JXB Advance Access originally published online on October 24, 2005
Journal of Experimental Botany 2005 56(422):3093-3101; doi:10.1093/jxb/eri306
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RESEARCH PAPER |
Water stress-induced modifications of leaf hydraulic architecture in sunflower: co-ordination with gas exchange
Dipartimento di Biologia, Università di Trieste, Via L. Giorgieri 10, 34127 Trieste, Italia
* To whom correspondence should be addressed. Fax: +39 040 568855. E-mail: nardini{at}univ.trieste.it
Received 30 May 2005; Accepted 31 August 2005
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Abstract |
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The hydraulic architecture, water relationships, and gas exchange of leaves of sunflower plants, grown under different levels of water stress, were measured. Plants were either irrigated with tap water (controls) or with PEG600 solutions with osmotic potential of –0.4 and –0.8 MPa (PEG04 and PEG08 plants, respectively). Mature leaves were measured for hydraulic resistance (Rleaf) before and after making several cuts across minor veins, thus getting the hydraulic resistance of the venation system (Rvenation). Rleaf was nearly the same in controls and PEG04 plants but it was reduced by about 30% in PEG08 plants. On the contrary, Rvenation was lowest in controls and increased in PEG04 and PEG08 plants as a likely result of reduction in the diameter of the veins' conduits. As a consequence, the contribution of Rvenation to the overall Rleaf markedly increased from controls to PEG08 plants. Leaf conductance to water vapour (gL) was highest in controls and significantly lower in PEG04 and PEG08 plants. Moreover, gL was correlated to Rvenation and to leaf water potential (
leaf) with highly significant linear relationships. It is concluded that water stress has an important effect on the hydraulic construction of leaves. This, in turn, might prove to be a crucial factor in plant–water relationships and gas exchange under water stress conditions. Key words: Gas exchange, leaf hydraulic architecture, sunflower, water relations, water stress
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Introduction |
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Several attempts at measuring the hydraulics of plant organs have been reported in the past, but only over the last 25 years has the concept of ‘plant hydraulic architecture’, as introduced by Zimmermann (1978)
, been widely adopted, with the purpose of interpreting the water balance of plants as the integrated result of the hydraulic properties of their organs (Tyree and Zimmermann, 2002). Over this time, changes in the hydraulics of roots, stems, and leaves of several species have been reported to occur in response to changing environmental factors. The discovery of aquaporins in plant cells (Maurel, 1997; Luu and Maurel, 2005) has made a powerful contribution to the explanation of why cell membranes have a high permeability to water and has elucidated some of its sources of variation (Henzler et al., 1999; Nardini et al., 2005). In particular, studies of the water pathways within the root (Steudle et al., 1987; Steudle and Peterson, 1998) have made it possible to discriminate between the contribution of the symplastic and that of the apoplastic hydraulic resistance along the soil-to-stele xylem path and have provided a solid biophysical and molecular basis for analogous studies at the leaf level.
The hydraulic properties of the leaf, however, are poorly understood at present, although this organ has been (and still is) widely studied for gas exchange, water status and, of course, photosynthesis. Leaf hydraulics are intrinsically difficult to study because: (i) the extreme morpho-anatomical heterogeneity of this organ, even within one individual, generates analogous heterogeneity in the data and makes them difficult to generalize; (ii) liquid and gaseous water flow in a leaf are hard to discriminate from each other using the techniques presently available for measuring hydraulic variables, like the vacuum chamber (Kolb et al., 1996; Nardini et al., 2001) or the high pressure flow meter (Tyree et al., 1995; Sack et al., 2002); (iii) a typical leaf of an angiosperm consists of a highly branched xylem system connected to the photosynthetic tissues through the vein living cells (the bundle sheath) about whose functional features very little is known at present. In addition, liquid water flows within the leaf lamina both in the vascular compartment and through the mesophyll living cells (the extra-vascular compartment). Each of these leaf compartments has its own hydraulic properties, the former mainly depending on the geometry of the xylem conduits (Canny, 1990; Cochard et al., 2004) and water permeability of the pits (Sperry et al., 2005), whereas the latter are closely dependent on water permeability of cell membranes and, ultimately, on cell metabolism (Morillon and Chrispeels, 2001). The present understanding of leaf hydraulic construction is still limited and data from the literature are sometimes contradictory (Zwieniecki et al., 2002; Cochard et al., 2004), especially when referring to the partitioning of leaf hydraulic resistance into venation and extra-vascular components.
