiency of energy transfer between the pigments predominantly absorbing blue light (the carotenoids) and the chlorophylls. This correction reduced by about 50% the effect of blue light on g_m. However, the residual reduction of g_m under blue light was real and significant, although it did not appear to limit the chloroplast CO₂ concentration and, consequently, photosynthesis. Reduction of g_m might be caused by chloroplast movement to avoid photodamage, in turn affecting the chloroplast surface exposed to intercellular spaces. However, g_m reduction occurred immediately after exposure to blue light and was complete after less than 3 min, whereas chloroplast relocation was expected to occur more slowly. In addition, fast g_m reduction was also observed after inhibiting chloroplast movement by cytochalasin. It is therefore concluded that g_m reduction under blue light is unlikely to be caused by chloroplast movement only, and must be elicited by other, as yet unknown, factors.

Light and CO2 do not affect the mesophyll conductance to CO2 diffusion in wheat leaves

Tazoe, Y., von Caemmerer, S., Badger, M. R., Evans, J. R. — 2009-05-20

In C₃ plants, diffusion of CO₂ into leaves is restricted by stomata and subsequently by the intercellular airspaces and liquid phase into chloroplasts. While considerable information exists on the effect of environmental conditions on stomatal conductance (g_s), little is known on whether the mesophyll conductance to CO₂ diffusion (g_m) changes with respect to photon flux density (PFD) and CO₂ partial pressure (pCO₂). In this study, the effects of PFD and/or pCO₂ on g_m were examined in wheat leaves by combining gas exchange with carbon isotope discrimination measurements using a membrane inlet mass spectrometer. Measurements were made in 2% O₂ to reduce the fractionation associated with photorespiration. The magnitude of g_m was estimated using the observed carbon isotope discrimination (), ambient and intercellular pCO₂, CO₂ assimilation and respiration rates, either from an individual measurement made under one environmental condition or from a global fit to multiple measurements where PFD was varied. It was found that respiration made a significant and variable contribution to the observed discrimination, which associated with the difference in isotopic composition between CO₂ in the greenhouse and that used for gas exchange measurements. In wheat, g_m was independent of PFD between 200 and 1500 µmol m^–2 s^–1 and was independent of p_i between 80 and 500 µbar.

Influence of leaf dry mass per area, CO2, and irradiance on mesophyll conductance in sclerophylls

Hassiotou, F., Ludwig, M., Renton, M., Veneklaas, E. J., Evans, J. R. — 2009-05-20

Leaf photosynthesis (A) is limited by mesophyll conductance (g_m), which is influenced by both leaf structure and the environment. Previous studies have indicated that the upper bound for g_m declines as leaf dry mass per area (LMA, an indicator of leaf structure) increases, extrapolating to zero at a LMA of about 240 g m^–2. No data exist on g_m and its response to the environment for species with LMA values higher than 220 g m^–2. In this study, laboratory measurements of leaf gas exchange and in vivo chlorophyll a fluorescence were used concurrently to derive estimates of g_m in seven species of the Australian sclerophyllous genus Banksia covering a wide range of LMA (130–480 g m^–2). Irradiance and CO₂ were varied during those measurements to gauge the extent of environmental effects on g_m. A significant decrease of g_m with increasing LMA was found. g_m declined by 35–60% in response to increasing atmospheric CO₂ concentrations at high irradiance, with a more variable response (0–60%) observed at low irradiance, where g_m was, on average, 22% lower than at high irradiance at ambient CO₂ concentrations. Despite considerable variation in A and LMA between the Banksia species, the CO₂ concentrations in the intercellular air spaces (C_i, 262±5 µmol mol^–1) and in the chloroplasts (C_c, 127±4 µmol mol^–1) were remarkably stable.

Mesophyll conductance to CO2 transport estimated by two independent methods: effect of variable CO2 concentration and abscisic acid

Vrabl, D., Vaskova, M., Hronkova, M., Flexas, J., Santrucek, J. — 2009-05-20

Mesophyll conductance (g_m) and stomatal conductance (g_s) are two crucial components of the diffusive limitation of photosynthesis. Variation of g_m in response to CO₂ concentration was evaluated by using two independent methods based on measurements of variable electron transport rate (J) and instantaneous carbon isotope discrimination, respectively. Both methods of g_m estimation showed a very similar shape of the g_m/C_i relationship, with an initial increase at low substomatal CO₂ concentrations (C_i), a peak at 180–200 µmol mol^–1 C_i, and a subsequent decrease at higher C_i. A good correlation was observed between values of g_m estimated from the two methods, except when C_i <200 µmol mol^–1, suggesting that the initial increase of g_m at low C_i was probably due to unreliable estimates over that range of C_i. Plants were also treated with abscisic acid (ABA), which induced a reduction in g_s without significantly affecting the rate of photosynthesis, g_m or the photosynthetic capacity. The present results confirm, using two independent methods, that g_m is strongly sensitive to C_i, and that the relationship between g_s and g_m is not conservative, differing between control and ABA-treated plants.

Leaf gas exchange, carbon isotope discrimination, and grain yield in contrasting rice genotypes subjected to water deficits during the reproductive stage

Centritto, M., Lauteri, M., Monteverdi, M. C., Serraj, R. — 2009-05-20

Genotypic variations in leaf gas exchange and yield were analysed in five upland-adapted and three lowland rice cultivars subjected to a differential soil moisture gradient, varying from well-watered to severely water-stressed conditions. A reduction in the amount of water applied resulted in a significant decrease in leaf gas exchange and, subsequently, in above-ground dry mass and grain yield, that varied among genotypes and distance from the line source. The comparison between the variable J and the values in recently synthesized sugars methods, yielded congruent estimations of mesophyll conductance (g_m), confirming the reliability of these two techniques. Our data demonstrate that g_m is a major determinant of photosynthesis (A), because rice genotypes with inherently higher g_m were capable of keeping higher A in stressed conditions. Furthermore, A, g_s, and g_m of water-stressed genotypes rapidly recovered to the well-watered values upon the relief of water stress, indicating that drought did not cause any lasting metabolic limitation to photosynthesis. The comparisons between the A/C_i and corresponding A/C_c curves, measured in the genotypes that showed intrinsically higher and lower instantaneous A, confirmed this finding. Moreover, the effect of drought stress on grain yield was correlated with the effects on both A and total diffusional limitations to photosynthesis. Overall, these data indicate that genotypes which showed higher photosynthesis and conductances were also generally more productive across the entire soil moisture gradient. The analysis of revealed a substantial variation of water use efficiency among the genotypes, both on the long-term (leaf pellet analysis) and short-term scale (leaf soluble sugars analysis).

Internal conductance under different light conditions along the plant profile of Ethiopian mustard (Brassica carinata A. Brown.)

