JXB Advance Access published online on August 28, 2007
Journal of Experimental Botany, doi:10.1093/jxb/erm156
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RESEARCH PAPER |
An assessment of the role of ethylene in mediating lettuce (Lactuca sativa) root growth at high temperatures
1Natural Sciences and Science Education Academic Group, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637 616
2Department of Biological Sciences, Lancaster Environment Centre, University of Lancaster, Lancaster LA1 4YQ, UK
* To whom correspondence should be addressed. E-mail: jie.he{at}nie.edu.sg
Received 18 April 2007; Revised 24 May 2007 Accepted 13 June 2007
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Abstract |
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Growth of temperate lettuce (Lactuca sativa) plants aeroponically in tropical greenhouses under ambient root-zone temperatures (A-RZTs) exposes roots to temperatures of up to 40 °C during the middle of the day, and severely limits root and shoot growth. The role of ethylene in inhibiting growth was investigated with just-germinated (24-h-old) seedlings in vitro, and 10-d-old plants grown aeroponically. Compared with seedlings maintained at 20 °C, root elongation in vitro was inhibited by 39% and root diameter increased by 25% under a temperature regime (38 °C/24 °C for 7 h/17 h) that simulated A-RZT in the greenhouse. The effects on root elongation were partially alleviated by supplying the ethylene biosynthesis inhibitors aminooxyacetic acid (100–500 µM) or aminoisobutyric acid (5–100 µM) to the seedlings. Application of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid to seedlings grown at 20 °C mimicked the high temperature effects on root elongation (1 µM) and root diameter (1 mM). Compared with plants grown at a constant 20 °C root-zone temperature, A-RZT plants showed decreased stomatal conductance, leaf relative water content, photosynthetic CO2 assimilation, shoot and root biomass, total root length, the number of root tips, and root surface area, but increased average root diameter. Addition of 10 µM ACC to the nutrient solution of plants grown at a constant 20 °C root-zone temperature mimicked the effects of A-RZT on these parameters but did not influence relative water content. Addition of 30 µM aminoisobutyric acid or 100 µM aminooxyacetic acid to the nutrient solution of A-RZT plants increased stomatal conductance and relative water content and decreased average root diameter, but had no effect on other root parameters or root and shoot biomass or photosynthetic CO2 assimilation. Although ethylene is important in regulating root morphology and elongation at A-RZT, the failure of ethylene biosynthesis inhibitors to influence shoot carbon gain limits their use in ameliorating the growth inhibition induced by A-RZT.
Key words: Ethylene, Lactuca sativa L., photosynthesis, root morphology, root-zone temperature, water relations
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Introduction |
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Each plant species has a minimum, optimum, and maximum temperature for growth, and differences in these temperature responses classify plants as either temperate or tropical. Growth of temperate plants under tropical conditions or vice versa inhibits growth and decreases crop production via a number of physiological mechanisms. Lettuce plants grown aeroponically in the tropics under ambient root-zone temperatures (A-RZTs) grew poorly and had lower photosynthetic CO2 assimilation (A), stomatal conductance (gs), and midday relative water content (RWC) than plants grown at a constant root-zone temperature of 20 °C (20 °C-RZT) (He et al., 2001), even though the shoots of all plants were exposed to similar environmental conditions. The decreased A of A-RZT plants was caused by both stomatal limitation of photosynthesis as gs and leaf RWC declined and also non-stomatal limitation of A due to decreased nutrient uptake (He et al., 2001). Decreased leaf water status (He et al., 2001) and nutrient uptake of A-RZT plants suggested impaired root functioning at A-RZT (Tan et al., 2002), and A-RZT also severely reduced root elongation and increased average root diameter (Qin et al., 2002; Tan et al., 2002).
Several lines of evidence suggested that the characteristic root morphology of A-RZT plants might be associated with excessive ethylene production. First, ethylene production may be stimulated by temperatures higher than those considered optimal (15–25 °C) for growth of temperate species (Field, 1985; Abeles et al., 1992). Secondly, exogenous ethylene induced root swelling (Goeschl and Kays, 1975; Moss et al., 1988; Abeles et al., 1992) and strongly inhibited root elongation (Smith and Robertson, 1971; Abeles et al., 1992) in several plant species. Thirdly, environmental or genetic conditions that increased average root diameter have been associated with increased root ethylene production (Romera et al., 1999; Spollen et al., 2000). Finally, at the cellular level, ethylene may change the direction of cell expansion in terrestrial plants from a mainly longitudinal to a mainly lateral direction (Abeles et al., 1992; Shibaoka, 1994).