Among the environmental factors that may influence leaf hydraulics, water stress can be safely expected to be a major one. Water stress is well known to impair the conductive efficiency of the plant vascular system through xylem embolism (Tyree and Sperry, 1989) and it is now known that the leaf xylem also undergoes cavitation-induced hydraulic failure (Salleo et al., 2001). As an example, Kikuta et al. (1997) measured the threshold values of leaf water potential (leaf) triggering vein cavitation in leaves of several deciduous and evergreen trees, on the basis of ultrasound acoustic emissions. Vein cavitation was found to occur at leaf values between –0.5 and –2.0 MPa, i.e. well within the range of minimum leaf experienced by plants in the field. More recently, hydraulic measurements of leaf blades of different species have revealed the potential impact of vein cavitation on leaf hydraulics (Nardini et al., 2001, 2003; Brodribb and Holbrook, 2003) and gas exchange (Lo Gullo et al., 2003; Brodribb and Holbrook, 2004). However, knowledge about the influence of middle- to long-term reduction of water availability on the hydraulic construction of leaves is still limited, although it is known that other environmental factors influencing leaf growth (e.g. light) have important effects on their hydraulic features (Sack et al., 2003, 2005).
The present study investigates the effects of moderate water stress developing during plant growth and leaf maturation on leaf hydraulic architecture, in terms of the contribution of the hydraulic resistance of the leaf vasculature Rvenation to Rleaf, as well as of the impact of eventual changes in Rvenation on leaf gas exchange. Studies were conducted on sunflower because plants of the modern cultivars of this species are fairly stable genetically and they have a sufficiently uniform leaf structure.
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Materials and methods |
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Plant material and growth conditions
All experiments were conducted on 21 sunflower (Helianthus annuus L. cv. Margot) plants 7–9 weeks old. Seeds (provided by Maisadour Semences Italy srl) were planted in greenhouse trays and, after cotyledons were fully expanded, seedlings were transferred to 1.5 l pots filled with a mixture (1:1) of peat and sand (one seedling per pot). Plants were grown in a room where air temperature was adjusted to vary between 23 °C and 16 °C (day/night), relative humidity was set at 50±5% and light was provided by lamps (HQI-T 1000 W/D; Osram GmbH, München, Germany) with a photosynthetically active radiation (PAR) of 400±50 µmol m–2 s–1. The photoperiod was set at 12 h. Plants were irrigated daily for 20 d with 200 ml of tap water. After this time (when plants were bearing two pairs of leaves, excluding cotyledons), they were randomly divided into three groups of seven plants each. One group continued to be irrigated with tap water (controls). Increasing water stress levels were imposed on the other two groups of plants by irrigating them daily with 200 ml of polyethylene glycol (PEG600; Sigma-Aldrich) at a concentration of 0.18 M and 0.36 M, resulting in solutions with osmotic potential (
) of –0.4 and –0.8 MPa, respectively, as measured using a dew-point hygrometer (WP4; Decagon Devices, Pullman, USA). Although PEG600 is thought to enter cell membranes with time, use of higher-size glycols is possibly not the most convenient way to depress aquaporin activity and decrease cell hydraulic conductivity (Ye et al., 2004). The above PEG concentrations were also used because previous studies (Lo Gullo et al., 2004) had shown that leaf water potential at the turgor loss point (tlp) of sunflower plants grown under similar environmental conditions was about –1.0 MPa. Therefore, the PEG solutions used in the present study were planned to correspond approximately to 40% and 80% of the expected tlp, i.e. to a mild and a severe water stress level, respectively. Hereafter, the two groups of plants irrigated with the 0.18 M and 0.36 M PEG600 solutions are referred to as ‘PEG04’ and ‘PEG08’ plants, respectively. Every irrigation with PEG solution was always preceded by irrigation with 200 ml of tap water. This procedure was aimed at preventing a build-up of PEG600 concentration in the soil and, hence, soil water potential dropping below the desired, pre-set value. To avoid eventual differences in the availability of nutrients as caused by different watering treatments, each plant received 2.5 g of fertilizer (Nitrophoska Top, BASF Italia SpA; 15% N, 10% P2O5, 15% K2O, 2% MgO, 12% SO3, 0.02% B, 0.01% Zn) at 10 d intervals. Soil water potential (soil) was measured at regular intervals using a dew-point hygrometer (see above) on soil samples collected from one