Monti, A., Bezzi, G., Venturi, G. — 2009-05-20

This study focused on the internal conductance (g_i) along the plant profile of Ethiopian mustard under two light conditions: (i) light from the top only (I1); (ii) light from the top integrated by supplementary lateral light along the whole plant profile (I2). Lateral light strongly increased the productivity (e.g. +104% of seed oil) and net photosynthesis (A). The latter appeared more driven by g_i (r=0.78**) than by stomatal conductance (g_s) (r=0.51*). Importantly, irradiance also considerably shortened the time from leaf appearance to senescence, which means that corresponding leaves in I1 and I2 had different ages. Therefore, since leaf age and irradiance have counteracting effects on g_i, I1 sometimes showed higher g_i values than I2. With respect to irradiance, leaf age had clearly higher effects on g_i, which radically declined from the top to the basal leaves, even under constant light conditions. The internal conductance caused a significant drawdown of CO₂ from the sub-stomatal cavity (C_i) to the site of carboxylation (C_c) that, in turn, led to a substantial underestimation of V_cmax calculated using the A/C_i model. Again, the trends of g_i and g_s were not consistent along the plant profile, and so the ratio between stomatal and internal limitations to A changed from top to bottom leaves, accordingly. This study suggests that g_i may be a valuable trait for increasing photosynthetic capacity and productivity; nonetheless, it suggests caution in selecting leaves for high g_i, as the latter can considerably change along the plant profile due to leaf age and irradiance effects.

Light-saturated photosynthetic rate in high-nitrogen rice (Oryza sativa L.) leaves is related to chloroplastic CO2 concentration

Li, Y., Gao, Y., Xu, X., Shen, Q., Guo, S. — 2009-05-20

To identify the effect of nitrogen (N) nutrition on photosynthetic efficiency and mesophyll conductance of rice seedlings (Oryza sativa L., cv. ‘Shanyou 63’ hybrid indica China), hydroponic experiments with different concentrations of N were conducted in a greenhouse. Although leaf N concentration on a dry mass basis increased with increasing supply of N, no significant differences in seedling biomass were observed. A higher light-saturated CO₂ assimilation rate (A) with a high concentration of supplied N was associated with a higher carboxylation efficiency (CE), but not a higher apparent quantum yield (). Based on classic photosynthetic models, both the Rubisco content and the ribulose bisphosphate (RuBP) regeneration rate were sufficient for light-saturated photosynthesis in rice seedlings; the estimated chloroplastic CO₂ concentration (C_c) and mesophyll conductance (g_m) demonstrated that a low C_c was the ultimate limiting factor to photosynthetic efficiency with a higher N supply. Due to a greater chloroplast size (i.e. a shorter distance to the plasma membrane) with a higher supply of N, the CO₂ transport resistance in the liquid phase (g_liq) in high-N leaves was lower than that in low-N leaves, which resulted in higher g_m and C_c in high-N leaves. Although CE_A/Ci was higher with a high supply of N, there were no differences in CE_A/Cc between plants grown with different concentrations of N, indicating that the carboxylation capacity of Rubisco between plants grown at different N concentrations was constant. The enhanced photosynthetic rate with supply of a high N concentration was attributed to a higher CO₂ concentration in the chloroplasts, related to a higher mesophyll conductance due to an increased chloroplast size.

Photosynthesis limitations during water stress acclimation and recovery in the drought-adapted Vitis hybrid Richter-110 (V. berlandierixV. rupestris)

2009-05-20

The hybrid Richter-110 (Vitis berlandierixVitis rupestris) has the reputation of being a genotype strongly adapted to drought. A study was performed with plants of R-110 subjected to sustained water-withholding to induce acclimation to two different levels of water stress, followed by rewatering to induce recovery. The goal was to analyse how photosynthesis is regulated during acclimation to water stress and recovery. In particular, the regulation of stomatal conductance (g_s), mesophyll conductance to CO₂ (g_m), leaf photochemistry (chlorophyll fluorescence and thermoluminescence), and biochemistry (V_c,max) were assessed. During water stress, g_s declined to 0.1 and less than 0.05 mol CO₂ m^–2 s^–1 in moderately and severely water-stressed plants, respectively, and was kept quite constant during an acclimation period of 1-week. Leaf photochemistry proved to be very resistant to the applied water-stress conditions. By contrast, g_m and V_c,max were affected by water stress, but they were not kept constant during the acclimation period. g_m was initially unaffected by water stress, and V_c,max even increased above control values. However, after several days of acclimation to water stress, both parameters declined below (g_m) or at (V_c,max) control values. For the latter two parameters there seemed to be an interaction between water stress and cumulative irradiance, since both recovered to control values after several cloudy days despite water stress. A photosynthesis limitation analysis revealed that diffusional limitations and not biochemical limitations accounted for the observed decline in photosynthesis during water stress and slow recovery after rewatering, both in moderately and severely stressed plants. However, the relative contribution of stomatal (SL) and mesophyll conductance (MCL) limitations changes during acclimation to water stress, from predominant SL early during water stress to similar SL and MCL after acclimation. Finally, photosynthesis recovery after rewatering was mostly limited by SL, since stomatal closure recovered much more slowly than g_m.

The role of mesophyll conductance during water stress and recovery in tobacco (Nicotiana sylvestris): acclimation or limitation?

2009-05-20

While the responses of photosynthesis to water stress have been widely studied, acclimation to sustained water stress and recovery after re-watering is poorly understood. In particular, the factors limiting photosynthesis under these conditions, and their possible interactions with other environmental conditions, are unknown. To assess these issues, changes of photosynthetic CO₂ assimilation (A_N) and its underlying limitations were followed during prolonged water stress and subsequent re-watering in tobacco (Nicotiana sylvestris) plants growing under three different climatic conditions: outdoors in summer, outdoors in spring, and indoors in a growth chamber. In particular, the regulation of stomatal conductance (g_s), mesophyll conductance to CO₂ (g_m), leaf photochemistry (chlorophyll fluorescence), and biochemistry (V_c,max) were assessed. Leaf gas exchange and chlorophyll fluorescence data revealed that water stress induced a similar degree of stomatal closure and decreased A_N under all three conditions, while V_c,max was unaffected. However, the behaviour of g_m differed depending on the climatic conditions. In outdoor plants, g_m strongly declined with water stress, but it recovered rapidly (1–2 d) after re-watering in spring while it remained low many days after re-watering in summer. In indoor plants, g_m initially declined with water stress, but then recovered to control values during the acclimation period. These differences were reflected in different velocities of recovery of A_N after re-watering, being the slowest in outdoor summer plants and the fastest in indoor plants. It is suggested that these differences among the experiments are related to the prevailing climatic conditions, i.e. to the fact that stress factors other than water stress have been superimposed (e.g. excessive light and elevated temperature). In conclusion, besides g_s, g_m contributes greatly to the limitation of photosynthesis during water stress and during recovery from water stress, but its role is strongly dependent on the impact of additional environmental factors.