One approach to discern a role for ethylene in different stress responses has been to use inhibitors of ethylene synthesis and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) to decrease and increase ethylene synthesis, respectively. The inhibitors aminooxyacetic acid (AOA) and aminoisobutyric acid (AIB), respectively, inhibit the enzymes ACC synthase and ACC oxidase involved in ethylene biosynthesis (Satoh and Esashi, 1980; Yang and Hoffman, 1984; Abeles et al., 1992). Preliminary experiments determined physiologically active concentrations of ACC, AOA, and AIB, using a lettuce root growth assay (Abeles and Wydoski, 1987) prior to adding these chemicals to aeroponic culture. The objectives were to investigate the environmental and hormonal influences on the root growth response of A-RZT plants, and determine the physiological consequences of modifying ethylene synthesis on root system morphology, root and shoot growth, leaf water relations, and photosynthetic performance of aeroponically grown lettuce.
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Materials and methods |
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Seedling root growth in vitro
Lettuce seeds (Lactuca sativa cv. Baby Butterhead) were allowed to germinate in a 14 cm Petri dish on moist filter paper. After 1 d, seedlings with roots 1.5–2.5 mm in length were selected. Ten seedlings were placed in a 9 cm Petri dish on filter paper (Whatman no. 1) wetted with 4 ml of one-quarter strength Netherlands standard composition nutrient solution (pH 6.5). The Petri dishes were covered and sealed with parafilm (to minimize evaporative losses), transported to incubators, and exposed to the following temperatures: 20 °C (control; model 3401 programmable cooled incubator; Rumed, Germany), 25, 30, 35, and 38 °C (model J-MIC1 multi-chamber incubator; Seoul, Korea). Control seedlings were held at 20 °C for the whole 24 h cycle. For the other temperature treatments, seedlings were subjected to different temperature regimes for the first 7 h and then all were cooled to the room temperature (24±2 °C) for another 17 h by manually switching off the incubator. It took 130–140 min for temperatures above 30 °C to cool to 24 °C (± 2 °C). The variable temperature regimes of the incubator simulated greenhouse conditions in Singapore. During the hottest months of the year (April–July), the root-zone temperature (RZT) of aeroponically grown plants at A-RZT reached 33–41 °C for about 7 h during the middle of the day (He and Lee, 1998). From late afternoon to early morning, A-RZT ranged from 27 °C to 22 °C; thus 24 °C was selected to simulate the coolest part of the day.
Root morphology was analysed immediately after a second 24 h temperature cycle (72 h after imbibition). Small root size precluded the analysis of root morphology after the first 24 h temperature cycle, while preliminary experiments showed that the hypocotyl had emerged and elongated after a third temperature cycle. Each experiment was repeated three times.
To analyse the effects of the ethylene precursor (ACC) or ethylene biosynthesis inhibitors (AIB, AOA) on root elongation, these chemicals were dissolved in one-quarter strength Netherlands standard composition nutrient solution (pH 6.5). Solution pH was adjusted to 6.5 with 1 M HNO3 or 1 M NaOH. Seedlings were selected as described above and placed on filter paper moistened with different concentrations of the various solutions (ACC, AIB, or AOA) or nutrient solution (control). Seedlings were incubated in the dark at 20 °C (control and ACC treatment) or at 37 °C/24 °C for 7 h/17 h (control, AIB, and AOA treatments; model 700 programmable incubator; Memert, GmbH & Co, KG, Germany) for 48 h prior to root morphology analysis. Each experiment was repeated three times.
Root morphology was analysed with the WIN MAC RHIZO V3.9 program (Regent Instruments, Canada) equipped with a WIN MAC RHIZO scanner (Québec, Canada) to determine total length, surface area, number of root tips, and average root diameter (total surface area/total root length).