pot per group. In particular, soil was measured 2, 4, 8, and 16 d after the beginning of the water stress treatments. In each case, soil samples were collected 2 h after the last irrigation, i.e. after excessive water had drained out. Plants growing in the pots from which soil samples were taken were not used for subsequent experiments. All other measurements started 21 d after the beginning of the water stress treatment and were completed within the following 15 d. Measurements were performed on mature leaves sampled from the two most apical nodes. These leaves were chosen because a previous study (Lo Gullo et al., 2004) had shown that apical leaves of mature sunflower plants have maximum hydraulic conductance.
Measurements of leaf gas exchange, water potential, and pressure–volume curves
Leaf conductance to water vapour (gL) was measured using a steady-state porometer (LI-1600; Li-Cor Inc., Lincoln, NE, USA). Measurements were taken at the middle of the light period, i.e. between 5 and 7 h after lights had been turned on. A leaf chamber, 2 cm2 in surface area, was used, and gL was measured at the central portion of the leaf blade, close to the middle of the major vein. Immediately after gL measurements, leaves were cut off and their hydraulic architecture was measured as described below. Six leaves per group (one leaf per plant) were measured for gL and hydraulic architecture. This procedure was adopted to get information about eventual relationships existing between leaf gas exchange rate and the corresponding hydraulic characteristics. To obtain information about relationships between gL and the leaf water potential (leaf), another set of leaves (six leaves per group, one leaf per plant) was measured for gL as described above. In this case, however, immediately after gL measurements, leaves were cut off, immediately enclosed in dark plastic bags, and their leaf was measured using a pressure chamber (model 3005; Soilmoisture Equipment Corp., Santa Barbara, CA, USA).
To check changes in leaf osmotic potential at full turgor (o) and water potential at the turgor loss point (tlp), as induced by water stress, four leaves per group from different plants were collected and rehydrated overnight by placing the cut end of the petiole in contact with distilled water. Leaf water potential isotherms (Tyree and Hammel, 1972) were measured using the procedure described by Salleo (1983) and Salleo et al. (1997). From pressure–volume curves, 0 and tlp were calculated.
Measurements of leaf hydraulic architecture
Leaf hydraulic resistance (Rleaf) was measured using a high pressure flow meter (HPFM) (Tyree et al., 1995). The instrument has proved to yield values of Rleaf consistent with those obtained using independent methods (Sack et al., 2002; Salleo et al., 2003). Leaves were cut off immediately after gL measurements had been completed (see above) leaving about 30 mm of petiole for connection to the HPFM. Leaves were then connected to the instrument using compression fittings, within 5 min from cutting. Degassed water filtered at 0.1 µm, was forced into the petiole at a pressure of 0.15 MPa and R was measured as the pressure-to-flow ratio at 16 s intervals until values became stable (i.e. the coefficient of variation of the last 20 readings was <3%), which usually took 10–20 min. During measurements, leaves were maintained under usual laboratory irradiance (PAR <10 µmol m–2 s–1). Although Rleaf is known to be reduced upon illumination at PAR 
1200 µmol m–2 s–1 in several species (Sack et al., 2002; Tyree et al., 2005), this is not the case of sunflower leaves when collected during their light period, as recently reported in a study by Nardini et al. (2005). The hydraulic resistance of the leaf vasculature (Rvenation) was estimated using the technique first proposed by Sack et al. (2004) consisting of serial measurements of leaf hydraulic resistance after increasing numbers of minor veins of the fourth order or higher had been cut open in order to by-pass the extra-vascular leaf compartment. Minor veins were cut at random locations throughout the lamina by making 1.5–2 mm incisions with a scalpel. This procedure was basically similar to that originally proposed by Sack et al. (2004) and repeated by Gascò et al. (2004) and Nardini et al. (2005). In the study by Sack et al. (2004), 120–150 cuts were reported to be sufficient to yield stable R values. In the case of sunflower, however, higher numbers of cuts (up to 500 cuts per leaf, corresponding to approximately 3–5 cuts cm–2) were necessary to achieve stable and relatively invariant R values. At the end of measurements, the leaf surface area (Aleaf) was measured using a leaf area meter (LI-3000A; Li-Cor Inc.), and both Rleaf and Rvenation were normalized by Aleaf (one side only).