Interactive effects of soil water deficit and air vapour pressure deficit on mesophyll conductance to CO2 in Vitis vinifera and Olea europaea

2009-05-20

The present work aims to study the interactive effect of drought stress and high vapour pressure deficit (VPD) on leaf gas exchange, and especially on mesophyll conductance to CO₂ (g_m), in two woody species of great agronomical importance in the Mediterranean basin: Vitis vinifera L. cv. Tempranillo and Olea europaea L. cv. Manzanilla. Plants were grown in specially designed outdoor chambers with ambient and below ambient VPD, under both well-irrigated and drought conditions. g_m was estimated by the variable J method from simultaneous measurements of gas exchange and fluorescence. In both species, the response to soil water deficit was larger in g_s than in g_m, and more important than the response to VPD. Olea europaea was apparently more sensitive to VPD, so that plants growing in more humid chambers showed higher g_s and g_m. In V. vinifera, in contrast, soil water deficit dominated the response of g_s and g_m. Consequently, changes in g_m/g_s were more related to VPD in O. europaea and to soil water deficit in V. vinifera. Most of the limitations of photosynthesis were diffusional and especially due to stomatal closure. No biochemical limitation was detected. The results showed that structural parameters played an important role in determining g_m during the acclimation process. Although the relationship between leaf mass per unit area (M_A) with g_m was scattered, it imposed a limitation to the maximum g_m achievable, with higher values of M_A in O. europaea at lower g_m values. M_A decreased under water stress in O. europaea but it increased in V. vinifera. This resulted in a negative relationship between M_A and the CO₂ draw-down between substomatal cavities and chloroplasts in O. europaea, while being positive in V. vinifera.

Seasonal time-course of gradients of photosynthetic capacity and mesophyll conductance to CO2 across a beech (Fagus sylvatica L.) canopy

Montpied, P., Granier, A., Dreyer, E. — 2009-05-20

Leaf photosynthesis is known to acclimate to the actual irradiance received by the different layers of a canopy. This acclimation is usually described in terms of changes in leaf structure, and in photosynthetic capacity. Photosynthetic capacity is likely to be affected by mesophyll conductance to CO₂ which has seldom been assessed in tree species, and whose plasticity in response to local irradiance is still poorly known. Structural [N and chlorophyll content, leaf mass to area ratio (LMA)] and functional leaf traits [maximum carboxylation rate (V_cmax), maximum light-driven electron flux (J_max), and mesophyll conductance (g_i)] were assessed by measuring leaf response curves of net CO₂ assimilation versus intercellular CO₂ partial pressure, along a vertical profile across a beech canopy, and by fitting a version of the Farquhar model including g_i. The measurements were repeated five times during a growth season to catch potential seasonal variation. Irradiance gradients resulted in large decreasing gradients of LMA, g_i, V_cmax, and J_max. Relative allocation of leaf N to the different photosynthetic processes was only slightly affected by local irradiance. Seasonal changes after leaf expansion and before induction of leaf senescence were only minor. Structural equation modelling confirmed that LMA was the main driving force for changes in photosynthetic traits, with only a minor contribution of leaf Nitrogen content. In conclusion, mesophyll conductance to CO₂ displays a large plasticity that scales with photosynthetic capacity across a tree canopy, and that it is only moderately (if at all) affected by seasonal changes in the absence of significant soil water depletion.

The diversity of 13C isotope discrimination in a Quercus robur full-sib family is associated with differences in intrinsic water use efficiency, transpiration efficiency, and stomatal conductance

Roussel, M., Dreyer, E., Montpied, P., Le-Provost, G., Guehl, J.-M., Brendel, O. — 2009-05-20

¹³C discrimination in organic matter with respect to atmospheric CO₂ (¹³C) is under tight genetic control in many plant species, including the pedunculate oak (Quercus robur L.) full-sib progeny used in this study. ¹³C is expected to reflect intrinsic water use efficiency, but this assumption requires confirmation due to potential interferences with mesophyll conductance to CO₂, or post-photosynthetic discrimination. In order to dissect the observed ¹³C variability in this progeny, six genotypes that have previously been found to display extreme phenotypic values of ¹³C [either very high (‘high ’) or low (‘low ’) phenotype] were selected, and transpiration efficiency (TE; accumulated biomass/transpired water), net CO₂ assimilation rate (A), stomatal conductance for water vapour (g_s), and intrinsic water use efficiency (W_i=A/g_s) were compared with ¹³C in bulk leaf matter, wood, and cellulose in wood. As expected, ‘high ’ displayed higher values of ¹³C not only in bulk leaf matter, but also in wood and cellulose. This confirmed the stability of the genotypic differences in ¹³C recorded earlier. ‘High ’ also displayed lower TE, lower W_i, and higher g_s. A small difference was detected in photosynthetic capacity but none in mesophyll conductance to CO₂. ‘High ’ and ‘low ’ displayed very similar leaf anatomy, except for higher stomatal density in ‘high ’. Finally, diurnal courses of leaf gas exchange revealed a higher g_s in ‘high ’ in the morning than in the afternoon when the difference decreased. The gene ERECTA, involved in the control of water use efficiency, leaf differentiation, and stomatal density, displayed higher expression levels in ‘low ’. In this progeny, the variability of ¹³C correlated closely with that of W_i and TE. Genetic differences of ¹³C and W_i can be ascribed to differences in stomatal conductance and stomatal density but not in photosynthetic capacity.

Leaf mesophyll diffusion conductance in 35 Australian sclerophylls covering a broad range of foliage structural and physiological variation

Niinemets, U., Wright, I. J., Evans, J. R. — 2009-05-20

Foliage structure, chemistry, photosynthetic potentials (V_cmax and J_max), and mesophyll diffusion conductance (g_m) were quantified for 35 broad-leaved species from four sites with contrasting rainfall and soil fertility in eastern Australia. The aim of the study was to estimate the extent to which g_m and related leaf properties limited photosynthesis (A), focusing on highly sclerophyllous species typical of the ‘slow-return’ end of the leaf economics spectrum. Leaf dry mass per unit area (M_A) varied ~5-fold, leaf life span (L_L) and N (N_M) and P (P_M) contents per dry mass ~8-fold, and various characteristics of foliage photosynthetic machinery 6- to 12-fold across the data set. As is characteristic of the ‘leaf economics spectrum’, more robust leaves with greater M_A and longevity were associated with lower nutrient contents and lower foliage photosynthetic potentials. g_m was positively correlated with V_cmax and J_max, and these correlations were stronger on a mass basis. Only g_m/mass was negatively associated with M_A. CO₂ drawdown from substomatal cavities to chloroplasts (C_i–C_C) characterizing mesophyll CO₂ diffusion limitations was larger in leaves with greater M_A, lower g_m/mass, and lower photosynthetic potentials. Relative limitation of A due to finite mesophyll diffusion conductance, i.e. 1–A(infinite g_m)/A(actual g_m), was always >0.2 and up to 0.5 in leaves with most robust leaf structure, demonstrating the profound effect of finite g_m on realized photosynthesis rates. Data from different sites were overlapping in bivariate relationships, and the variability of average values between the sites was less than among the species within the sites. Nevertheless, photosynthesis was more strongly limited by g_m in low rain/high nutrient and high rain/low nutrient sites that supported vegetation with more sclerophyllous foliage. These data collectively highlight a strong relationship between leaf structure and g_m, and demonstrate that realized photosynthesis rates are strongly limited by g_m in this highly sclerophyllous flora.