Growth of plants aeroponically at two different RZTs in the greenhouse
Seedlings were germinated as described above. After 3 d, those with open cotyledons were transplanted into polyurethane cubes (24x24x24 mm) soaked in water and placed in trays. These trays were then transferred to the greenhouse after 4–5 d when the seedlings had established. Seedlings were then transplanted to the aeroponic system previously described by Lee (1993). Polystyrofoam planks, in which the polyurethane cubes were anchored, insulated the top of each trough. Nutrient solution was misted over the roots of the plants for 30 s every minute. The nutrient solution was based on full-strength Netherlands Standard Composition (electrical conductivity of 2.2 mS). In this experiment, the aerial part of the lettuce plants was exposed to fluctuating ambient temperatures (40 °C/25 °C) under 100% of prevailing solar radiation, while their roots were grown at two different temperatures: A-RZT and 20 °C (20 °C-RZT). The latter temperature was optimal for lettuce grown in the tropics (He et al., 2001) and was achieved by cooling the reservoir of nutrient solution to the required temperature before spraying it on the roots of the plants. Dissolved oxygen concentration (DOC) and temperature in nutrient solution were measured using the YSI 550 DO Handheld dissolved oxygen and temperature system (YSI Incorporated, Yellow Springs, OH, USA) at 1700 h.
The ethylene precursor (10 µM ACC) and ethylene biosynthesis inhibitors (100 µM AOA or 30 µM AIB) were added to nutrient solutions of some 20 °C-RZT and A-RZC plants, respectively, at 10 d after transplanting. Plants maintained at the different RZTs in unamended nutrient solution acted as controls.
Every 5 d, plants from each treatment were harvested for determination of root and shoot fresh weight, and root morphology as described above. After 13 d of treatment, leaf RWC and gas exchange were determined. A and gs were measured on the youngest fully expanded leaves (2.5 cm2) using an infra-red gas analyser (CIRAS-1; PP Systems, Hitchin, UK). A and gs were measured between 1000 h and 1100 h with prevailing solar radiation (average light intensity of 900±30 µmol m–2 s–1) on a sunny day at a CO2 concentration of 400 ppm.
Ten leaf discs, 1 cm in diameter, were cut and immediately weighed with an analytical balance for the field fresh weight. The leaf discs were then floated on distilled water in the dark for 24 h to determine their turgid weight. Dry weights were obtained after wrapping the leaf discs in aluminium foil and oven-drying at 105 °C for 48 h. RWC was determined as: (fresh weight–oven dry weight)/(turgid weight–oven dry weight)x100 (He et al., 2001).
Statistical analysis
Single factor analysis of variance (ANOVA) tested for significant (P <0.05) differences between RZT and chemical treatments.
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Results |
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Seedling root growth in vitro
Exposure of seedlings to temperatures ranging from 25 °C to 35 °C for 7 h in each 24 h period had no effect on total root length or root surface area compared with control seedlings maintained at 20 °C (Fig. 1). However, exposure to 38 °C decreased total root length and root surface area by 39% and 33%, respectively, compared with control seedlings. Increasing temperature from 25 °C to 38 °C significantly increased the average root diameter of seedlings compared with controls at temperatures of
30 °C (Fig. 1). Preliminary experiments with different numbers of seedlings (5, 10, and 15) in each Petri dish at 38 °C showed no significant differences in root morphology among all seedlings (data not shown).
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In seedlings maintained at 20 °C, 1 µM ACC was sufficient to decrease total root length, and total root length decreased further as ACC concentration increased (Fig. 2A). Significant decreases in root surface area and increases in average root diameter were observed at 100 µM and 1000 µM ACC, respectively (Fig. 2A).
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In seedlings grown at 37 °C/24 °C (7 h/17 h), AOA at 100 µM or 500 µM partially alleviated the inhibition of root elongation caused by high temperature but had no significant effects on root surface area (Fig. 2B). Higher concentrations of AOA (2500 µM) significantly decreased total root length and root surface area compared with seedlings grown without AOA (Fig. 2B), indicating a toxic effect. AOA had no significant effect on average root diameter at any concentration tested.
In seedlings grown at 37 °C/24 °C (7 h/17 h), AIB at 5–100 µM and 30–100 µM increased root total length and root surface area, respectively, compared with control seedlings grown without AIB (Fig. 2C). Toxic effects of AIB were observed at high concentrations (5000 µM), as evidenced by significant decreases of total root length and root surface area (Fig. 2C). AIB had no significant effect on average root diameter at any concentration tested.