Anatomical measurements
Because Rvenation is likely to be related to anatomical features of the leaf veins as eventually modified by water stress, the diameter of xylem conduits of the midrib was measured for each leaf used for HPFM measurements. Cross-sections of the midrib (one section per leaf) were taken at the proximal third of the leaf length using fresh razor blades. Sections were immediately observed under a light microscope and the total number of conduits per midrib was recorded. Then, the diameter of the 10–15 widest conduits per section was measured. The diameter of conduits that were elliptical in shape was computed by averaging the major and minor axes. The authors are aware that dimensions of the xylem conduits in the midrib may not necessarily give information of the hydraulic efficiency of the whole leaf venation system but they at least provide an idea of the likely reduction of conduit dimensions in higher order veins, because hydraulic traits of midrib, major veins, and minor veins are known to be correlated with each other (Sack et al., 2005).
Statistics
Data were analysed with the SigmaStat 2.0 (SPSS, Chicago, IL, USA) statistics package. One-way ANOVA was used to test differences between experimental groups. The statistical significance of correlations between parameters was tested using the Pearson Product Moment Correlation.
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Results |
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Repeated plant irrigations with PEG aqueous solutions caused soil water potential (
soil) to decrease from –0.22 MPa (controls) to –0.58 MPa for plants irrigated with PEG solution at =–0.4 MPa and further to –0.94 MPa for plants irrigated with PEG at =–0.8 MPa (Table 1). The main morphological/anatomical effects of the two levels of water stress applied to growing sunflower plants are summarized in Table 2, and an example of the general appearance of plants is given in Fig. 1. PEG04 plants grew less and had thinner stems than controls, i.e. their height above the ground was 0.57 m versus 0.73 m recorded for controls and their base stem diameter was about 8.0 mm versus 9.7 mm for controls. PEG08 plants showed an even larger reduction in height and stem diameters which were about 34% and 28% less than controls, respectively. Also the total leaf surface area was consistently reduced as a consequence of the water stress applied, in terms of both number of leaves per plant and mean leaf surface area (Aleaf). In particular, Aleaf of mature, fully expanded leaves was about 113 and 105 cm2 in PEG04 and PEG08 plants, respectively, versus about 140 cm2 as recorded for control leaves, i.e. leaves from water-stressed plants were 19–25% smaller than leaves from well-hydrated plants. In addition, the widest xylem conduits in the midrib were significantly narrower in PEG04 and PEG08 plants (37.6 and 35.1 µm in diameter, respectively) than those of controls (43.2 µm in diameter).