Food production: reducing water consumption by manipulating long-distance chemical signalling in plants

Wilkinson, S., Hartung, W. — 2009-05-14

Symbolism of plants: examples from European-Mediterranean culture presented with biology and history of art: JUNE: Lilies

Kandeler, R., Ullrich, W. R. — 2009-05-14

Preface

Sparkes, D. — 2009-05-14

Raising yield potential in wheat

2009-05-14

Recent advances in crop research have the potential to accelerate genetic gains in wheat, especially if co-ordinated with a breeding perspective. For example, improving photosynthesis by exploiting natural variation in Rubisco's catalytic rate or adopting C₄ metabolism could raise the baseline for yield potential by 50% or more. However, spike fertility must also be improved to permit full utilization of photosynthetic capacity throughout the crop life cycle and this has several components. While larger radiation use efficiency will increase the total assimilates available for spike growth, thereby increasing the potential for grain number, an optimized phenological pattern will permit the maximum partitioning of the available assimilates to the spikes. Evidence for underutilized photosynthetic capacity during grain filling in elite material suggests unnecessary floret abortion. Therefore, a better understanding of its physiological and genetic basis, including possible signalling in response to photoperiod or growth-limiting resources, may permit floret abortion to be minimized for a more optimal source:sink balance. However, trade-offs in terms of the partitioning of assimilates to competing sinks during spike growth, to improve root anchorage and stem strength, may be necessary to prevent yield losses as a result of lodging. Breeding technologies that can be used to complement conventional approaches include wide crossing with members of the Triticeae tribe to broaden the wheat genepool, and physiological and molecular breeding strategically to combine complementary traits and to identify elite progeny more efficiently.

Capture and use of solar radiation, water, and nitrogen by sugar beet (Beta vulgaris L.)

Jaggard, K. W., Qi, A., Ober, E. S. — 2009-05-14

Sugar beet is spring-sown for sugar production in most sugar beet-growing countries. It is grown as a vegetative crop and it accumulates yield (sugar) from very early in its growth cycle. As long as the sugar beet plants do not flower, the sugar accumulation period is indefinite and yield continues to increase. This paper reviews the success of the sugar beet crop in capturing and using solar radiation, water and mineral nitrogen resources. The prospects for improved resource capture and therefore increased sugar yield are also considered, particularly the potential to increase solar radiation interception in the future by sowing the crop in the autumn.

Plant phenology: a critical controller of soil resource acquisition

Nord, E. A., Lynch, J. P. — 2009-05-14

Plant phenology, the timing of plant growth and development, is changing in response to global climate change. Changing temperature, soil moisture, nitrogen availability, light, and elevated CO₂ are all likely to affect plant phenology. Alteration of plant phenology by global climate change may alter the ability of plants to acquire soil resources (water and nutrients) by altering the timing and duration of the deployment of roots and leaves, which drive resource acquisition. The potential importance of phenologically-driven changes in soil resource acquisition for plant fitness and productivity have received little attention. General hypotheses are proposed for how plant acquisition of soil resources may be affected by the alteration of phenology. It is expected that the acquisition of mobile resources will be approximately proportional to total transpiration. Alteration of phenology that increases total transpiration should increase, while changes in phenology that reduce transpiration should decrease the acquisition of mobile resources. The acquisition of immobile resources will be approximately proportional to root length duration, thus changes in phenology that increase growth duration should increase the acquisition of immobile resources and vice versa. For both groups of resources, longer growing seasons would tend to increase resource acquisition, and shorter growing seasons would tend to decrease resource acquisition. In the case of resources that exhibit seasonal variability in availability, the synchrony of resource availability and acquisition capacity is important, and subject to disturbance by the alteration of phenology.

Analysing nitrogen responses of cereals to prioritize routes to the improvement of nitrogen use efficiency

Sylvester-Bradley, R., Kindred, D. R. — 2009-05-14

The efficient use of fertilizer nitrogen (N) is crucial to sustainable human nutrition. All crops receive significant amounts of additional N in temperate environments, through fixation or fertilizer use. This paper reviews progress towards the efficient use of fertilizer N by winter wheat (Triticum aesitivum L.) and spring barley (Hordeum vulgare L.) in the UK, acknowledging that on-farm this is governed by economics. Recent multi-site N response experiments on old and modern varieties show that yield improvements since the 1980s have been accompanied by increases in economic optimum N amounts for wheat but not for spring barley. On-farm N use efficiency (NUE) has increased for barley because increased yields with optimum N were associated with compensatory decreases in grain N concentration, whereas on-farm NUE has not increased for wheat because grain N concentration has not changed and improvements in N capture were insufficient to make up for the increased yield. Genetic effects on NUE are shown to differ markedly depending on whether they are determined at a single N rate, as in variety trials, or with optimum N amounts. It is suggested that, in order to elicit faster improvement in NUE on farms, breeding and variety testing should be conducted at some sites with more than one level of applied N, and that grain N%, N harvest index, and perhaps canopy N ratio (kg N ha^–1 green area) should be measured more widely. It is also suggested that, instead of using empirical functions, N responses might be analysed more effectively using functions based on explanations of yield determination for which the parameters have some physiological meaning.

Shoot yield drives phosphorus use efficiency in Brassica oleracea and correlates with root architecture traits

2009-05-14

The environmental and financial costs of using inorganic phosphate fertilizers to maintain crop yield and quality are high. Breeding crops that acquire and use phosphorus (P) more efficiently could reduce these costs. The variation in shoot P concentration (shoot-P) and various measures of P use efficiency (PUE) were quantified among 355 Brassica oleracea L. accessions, 74 current commercial cultivars, and 90 doubled haploid (DH) mapping lines from a reference genetic mapping population. Accessions were grown at two or more external P concentrations in glasshouse experiments; commercial and DH accessions were also grown in replicated field experiments. Within the substantial species-wide diversity observed for shoot-P and various measures of PUE in B. oleracea, current commercial cultivars have greater PUE than would be expected by chance. This may be a consequence of breeding for increased yield, which is a significant component of most measures of PUE, or early establishment. Root development and architecture correlate with PUE; in particular, lateral root number, length, and growth rate. Significant quantitative trait loci associated with shoot-P and PUE occur on chromosomes C3 and C7. These data provide information to initiate breeding programmes to improve PUE in B. oleracea.

Understanding phototropism: from Darwin to today

Holland, J. J., Roberts, D., Liscum, E. — 2009-05-14

Few individuals have had the lasting impact on such a breadth of science as Charles Darwin. While his writings about time aboard the HMS Beagle, his study of the Galapagos islands (geology, fauna, and flora), and his theories on evolution are well known, less appreciated are his studies on plant growth responses to a variety of environmental stimuli. In fact, Darwin, together with the help of his botanist son Francis, left us an entire book, ‘The power of movements in plants’, describing his many, varied, and insightful observations on this topic. Darwin's findings have provided an impetus for an entire field of study, the study of plant tropic responses, or differential growth (curvature) of plant organs in response to directional stimuli. One tropic response that has received a great deal of attention is the phototropic response, or curvature response to directional light. This review summarizes many of the most significant advancements that have been made in our understanding of this response and place these recent findings in the context of Darwin's initial observations.