Growth of plants aeroponically at two different RZTs in the greenhouse
As in previous work (He et al., 2001), A-RZT significantly decreased shoot and root fresh weights compared with 20 °C-RZT plants. Addition of 10 µM ACC to the nutrient solution of 20 °C-RZT plants significantly decreased shoot and root fresh weights after 10 d of treatment (Fig. 3). Compared with A-RZT plants, adding 100 µM AOA or 30 µM AIB to the nutrient solution did not affect shoot and root fresh weights of A-RZT plants during the 15 d of treatment (Fig. 3).
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As in previous work (Qin et al., 2002), A-RZT significantly decreased total root length, number of root tips, and root surface area compared with 20 °C-RZT plants. In 20 °C-RZT plants, exogenous ACC significantly decreased total root length (Fig. 4A), number of root tips (Fig. 4B), and root surface area (Fig. 4C) within 5 d of its addition to the nutrient solution. Root diameter was increased 15 d after ACC addition and at the end of the experiment was similar to that of A-RZT plants (Fig. 4D).
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In A-RZT plants, exogenous AOA or AIB had no effect on total root length (Fig. 4A), number of root tips (Fig. 4B), and root surface area (Fig. 4C). Adding 100 µM AOA to the nutrient solution significantly decreased average root diameter throughout the experiment, while 30 µM AIB only transiently (plants sampled at 5 d) decreased average root diameter (Fig. 4D).
As in previous work (He et al., 2001), A, gs, and leaf RWC of A-RZT plants were significantly less than in 20 °C-RZT plants. Supplying 10 µM ACC to the nutrient solution of 20 °C-RZT plants significantly decreased both A (Fig. 5A) and gs (Fig. 5B), with the former decreasing to levels similar to those of A-RZT plants. However, ACC treatment did not change leaf RWC of 20 °C-RZT plants.
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In plants grown at A-RZT, adding 100 µM AOA or 30 µM AIB to the nutrient solution significantly increased gs (Fig. 5B) and leaf RWC (Fig. 5C) but had no effect on A (Fig. 5A). Despite these increases, gs and leaf RWC of A-RZT plants treated with AOA or AIB were still much lower than in 20 °C-RZT plants.
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Discussion |
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In vitro assays showed that high temperature (7 h of 38 °C in a 24 h cycle) inhibited primary root growth of lettuce by 39% and increased root diameter by 25% (Fig. 1). Similarly, increasing the temperature from 15 °C to 29 °C decreased elongation of primary roots of temperate pea (Pisum sativum) plants grown in vermiculite (Gladish and Rost, 1993). However, there is considerable species variation in temperature optima for root growth, as elongation of primary roots of maize (Zea mays) grown in vermiculite increased 3-fold as the temperature was increased from 16 °C to 29 °C (Pahlavanian and Silk, 1988). Given that these root growth responses may be partially responsible for the differential sensitivity of temperate and tropical plants to high temperature (Cooper, 1973), it is important to crop production to investigate their regulation.
Ethylene may be involved in growth regulation at high temperature since supply of its immediate precursor (ACC) to seedling roots grown at 20 °C, or 10-d-old plants grown aeroponically at 20 °C-RZT, resembled the high temperature effects on root elongation (Figs 2A, 4A) and radial expansion (Figs 2A, 4D). These effects were consistent with the inhibition of longitudinal growth and increase of root diameter in pea (Goeschl and Kays, 1975) and maize (Moss et al., 1988) primary roots exposed to exogenous ethylene at optimal growth temperatures. Although ethylene caused similar effects on both processes in these species, in other species (cotton, bean, peanut) longitudinal growth was inhibited with minimal effects on root diameter (Goeschl and Kays, 1975), suggesting that the two processes can be differentially sensitive to ethylene. This appeared to be the case in lettuce. In vitro, root diameter was less responsive (in terms of minimum concentration required to induce an effect) to exogenous ACC than root length (Fig. 2A). Similarly, in aeroponically grown plants, root diameter was less responsive (in terms of duration of exposure required to induce an effect) to exogenous ACC than root length (cf. Fig. 4A, D). Given that root diameter is less sensitive to ACC than elongation, the case that ethylene is responsible for the high temperature effects on root growth is weakened by the observation that root thickening occurred at substantially lower temperatures than the inhibition of longitudinal growth (Fig. 1). Consequently, the role of ethylene at high temperature was further investigated using ethylene biosynthesis inhibitors.