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) measured 5 cm above the soil, number of leaves per plant, mean leaf surface area (AL) and mean diameter of the midrib's widest conduits as measured in control plants (controls) and in water-stressed plants (PEG04 and PEG08)
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In response to the osmotic stress applied to roots, plants showed increasingly larger drops in
leaf (Table 1), from –0.65 MPa as recorded for controls at the middle of the light period, to –1.26 MPa for PEG04 plants and to –1.59 MPa for PEG08 plants. The driving force for liquid water flow through the plant body corresponding to (soil–leaf), turned out to be higher for water-stressed plants (0.68 and 0.65 MPa for PEG04 and PEG08 plants, respectively) than for controls (0.43 MPa). It is of interest to note that the lower level of water stress applied (PEG04) allowed leaves to retain most of their turgor pressure. Isotherms of leaf water potential gave information of leaf at the turgor loss point (tlp) from which the average leaf turgor pressure (Pt=leaf–tlp) can be computed. This was of the order of 0.37 MPa in controls and still as high as 0.31 MPa in leaves from PEG04 plants. Plants growing at soil =–0.94 MPa (PEG08) still retained some turgor, i.e. Pt was estimated to be about 0.12 MPa. Really, leaf was found not to be significantly different from tlp in this case, but the statistical power of this comparison was likely to be affected by the difference in the numbers of replications (n=4 for tlp and n=6 for leaf). Therefore, the lack of statistical significance of the difference between leaf and tlp for PEG08 plants remained dubious. The general appearance of leaves of these plants, in fact, indicated that their turgor was not zero (Fig. 1). Data from Table 1 also show that water-stressed plants had adjusted leaf osmotic potential because this variable, when estimated at full turgor, was found to be significantly more negative in PEG04 plants than in controls (o was about –1.2 MPa in PEG04 plants versus –0.83 MPa in controls). In turn, PEG08 plants showed leaf osmoregulation basically similar to that measured for PEG04 ones.
Plants growing in soils with experimentally decreased showed a significant reduction in their maximum stomatal aperture, as indicated by gL that was found to be reduced by about 28% in PEG04 plants with respect to controls, i.e. from 250 to 180 mmol m–2 s–1 and further to 150 mmol m–2 s–1 in PEG08 plants (Fig. 2). It has to be noted, however, that PEG08 plants still maintained significant gas exchange, although gL of these plants was 40–50% less than that of controls.
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Several serial cuttings of leaf veins of the fourth or higher order (Fig. 3), followed by repeated measurements of leaf hydraulic resistance (Rleaf), allowed the resistance of the leaf venation system (Rvenation) to be estimated in terms of the residual R once this variable had become approximately constant (Sack et al., 2004
). This condition was achieved after about 500 cuts had been made across minor veins. All plants subjected to water stress had Rvenation values higher than those of controls with no difference between plants growing at the two soil levels tested. The initial Rleaf values of Fig. 3, corresponding to the R of leaves with intact lamina, are reported in Fig. 4A. It can be noted that the initial Rleaf of controls and that of PEG04 plants were very similar to each other, while Rleaf recorded in PEG08 plants was one-third less than that of controls (2.4 e+3 versus about 3.2 e+3 MPa s m2 kg–1). By contrast, Rvenation was lower in controls (about 0.6 e+3 MPa s m2 kg–1) than in water-stressed plants (about 0.8 e+3 MPa s m2 kg–1) with no difference between PEG04 and PEG08 plants in this regard (Fig. 4B). The fractional amount of Rvenation with respect to total Rleaf was found to increase as a consequence of the water stress applied (Fig. 4C). The Rvenation:Rleaf ratio, in fact, was about 0.2 for leaves from plants irrigated with water and as high as 0.4 in plants irrigated with PEG solutions at =–0.8 MPa. Controls and PEG04 plants did not differ from each other for the above ratio, statistically, although a possible tendency to higher Rvenation:Rleaf ratios might exist in the latter. When Rvenation values from all the plants studied were plotted against the diameters of the 10–15 widest conduits in the midrib, a linear, negative relationship appeared to exist between the two variables (Fig. 5) with r=–0.778 and high significance (P <0.001). This strongly suggests that the higher Rvenation recorded in water-stressed plants, with respect to controls, was at least partly due to the narrower xylem conduits in the veins of the former.