Gibberellin as a factor in floral regulatory networks

Mutasa-Gottgens, E., Hedden, P. — 2009-05-14

Gibberellins (GAs) function not only to promote the growth of plant organs, but also to induce phase transitions during development. Their involvement in flower initiation in long-day (LD) and biennial plants is well established and there is growing insight into the mechanisms by which floral induction is achieved. The extent to which GAs mediate the photoperiodic stimulus to flowering in LD plants is, with a few exceptions, less clear. Despite evidence for photoperiod-enhanced GA biosynthesis in leaves of many LD plants, through up-regulation of GA 20-oxidase gene expression, a function for GAs as transmitted signals from leaves to apices in response to LD has been demonstrated only in Lolium species. In Arabidopsis thaliana, as one of four quantitative floral pathways, GA signalling has a relatively minor influence on flowering time in LD, while in SD, in the absence of the photoperiod flowering pathway, the GA pathway assumes a major role and becomes obligatory. Gibberellins promote flowering in Arabidopsis through the activation of genes encoding the floral integrators SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), LEAFY (LFY), and FLOWERING LOCUS T (FT) in the inflorescence and floral meristems, and in leaves, respectively. Although GA signalling is not required for floral organ specification, it is essential for the normal growth and development of these organs. The sites of GA production and action within flowers, and the signalling pathways involved are beginning to be revealed.

The {alpha}-subunit of the heterotrimeric G-protein affects jasmonate responses in Arabidopsis thaliana

Okamoto, H., Gobel, C., Capper, R. G., Saunders, N., Feussner, I., Knight, M. R. — 2009-05-14

Heterotrimeric G-proteins have been implicated in having a role in many plant signalling pathways. To understand further the role of G-proteins, a preliminary experiment was performed to assess the impact of the G subunit loss-of-function mutation gpa1-1 on the Arabidopsis transcriptome. The analysis indicated that the G subunit may play a role in response to jasmonic acid (JA). Consistent with this, G mutants showed a reduced response to JA in inhibition of chlorophyll accumulation and root growth, whilst G gain-of-function plants overexpressing G showed the opposite phenotype. The levels of JA and related compounds were unaffected in the gpa1-1 mutant, as was autoregulation of the Allene Oxide Synthase (AOS) gene that encodes a key enzyme for JA biosynthesis. In contrast, further analyses using G loss- and gain-of-function Arabidopsis lines indicated that G positively modulates the expression of the Vegetative Storage Protein (VSP) gene. This indicates that the G subunit regulates a subset of JA-regulated genes defining a branch point in this signalling pathway in Arabidopsis. Further analysis of the impact of G loss of function upon the JA-regulated transcriptome using Arabidopsis full genome arrays indicated that up to 29% of genes that are >2-fold regulated by JA in the wild type are misregulated in the G mutant. This supports the observation that a significant proportion of, but not all, JA-regulated gene expression is mediated by G.

Proteomic analysis of salt-stressed tomato (Solanum lycopersicum) seedlings: effect of genotype and exogenous application of glycinebetaine

Chen, S., Gollop, N., Heuer, B. — 2009-05-14

An investigation aimed at a better understanding of the molecular adaptation mechanisms of salt stress was carried out in 7-d-old tomato Solanum lycopersicum (L.) Mill cultivars Patio and ‘F144’, using a proteomic approach. Total proteins were extracted from radicles and hypocotyls collected from both non-saline control and salt-stressed seedlings, and separated by two-dimensional gel electrophoresis. Liqud chromatography-electron spray ionization tandem mass spectrometry (LC-ESI-MS/MS) identified 23 salt stress response proteins, classified into six functional categories. The effect of exogenously applied glycinebetaine (GB) on the salt stress-induced inhibition of growth in tomato seedlings of cultivars Patio and ‘F144’ and on the protein profile was investigated. It was found that GB could alleviate the inhibition of tomato growth induced by salt stress through changing the expression abundance of six proteins in Patio and two proteins in ‘F144’ more than twice compared with salt-stressed seedlings. Furthermore, the interaction analysis based on computational bioinformatics reveals major regulating networks: photosystem II (PSII), Rubisco, and superoxide dismutase (SOD). The results suggest that it is likely that improvement of salt tolerance in tomato might be achieved through the application of exogenous compatible solutes, such as GB. Moreover, quantitative and qualitative analysis of the differentially expressed proteins of tomato under salt stress is an important step towards further elucidation of mechanisms of salt stress resistance.

Expression patterns of cell wall-modifying genes from banana during fruit ripening and in relationship with finger drop

2009-05-14

Few molecular studies have been devoted to the finger drop process that occurs during banana fruit ripening. Recent studies revealed the involvement of changes in the properties of cell wall polysaccharides in the pedicel rupture area. In this study, the expression of cell-wall modifying genes was monitored in peel tissue during post-harvest ripening of Cavendish banana fruit, at median area (control zone) and compared with that in the pedicel rupture area (drop zone). To this end, three pectin methylesterase (PME) and seven xyloglucan endotransglycosylase/hydrolase (XTH) genes were isolated. The accumulation of their mRNAs and those of polygalaturonase, expansin, and pectate lyase genes already isolated from banana were examined. During post-harvest ripening, transcripts of all genes were detected in both zones, but accumulated differentially. MaPME1, MaPG1, and MaXTH4 mRNA levels did not change in either zone. Levels of MaPME3 and MaPG3 mRNAs increased greatly only in the control zone and at the late ripening stages. For other genes, the main molecular changes occurred 1–4 d after ripening induction. MaPME2, MaPEL1, MaPEL2, MaPG4, MaXTH6, MaXTH8, MaXTH9, MaEXP1, MaEXP4, and MaEXP5 accumulated highly in the drop zone, contrary to MaXTH3 and MaXTH5, and MaEXP2 throughout ripening. For MaPG2, MaXET1, and MaXET2 genes, high accumulation in the drop zone was transient. The transcriptional data obtained from all genes examined suggested that finger drop and peel softening involved similar mechanisms. These findings also led to the proposal of a sequence of molecular events leading to finger drop and to suggest some candidates.

Transcriptional activation of a 37 kDa ethylene responsive cysteine protease gene, RbCP1, is associated with protein degradation during petal abscission in rose

Tripathi, S. K., Singh, A. P., Sane, A. P., Nath, P. — 2009-05-14

Cysteine proteases play an important role in several developmental processes in plants, particularly those related to senescence and cell death. A cysteine protease gene, RbCP1, has been identified that encodes a putative protein of 357 amino acids and is expressed in the abscission zone (AZ) of petals in rose. The gene was responsive to ethylene in petals, petal abscission zones, leaves, and thalamus. The expression of RbCP1 increased during both ethylene-induced as well as natural abscission and was inhibited by 1-MCP. Transcript accumulation of RbCP1 was accompanied by the appearance of a 37 kDa cysteine protease, a concomitant increase in protease activity and a substantial decrease in total protein content in the AZ of petals. Agro-injection of rose petals with a 2.0 kb region upstream of the RbCP1 gene could drive GUS expression in an abscission zone-specific manner and was blocked by 1-MCP. It is concluded that petal abscission is associated with a decrease in total protein content resulting from rapid transcription of RbCP1 and the expression of a 37 kDa protease.