Again, there was some support for a role for ethylene in growth regulation at high temperature. Ethylene biosynthesis inhibitors could partially alleviate the effects of high temperature on root elongation in vitro (Fig. 2B, C) and on radial expansion in aeroponically grown crops (Fig. 4D). These divergent responses may reflect developmental differences in sensitivity of the two processes to ethylene at the two growth stages. Just after germination, root elongation may be especially sensitive to ethylene, while later in development, radial expansion may be more sensitive.
A common difficulty of using inhibitors is inconsistency of response related to non-specific effects on metabolism, such that different compounds induced different effects on lettuce root elongation despite similar effects on root ethylene production. While 0.1–1.0 µM aminoethoxyvinylglycine (AVG) stimulated lettuce root elongation, AOA had no effect at concentrations up to 100 µM at optimal temperatures (Abeles and Wydoski, 1987), indicating that AOA imposed additional growth limitations. In contrast, AOA was effective at high temperatures in promoting root elongation in vitro (Fig. 2B) and decreasing root diameter in aeroponically grown plants (Fig. 4D). Previously, the ability of ethylene biosynthesis inhibitors to prevent root thickening has varied according to the stress under consideration. AVG and AOA were both able to alleviate root thickening of Fe-deficient plants (Romera and Alcantara, 1994) and ABA-deficient plants grown at low water potential (Spollen et al., 2000), suggesting that ethylene was responsible for thickening under these conditions. In contrast, ethylene biosynthesis inhibitors were ineffective at alleviating root thickening under mechanical impedance (0.75 µM AVG; Moss et al., 1988) and nutrient solution causing thick root syndrome (5 µM AIB; Pierik et al., 1999), suggesting that factors other than ethylene are important in mediating root growth under these conditions. Since environmental conditions are likely to change both hormone concentrations as well as sensitivity to these hormones, it is perhaps not surprising that inhibiting root ethylene production has produced variable responses.
It therefore becomes important to identify the environmental or endogenous signal(s) that the roots are responding to at high temperatures. In aeroponically grown plants, changes in root growth may be a direct response to the rhizosphere environment, or a secondary response to a decreased supply of photoassimilate from the shoot, since A-RZT decreased photosynthesis (Fig. 5A). However, the similar temperature response of dark-grown seedlings (in which primary root growth was still dependent on seedling reserves and not recently acquired photoassimilate) and aeroponically grown plants suggested a direct effect of the rhizosphere environment. High temperature may directly limit root growth by limiting the activity of enzymes involved in metabolism, or may decrease nutrient solution DOC or root nutrient status.
As water temperature increases from 20 °C to 35 °C, DOC decreases by 21% (Gevantman, 2005). Direct measurement of DOC in the nutrient solution of aeroponically grown plants confirmed the magnitude of this decrease, but absolute values exceeded 6.5 mg O2 l–1 even at A-RZT (Table 1). Typical values of DOC in aerated nutrient solution in hydroponics are 
7 mg l–1 (Kamaluddin and Zwiazek, 2000), so it seems unlikely that hypoxia is responsible for the observed changes in root growth. Furthermore, the similar temperature response of lettuce primary root elongation in filter paper culture (Fig. 1) and in pea grown in vermiculite (where oxygen concentration is unlikely to be limiting) (Gladish and Rost, 1993) discounts the notion that hypoxia caused this response. Furthermore, experiments evaluating the temperature response of lettuce seedlings grown on different substrates (both agar and filter paper culture) showed similar root growth responses (data not shown), confirming that the roots were responding to temperature per se.
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Another possibility is that nutrient deprivation of A-RZT plants stimulated root ethylene production and thus limited growth. Although aeroponically grown plants were continuously misted with a balanced, complete nutrient solution, lettuce plants grown under A-RZT developed multiple mineral deficiencies (including both P and Fe) in both roots and shoots (Tan et al., 2002). Mineral deficiency of P and Fe can increase both root ethylene production and radial expansion (Lynch and Brown, 1997; Romera et al., 1999). Most data concerning nutrient deprivation and root ethylene relations pertain to specific deficiencies of single elements, and there is limited information on the effects of general nutrient deprivation (Lynch and Brown, 1997), as experienced by A-RZT plants. Since general nutrient limitation can stimulate root elongation (Chapin, 1990; Ryser, 2006), in contrast to the inhibitory effect of A-RZT, it is unlikely to be responsible for the high temperature effects on root morphology. Furthermore, the similarity of root growth response in both 10-d-old plants (Fig. 4) and dark-grown seedlings reliant on seed reserves (Fig. 1) suggested that roots were not sensing nutrient deprivation.