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Leaf conductance to water vapour (gL) turned out to be related to Rvenation with a closely linear, negative correlation between the two variables (Fig. 6) and high significance (P <0.001). The highest gL values recorded (250–300 mmol m–2 s–1) corresponded to Rvenation values of about 0.6 e+3 MPa s m2 kg–1, whereas the lowest gL values recorded (about 125 mmol m–2 s–1) corresponded to Rvenation values of about 0.97 e+3 MPa s m2 kg–1. In the inset of Fig. 6, the relationship is reported between
leaf and Rvenation. Because leaf was not recorded on the same leaves where Rvenation was measured (see above), the correlation between these two variables was calculated on the basis of mean values recorded for each experimental group. As a consequence, the statistical power of the correlation is low. Nonetheless, the observed leaf-to-Rvenation relationship (with r=–0.986; inset of Fig. 6) suggests that leaf was closely related to Rvenation whose changes were apparently due to the water stress applied (Table 2; Fig. 5). In agreement with many similar reports (see below), gL also resulted to be a positive, linear function of leaf bulk water potential (Fig. 7).
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Discussion |
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The aim of the present study was to check whether moderate water stress had an impact on the partitioning of Rleaf into its vascular and non-vascular components and how changes in Rvenation influenced leaf hydraulics and gas exchange. The results of this study provide evidence that: (i) water stress developing during plant active growth increased Rvenation consistently; (ii) although Rvenation represented a minor fraction of Rleaf, both in well-hydrated and water-stressed plants of sunflower, Rleaf was the result of parallel (and opposite) changes in the R of the two leaf compartments occurring in response to water stress; (iii) changes in the hydraulic efficiency of the leaf venation system were co-ordinated with leaf gas exchange rates.
Several studies have appeared in classical, as well as recent literature, reporting the effects of experimentally induced water stress on plant hydraulic architecture and gas exchange (Sperry and Pockman, 1993; Salleo et al., 2000; Trifilò et al., 2004). In most studies, water stress was applied by depriving plants of irrigation until leaf or soil reached pre-established values (Trifilò et al., 2004). In the present case, sunflower plants were grown under favourable conditions from seeding to the production of the first two leaf pairs, which took about 20 d. Then, soil was decreased by repeatedly irrigating plants with PEG solutions for 20 more days. As a consequence, the decrease in soil occurred while plants were actively growing and producing most of their leaves and the inflorescence. Because sunflower typically takes about 50–60 d to complete its life cycle from seed germination to full anthesis, the water stress period applied covered about 40–60% of the plants' life and apparently affected the differentiation and maturation of most leaves, as well as the general structure of the plant.
Osmotic stress applied to roots induced the typical symptoms reported for plants growing under arid conditions, i.e. reduced plant size as due to reduced cell expansion and smaller photosynthetic surface area. Plants growing under imposed experimental water stress, showed apparent leaf osmoregulation (more negative o with respect to controls; Table 1). This allowed PEG04 plants to increase (soil–leaf), i.e. the driving force for water uptake and vertical conduction to leaves by >50% with respect to controls. This probably helped leaves from PEG04 plants to retain approximately the same turgor as control leaves did (leaf–tlp; Table 1). In spite of nearly equal average leaf turgor pressure in PEG04 plants with respect to controls and similar Rleaf (Fig. 4A), as well as a similar Rvenation:Rleaf ratio (Fig. 4C), recorded in the two plant groups, leaf gas exchange was significantly less in the former than in the latter, as indicated by gL of PEG04 plants that was 28% lower than that of controls (Fig. 2). This seemingly paradoxical stomatal behaviour can be explained if the increase by 20% in Rvenation of PEG04 plants with respect to controls is taken into account. This was likely to be due to the smaller diameter of xylem conduits in their leaf veins. Even if only the widest conduits in the midrib were measured for their diameter in the present study, it can be assumed that changes observed in the conduit lumen at the midrib level reflect analogous changes in the diameter of conduits of higher-order veins.
In the present study, the hydraulic resistance of roots (Rroot) and stem (Rstem) or of the whole plant were not measured. Any increase of Rroot (e.g. through impairment of aquaporins) or of Rstem (due to xylem cavitation) can be expected to contribute to a reduction in leaf gas exchange rates (Sperry, 2000; Meinzer, 2002). Therefore, the possibility that the observed gL-to-Rvenation relationship might reflect the general drop in plant hydraulic conductance (Kplant), as the result of the water stress applied, cannot be ruled out. Some insight can be gained by a rough estimate of Kplant based on the ratio of leaf-level transpiration rate (data not shown) and the water potential drop between soil and leaves as calculated on the basis of data reported in Table 1. This simple exercise suggests that Kplant was in the order of 12 mmol m–2 s–1 MPa–1 in controls and dropped to about 5 and 4.5 mmol m–2 s–1 MPa–1 in PEG04 and PEG08 plants, respectively. Although this calculation does not take into proper account the whole plant transpiration rate, it nonetheless suggests that the overall Kplant was probably affected by the water stress treatment.