The mechanics of explosive seed dispersal in orange jewelweed (Impatiens capensis)

Hayashi, M., Feilich, K. L., Ellerby, D. J. — 2009-05-14

Explosive dehiscence ballistically disperses seeds in a number of plant species. During dehiscence, mechanical energy stored in specialized tissues is transferred to the seeds to increase their kinetic and potential energies. The resulting seed dispersal patterns have been investigated in some ballistic dispersers, but the mechanical performance of a launch mechanism of this type has not been measured. The properties of the energy storage tissue and the energy transfer efficiency of the launch mechanism were quantified in Impatiens capensis. In this species the valves forming the seed pod wall store mechanical energy. Their mass specific energy storage capacity (124 J kg^–1) was comparable with that of elastin and spring steel. The energy storage capacity of the pod tissues was determined by their level of hydration, suggesting a role for turgor pressure in the energy storage mechanism. During dehiscence the valves coiled inwards, collapsing the pod and ejecting the seeds. Dehiscence took 4.2±0.4 ms (mean ±SEM, n=13). The estimated efficiency with which energy was transferred to the seeds was low (0.51±0.26%, mean ±SEM, n=13). The mean seed launch angle (17.4±5.2, mean ±SEM, n=45) fell within the range predicted by a ballistic model to maximize dispersal distance. Low ballistic dispersal efficiency or effectiveness may be characteristic of species that also utilize secondary seed dispersal mechanisms.

Complete blockage of the mevalonate pathway results in male gametophyte lethality

2009-05-14

Plants have two isoprenoid biosynthetic pathways: the cytosolic mevalonate (MVA) pathway and the plastidic 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway. Since the discovery of the MEP pathway, possible metabolic cross-talk between these pathways has prompted intense research. Although many studies have shown the existence of such cross-talk using feeding experiments, it remains to be determined if native cross-talk, rather than exogenously applied metabolites, can compensate for complete blockage of the MVA pathway. Previously, Arabidopsis mutants for HMG1 and HMG2 encoding HMG-CoA reductase (HMGR) were isolated. Although it was shown that HMGR1 is a functional HMGR, the enzyme activity of HMGR2 has not been confirmed. It is demonstrated here that HMG2 encodes a functional reductase with similar activity to HMGR1, using enzyme assays and complementation experiments. To estimate the contribution of native cross-talk, an attempt was made to block the MVA pathway by making double mutants lacking both HMG1 and HMG2, but no double homozygotes were detected in the progeny of self-pollinated HMG1/hmg1 hmg2/hmg2 plants. hmg1 hmg2 male gametophytes appeared to be lethal based on crossing experiments, and microscopy indicated that ~50% of the microspores from the HMG1/hmg1 hmg2/hmg2 plant appeared shrunken and exhibited poorly defined endoplasmic reticulum membranes. In situ hybridization showed that HMG1 transcripts were expressed in both the tapetum and microspores, while HMG2 mRNA appeared only in microspores. It is concluded that native cross-talk from the plastid cannot compensate for complete blockage of the MVA pathway, at least during male gametophyte development, because either HMG1 or HMG2 is required for male gametophyte development.

Plant cells oxidize hydroxylamines to NO

Rumer, S., Gupta, K. J., Kaiser, W. M. — 2009-05-14

Plants are known to produce NO via the reduction of nitrite. Oxidative NO production in plants has been considered only with respect to a nitric oxide synthase (NOS). Here it is shown that tobacco cell suspensions emitted NO when hydroxylamine (HA) or salicylhydroxamate (SHAM), a frequently used AOX inhibitor, was added. N^G-hydroxy-L-arginine, a putative intermediate in the NOS-reaction, gave no NO emission. Only a minor fraction (≤1%) of the added HA or SHAM was emitted as NO. Production of NO was decreased by anoxia or by the addition of catalase, but was increased by conditions inducing reactive oxygen (ROS) or by the addition of hydrogen peroxide. Cell-free enzyme solutions generating superoxide or hydrogen peroxide also led to the formation of NO from HA or (with lower rates) from SHAM, and nitrite was also an oxidation product. Unexpectedly, the addition of superoxide dismutase (SOD) to cell suspensions stimulated NO formation from hydroxylamines, and SOD alone (without cells) also catalysed the production of NO from HA or SHAM. NO production by SOD plus HA was higher in nitrogen than in air, but from SOD plus SHAM it was lower in nitrogen. Thus, SOD-catalysed NO formation from SHAM and from HA may involve different mechanisms. While our data open a new possibility for oxidative NO formation in plants, the existence and role of these reactions under physiological conditions is not yet clear.

Characterization of mitochondrial dynamics and subcellular localization of ROS reveal that HsfA2 alleviates oxidative damage caused by heat stress in Arabidopsis

Zhang, L., Li, Y., Xing, D., Gao, C. — 2009-05-14

Heat shock transcription factor A2 (HsfA2) participates in multiple stress responses. To provide new insights into the role of HsfA2 in the heat stress (HS) response, in vivo production and localization of reactive oxygen species (ROS) and mitochondrial dynamics were investigated during the onset of cell death induced by an HS (40 °C, 10 min) applied after a 2 d recovery at 24 °C following a conditioning treatment at 37 °C for 1 h. In response to the HS, generated ROS were significantly higher in hsfA2 than in wild-type (WT) protoplasts and did not return to the baseline level when compared with WT protoplasts. The uncontrolled ROS in hsfA2 protoplasts localized not only to mitochondria but also to chloroplasts. Microscopic observations also revealed that, prior to cell death, hsfA2 protoplasts underwent more severe alterations in mitochondrial dynamics than WT protoplasts, including mitochondrial swelling, transmembrane potential loss, and the cessation of mitochondrial movement. The lower cell viability in hsfA2 than in WT protoplasts suggested that—combined with the findings that antioxidants only partially blocked ROS generation and arrested cell death in hsfA2 protoplasts relative to WT protoplasts—ROS participated in HS-induced cell death. Also the disruption of HsfA2 resulted in more severe oxidative stress and more cell death which, together with the more severe alterations in mitochondrial dynamics, could be complemented by introducing a WT copy of HsfA2. These results represent the first subcellular evidence that HsfA2 protects plants against HS-induced oxidative damage, organelle dysfunction, and subsequent cell death.

Stilbene synthase gene transfer caused alterations in the phenylpropanoid metabolism of transgenic strawberry (Fragariaxananassa)

2009-05-14

The gene encoding stilbene synthase is frequently used to modify plant secondary metabolism with the aim of producing the self-defence phytoalexin resveratrol. In this study, strawberry (Fragariaxananassa) was transformed with the NS-Vitis3 gene encoding stilbene synthase from frost grape (Vitis riparia) under the control of the cauliflower mosaic virus 35S and the floral filament-specific fil1 promoters. Changes in leaf metabolites were investigated with UPLC-qTOF-MS (ultra performance liquid chromatography-quadrupole time of flight mass spectrometry) profiling, and increased accumulation of cinnamate, coumarate, and ferulate derivatives concomitantly with a decrease in the levels of flavonols was observed, while the anticipated resveratrol or its derivatives were not detected. The changed metabolite profile suggested that chalcone synthase was down-regulated by the genetic modification; this was verified by decreased chalcone synthase transcript levels. Changes in the levels of phenolic compounds led to increased susceptibility of the transgenic strawberry to grey mould fungus.