The ethylene-mediated limitation of root growth at high temperature also impacted on shoot growth and functioning. Adding the ethylene biosynthesis inhibitors AOA or AIB to the nutrient solution of aeroponically grown plants at A-RZT increased leaf RWC (Fig. 5C) and gs (Fig. 5B), suggesting an improvement in not only root growth, but also root function. However, this improvement in RWC was minimal (no more than 5%) in comparison with the drastic decrease in RWC caused by A-RZT (RWC of A-RZT plants was 20% lower than 20 °C-RZT plants), suggesting that the bulk of this temperature effect is ethylene-independent. A-RZT dramatically decreased root hydraulic conductivity (Lp) compared with 20 °C-RZT plants (Dodd et al., 2000). Decreased Lp is likely to decrease shoot water potential and RWC, thus directly inducing stomatal closure, especially under conditions of high evaporative demand characteristic of sunny days in Singapore greenhouses, when transpirational demands can exceed the capacity of the root system to supply water. The decrease in Lp under A-RZT is likely to be responsible for the temporal correlation between RWC and gs (and thus stomatal limitation of photosynthesis) in plants reciprocally transferred between 20 °C-RZT and A-RZT (He et al., 2001). Furthermore, RWC and gs were correlated in these experiments (Fig. 5), although 20 °C-RZT plants supplied with ACC did not fit this relationship (see below).
Alternatively, root temperature effects on root ethylene synthesis may be transmitted to the shoots via increased xylem concentrations of the soluble ethylene precursor ACC, as in other edaphic stresses (reviewed in Dodd, 2005). While ethylene is well known as a growth regulator, there is limited evidence that it controls stomatal behaviour, in contrast to the role of ABA in stomatal closure (reviewed in Dodd, 2003). However, ethylene may stimulate ABA synthesis, since applying 500 µM ethephon (an ethylene-releasing chemical) in hydroponic culture increased tomato leaf ABA concentration 1.6-fold (Hansen and Grossmann, 2000). Ethylene-stimulated ABA biosynthesis might explain stomatal closure of 20 °C-RZT plants in which ACC was added to the nutrient solution. Further investigation of the effects of ethylene biosynthesis inhibitors on plant ABA relations at different RZTs would seem warranted.
Irrespective of whether A-RZT limited gs by hydraulic (leaf RWC) or chemical signals, changes in stomatal physiology may not necessarily alter photosynthetic performance and biomass accumulation. Decreasing ethylene biosynthesis by addition of AOA or AIB to the nutrient solution of A-RZT plants did not show positive effects on biomass (Fig. 3) or A (Fig. 5A), probably due to the non-stomatal limitation of photosynthesis mediated by nutrient deficiency. In contrast, transferring A-RZT plants to 20 °C-RZT resulted in partial recovery of A and total leaf N content after 6 d (He et al., 2001), in association with changes in root morphology (Qin et al., 2002). Overcoming high-temperature-induced changes in nutrient uptake will be necessary if the positive effects of AOA or AIB on growth, leaf water status, and stomatal opening are to improve shoot growth.
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Conclusions |
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Supplying ethylene biosynthesis inhibitors to A-RZT plants affected root growth and had positive effects on leaf water status and stomatal opening. While economic reasons may preclude use of such chemicals in commercial production, the deployment of ethylene-insensitive lettuce (Saltveit et al., 2003; Kim and Botella, 2004) at A-RZT may provide an alternative means to alter crop responses. Increasing interest in the manipulation of ethylene biosynthesis or sensitivity as an agronomic tool to improve crop production (Stearns and Glick, 2003) suggests that future trials may be justified.
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Acknowledgements |
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LQ, JH, and SKL thank the Academic Research Fund, Ministry of Education, Singapore for support, while ICD thanks BBSRC for support. We thank two anonymous reviewers for their helpful comments on an earlier version of this manuscript.
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Abbreviations |
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A, photosynthetic CO2 assimilation; ACC, 1-aminocyclopropane-1-carboxylic acid; AIB, aminoisobutyric acid; AOA, aminooxyacetic acid; A-RZT, ambient root-zone temperature; AVG, aminoethoxyvinylglycine; DOC, dissolved oxygen concentration; gs, stomatal conductance; Lp, root hydraulic conductivity; RWC, relative water content; RZT, root-zone temperature.
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References |
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