Much to the authors' surprise, Rleaf of PEG08 plants was found to be significantly lower (by 25%) than that recorded for leaves of PEG04 and control plants, while their gL was about 40% less than that of controls (although only 17% less than that of leaves from PEG04 plants). Rvenation was nearly the same for leaves from PEG04 and PEG08 plants but the fractional amount of Rvenation was as high as 39% of the total in leaves from PEG08 plants versus only about 20% in controls. The substantially lower Rleaf recorded in PEG08 leaves might at first sight be interpreted as the result of some cellular death (Gascò et al., 2004), but this was not the case, as indicated not only by the general appearance of these leaves but also by some electrolyte leakage tests (not reported in this study). On the contrary, the lower Rleaf of PEG08 plants strongly suggests that some mechanisms were adopted by plants to favour leaf hydraulics when under severe water stress developing over the middle term. Because Rvenation was nearly the same for all water-stressed plants, it appears that the general reduction of Rleaf recorded in PEG08 leaves was entirely due to the strong reduction in the resistance of the non-vascular leaf compartment (if the vascular and the non-vascular compartments in a leaf are assumed to be arranged in series with each other; Cochard et al., 2004). The present understanding of leaf hydraulics is that the hydraulic resistance of the extra-vascular water pathway is about of the same order of magnitude as that of the vascular compartment, the former or the latter prevailing according to species-specific features (Salleo et al., 2003; Sack et al., 2005) and environmental factors (Gascò et al., 2004; Nardini et al., 2005). At present, it is not possible to discriminate between apoplastic and symplastic water pathways within the leaf extra-vascular compartment. Many years ago, Cruiziat et al. (1980) tried to get such information by comparing the time-course of rehydration of sunflower leaves previously dehydrated in a pressure chamber, with an ideal apoplastic or a symplastic hydraulic model. Experimental data fitted equally well with the two models indicating that both water pathways could be followed by water in the leaf extra-vascular compartment. It is also known that up-regulation or new expression of aquaporins can decrease leaf hydraulic resistance of sunflower during the day by about 30% (Nardini et al., 2005). Therefore, it is conceivable that the reduced Rleaf recorded in PEG08 plants was at least partly due to reduced cell membrane resistance. The possible involvement of aquaporins in the observed reduction of Rleaf in plants growing under severe water stress conditions deserves, in our opinion, further detailed studies.
Leaf conductance to water vapour resulted in being a linear negative function of both Rvenation (Fig. 6) and leaf (Fig. 7). In turn, leaf was related to Rvenation (inset of Fig. 6). These correlations do not necessarily indicate causal relationships between the above variables. According to Tyree and Hammel (1972) and Tyree and Zimmermann (2002), leaf, when measured using the pressure chamber, is a measure of the xylem pressure (Px) inside leaf veins. If this is the case, it can be concluded that stomata responded to changes in Px. In turn, Px is a function of vein hydraulic resistance according to the Ohm's law analogue. If stomata respond to Px and Px is partly determined by Rvenation, then gL can be expected to be closely related to Rvenation, as shown in Fig. 6. In conclusion, vein hydraulics and leaf gas exchange would be co-ordinated through changes in Px inside leaf veins. This interpretation of the present data would also provide an explanation for the responsiveness of stomata to leaf values approaching the vein cavitation threshold (Salleo et al., 2001; Lo Gullo et al., 2003; Trifilò et al., 2003). Clearly, the relationships between leaf hydraulic construction, plant water status, and gas exchange capacity still await better elucidation and deserve more extensive studies.
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Acknowledgements |
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We are grateful to Antonio Gascò and Emmanuelle Gortan for technical assistance.
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References |
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