Dynamic root exudation of sorgoleone and its in planta mechanism of action

Dayan, F. E., Howell, J., Weidenhamer, J. D. — 2009-05-14

The oily droplets exuded from the root hairs of sorghum are composed of a 1:1 ratio of sorgoleone and its lipid resorcinol analogue. The production of these droplets appears to be suppressed when c. 20 µg of exudate mg^–1 root dry weight accumulates at the tip of the root hairs. However, more exudate is produced following gentle washing of the roots with water, suggesting that the biosynthesis of lipid benzoquinones and resorcinols is a dynamic process. Sorgoleone interferes with several molecular target sites, including photosynthetic electron transport, in in vitro assays. However, the in planta mechanism of action of sorgoleone remains controversial because it is not clear whether this lipid benzoquinone exuding from the roots of sorghum is taken up by roots of the receiving plants and translocated to their foliage where it must enter the chloroplast and inhibit PSII in the thylakoid membrane. Experiments designed to test the in planta mode of action of sorgoleone demonstrated that it has no effect on the photosynthesis of older plants, but inhibits photosynthesis in germinating seedlings. Sorgoleone is not translocated acropetally in older plants, but can be absorbed through the hypocotyl and cotyledonary tissues. Therefore, the mode of action of sorgoleone may be the result of inhibition of photosynthesis in young seedlings in concert with inhibition of its other molecular target sites in older plants.

Regulation of intracellular pH during anoxia in rice coleoptiles in acidic and near neutral conditions

Kulichikhin, K. Y., Greenway, H., Byrne, L., Colmer, T. D. — 2009-05-14

Rice coleoptiles, renowned for anoxia tolerance, were hypoxically pretreated, excised, ‘healed’, and then exposed to a combination of anoxia and pH 3.5. The putative acid load was confirmed by net effluxes of K⁺ to the medium, with concurrent net decreases of H⁺ in the medium, presumably mainly due to H⁺ influx. Yet the coleoptiles survived the combination of anoxia and pH 3.5 for at least 90 h, and even for at least 40 h when the energy crisis, inherent to anoxia, had been aggravated by supplying the coleoptiles with 2.5 mM rather than 50 mM glucose. Even in the case of coleoptiles with 2.5 mM glucose, an accumulation ratio of 6 for Cl^– was attained at 4 h after the start of re-aeration, implying plasma membrane integrity was either maintained during anoxia, or rapidly restored after a return to aerated conditions. Cytoplasmic pH and vacuolar pH were measured using in vivo ³¹P nuclear magnetic resonance spectroscopy with 50 mM glucose in the basal perfusion medium. After 60 h in anoxia, external pH was suddenly decreased from 6.5 to 3.5, but cytoplasmic pH only decreased from 7.35 to 7.2 during the first 2 h and then remained steady for the next 16 h. During the first 3 h at pH 3.5, vacuolar pH decreased from 5.7 to 5.25 and then stabilized. After 18 h at pH 3.5, the initial values of cytoplasmic pH and vacuolar pH were rapidly restored, both upon a return to pH 6.5 while maintaining anoxia and after subsequent return to aerated solution. Summing up, rice coleoptiles exposed to a combination of anoxia and pH 3.5 retained pH regulation and cellular compartmentation, demonstrating tolerance to anoxia even during the acid load imposed by exposure to pH 3.5.

Intersection of two signalling pathways: extracellular nucleotides regulate pollen germination and pollen tube growth via nitric oxide

2009-05-14

Plant and animal cells release or secrete ATP by various mechanisms, and this activity allows extracellular ATP to serve as a signalling molecule. Recent reports suggest that extracellular ATP induces plant responses ranging from increased cytosolic calcium to changes in auxin transport, xenobiotic resistance, pollen germination, and growth. Although calcium has been identified as a secondary messenger for the extracellular ATP signal, other parts of this signal transduction chain remain unknown. Increasing the extracellular concentration of ATPS, a poorly-hydrolysable ATP analogue, inhibited both pollen germination and pollen tube elongation, while the addition of AMPS had no effect. Because pollen tube elongation is also sensitive to nitric oxide, this raised the possibility that a connection exists between the two pathways. Four approaches were used to test whether the germination and growth effects of extracellular ATPS were transduced via nitric oxide. The results showed that increases in extracellular ATPS induced increases in cellular nitric oxide, chemical agonists of the nitric oxide signalling pathway lowered the threshold of extracellular ATPS that inhibits pollen germination, an antagonist of guanylate cyclase, which can inhibit some nitric oxide signalling pathways, blocked the ATPS-induced inhibition of both pollen germination and pollen tube elongation, and the effects of applied ATPS were blocked in nia1nia2 mutants, which have diminished NO production. The concurrence of these four data sets support the conclusion that the suppression of pollen germination and pollen tube elongation by extracellular nucleotides is mediated in part via the nitric oxide signalling pathway.

Metabolic characterization of loci affecting sensory attributes in tomato allows an assessment of the influence of the levels of primary metabolites and volatile organic contents

2009-05-14

Numerous studies have revealed the extent of genetic and phenotypic variation between both species and cultivars of tomato. Using a series of tomato lines resulting from crosses between a cherry tomato and three independent large fruit cultivar (Levovil, VilB, and VilD), extensive profiling of both central primary metabolism and volatile organic components of the fruit was performed. In this study, it was possible to define a number of quantitative trait loci (QTLs) which determined the levels of primary metabolites and/or volatile organic components and to evaluate their co-location with previously defined organoleptic QTLs. Correlation analyses between either the primary metabolites or the volatile organic compounds and organoleptic properties revealed a number of interesting associations, including pharmaceutical aroma–guaiacol and sourness–alanine, across the data set. Considerable correlation within the levels of primary metabolites or volatile organic compounds, respectively, were also observed. However, there was relatively little association between the levels of primary metabolites and volatile organic compounds, implying that they are not tightly linked to one another. A notable exception to this was the strong association between the levels of sucrose and those of a number of volatile organic compounds. The combined data presented here are thus discussed both with respect to those obtained recently from wide interspecific crosses of tomato and within the framework of current understanding of the chemical basis of fruit taste.

Functional and chemical comparison of apoplastic barriers to radial oxygen loss in roots of rice (Oryza sativa L.) grown in aerated or deoxygenated solution

Kotula, L., Ranathunge, K., Schreiber, L., Steudle, E. — 2009-05-14

Radial oxygen loss (ROL) and root porosity of rice (Oryza sativa L.) plants grown in either aerated or deoxygenated (stagnant) conditions were combined for the first time with extensive histochemical and biochemical studies of the apoplastic barriers in the roots’ peripheral cell layers. Growth in stagnant solution significantly affected structural and, consequently, the physiological features of rice roots. It increased adventitious root porosity by about 20% and decreased the ROL towards the base to zero at a distance of 40 mm from the apex. By contrast, roots of plants grown in aerated solutions revealed the highest rates of ROL at 30 mm from the apex. Differences in the ROL pattern along the root were related to histochemical studies, which showed an early development of Casparian bands and suberin lamellae in the exodermis, and lignified sclerenchyma cells in roots of plants grown in deoxygenated solution. In agreement with anatomical studies, absolute contents of suberin and lignin in the outer part of the roots (OPR) were higher in plants grown in deoxygenated solution. Regardless of growth conditions, the levels of suberin and lignin increased along the roots towards the base. It is concluded that radial oxygen loss can be effectively restricted by the formation of a suberized exodermis and/or lignified sclerenchyma in the OPR. However, the relative contribution of suberin and lignin in the formation of a tight barrier is unclear. Knowing the permeability coefficient across OPR for roots of plants grown in both conditions will allow a more precise understanding of the mechanisms controlling ROL.

The influence of vernalization and daylength on expression of flowering-time genes in the shoot apex and leaves of barley (Hordeum vulgare).

2009-05-14

Responses to prolonged low-temperature treatment of imbibed seeds (vernalization) were examined in barley (Hordeum vulgare). These occurred in two phases: the perception of prolonged cold, which occurred gradually at low temperatures, and the acceleration of reproductive development, which occurred after vernalization. Expression of the VERNALIZATION1 gene (HvVRN1) increased gradually in germinating seedlings during vernalization, both at the shoot apex and in the developing leaves. This occurred in darkness, independently of VERNALIZATION2 (HvVRN2), consistent with the hypothesis that expression of HvVRN1 is induced by prolonged cold independently of daylength flowering-response pathways. After vernalization, expression of HvVRN1 was maintained in the shoot apex and leaves. This was associated with accelerated inflorescence initiation and with down-regulation of HvVRN2 in the leaves. The largest determinant of HvVRN1 expression levels in vernalized plants was the length of seed vernalization treatment. Daylength did not influence HvVRN1 expression levels in shoot apices and typically did not affect expression in leaves. In the leaves of plants that had experienced a saturating seed vernalization treatment, expression of HvVRN1 was higher in long days, however. HvFT1 was expressed in the leaves of these plants in long days, which might account for the elevated HvVRN1 expression. Long-day up-regulation of HvVRN1 was not required for inflorescence initiation, but might accelerate subsequent stages of inflorescence development. Similar responses to seed vernalization were also observed in wheat (Triticum aestivum). These data support the hypothesis that VRN1 is induced by cold during winter to promote spring flowering in vernalization-responsive cereals.

Ethylene regulates phosphorus remobilization and expression of a phosphate transporter (PhPT1) during petunia corolla senescence

Chapin, L. J., Jones, M. L. — 2009-05-14

The programmed degradation of macromolecules during petal senescence allows the plant to remobilize nutrients from dying to developing tissues. Ethylene is involved in regulating the timing of nucleic acid degradation in petunia, but it is not clear if ethylene has a role in the remobilization of phosphorus during petal senescence. To investigate ethylene's role in nutrient remobilization, the P content of petals (collectively called the corolla) during early development and senescence was compared in ethylene-sensitive wild type Petuniaxhybrida ‘Mitchell Diploid’ (MD) and transgenic petunias with reduced sensitivity to ethylene (35S::etr1-1). When compared to the total P content of corollas on the day of flower opening (the early non-senescing stage), P in MD corollas had decreased 74% by the late stage of senescence (advanced wilting). By contrast, P levels were only reduced by an average of 32% during etr1-1 corolla (lines 44568 and Z00-35-10) senescence. A high-affinity phosphate transporter, PhPT1 (PhPht1;1), was cloned from senescing petunia corollas by RT-PCR. PhPT1 expression was up-regulated during MD corolla senescence and a much smaller increase was detected during the senescence of etr1-1 petunia corollas. PhPT1 mRNA levels showed a rapid increase in detached corollas (treated at 1 d after flower opening) following treatment with low levels of ethylene (0.1 µl l^-1). Transcripts accumulated in the presence of the protein synthesis inhibitor, cycloheximide, indicating that PhPT1 is a primary ethylene response gene. PhPT1 is a putative phosphate transporter that may function in Pi translocation during senescence.

Light-induced vegetative anthocyanin pigmentation in Petunia

2009-05-14

The Lc petunia system, which displays enhanced, light-induced vegetative pigmentation, was used to investigate how high light affects anthocyanin biosynthesis, and to assess the effects of anthocyanin pigmentation upon photosynthesis. Lc petunia plants displayed intense purple anthocyanin pigmentation throughout the leaves and stems when grown under high-light conditions, yet remain acyanic when grown under shade conditions. The coloured phenotypes matched with an accumulation of anthocyanins and flavonols, as well as the activation of the early and late flavonoid biosynthetic genes required for flavonol and anthocyanin production. Pigmentation in Lc petunia only occurred under conditions which normally induce a modest amount of anthocyanin to accumulate in wild-type Mitchell petunia [Petunia axillarisx(Petunia axillarisxPetunia hybrida cv. ‘Rose of Heaven’)]. Anthocyanin pigmentation in Lc petunia leaves appears to screen underlying photosynthetic tissues, increasing light saturation and light compensation points, without reducing the maximal photosynthetic assimilation rate (A_max). In the Lc petunia system, where the bHLH factor Leaf colour is constitutively expressed, expression of the bHLH (Lc) and WD40 (An11) components of the anthocyanin regulatory system were not limited, suggesting that the high-light-induced anthocyanin pigmentation is regulated by endogenous MYB transcription factors.

Low temperature maximizes growth of Crocus vernus (L.) Hill via changes in carbon partitioning and corm development

Lundmark, M., Hurry, V., Lapointe, L. — 2009-05-14

In Crocus vernus, a spring bulbous species, prolonged growth at low temperatures results in the development of larger perennial organs and delayed foliar senescence. Because corm growth is known to stop before the first visual sign of leaf senescence, it is clear that factors other than leaf duration alone determine final corm size. The aim of this study was to determine whether reduced growth at higher temperatures was due to decreased carbon import to the corm or to changes in the partitioning of this carbon once it had reached the corm. Plants were grown under two temperature regimes and the amount of carbon fixed, transported, and converted into a storable form in the corm, as well as the partitioning into soluble carbohydrates, starch, and the cell wall, were monitored during the growth cycle. The reduced growth at higher temperature could not be explained by a restriction in carbon supply or by a reduced ability to convert the carbon into starch. However, under the higher temperature regime, the plant allocated more carbon to cell wall material, and the amount of glucose within the corm declined earlier in the season. Hexose to sucrose ratios might control the duration of corm growth in C. vernus by influencing the timing of the cell division, elongation, and maturation phases. It is suggested that it is this shift in carbon partitioning, not limited carbon supply or leaf duration, which is responsible for the smaller final biomass of the corm at higher temperatures.