Journal of Experimental Botany, Vol. 54, No. 391, pp. 2323-2330,
October 1, 2003
© 2003 Oxford University Press
Kinetics of recovery of leaf hydraulic conductance and vein functionality from cavitation-induced embolism in sunflower
Received 7 March 2003; Accepted 8 July 2003
1 Dipartimento di Biologia, Università di Trieste, Via L. Giorgieri 10, 34127 Trieste, Italia
2 UD Anatomía, Fisiología y Genética Vegetal, ETSI Montes, Ciudad Universitaria s/n, 28040 Madrid, España
* To whom correspondence should be addressed. Fax: +39 040 568855. E-mail: salleo{at}univ.trieste.it
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The kinetics of leaf vein recovery from cavitation-induced embolism was studied in plants of sunflower cv. Margot, together with the impact of vein embolism on the overall leaf hydraulic conductance (Kleaf). During the air-dehydration of leaves to leaf water potentials (
L) of –1.25 MPa, Kleaf was found to decrease by about 46% with respect to values recorded in well-hydrated leaves. When leaves, previously dehydrated to L= –1.1 MPa (corresponding to the turgor loss point), were put in contact with water, Kleaf recovered completely in 10 min and so did leaf water potential. Functional vein density was estimated in both dehydrating and rehydrating leaves in terms of total length of red-stained veins infiltrated with a Phloxine B solution per unit leaf surface area. Veins were found to embolize (unstained) with kinetics showing a linear relationship with Kleaf so that about a 70% loss of functional veins corresponded with a Kleaf loss of 46%. Cavitated veins recovered from embolism within 10 min from the beginning of leaf rehydration. These data indicate that: (a) leaves of sunflower underwent substiantial vein embolism during dehydration; (b) vein embolism and leaf hydraulic efficiency apparently recovered from dehydration completely and rapidly upon rehydration; (c) vein refilling occurred while conduits were still at more negative xylem pressures than those required for spontaneous bubble dissolution on the basis of Henry’s law. The possible consistent contribution of vital mechanisms for vein refilling is discussed. Key words: Helianthus annuus, hydraulic conductance, leaf, refilling, sunflower, vein embolism, xylem pressure.
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The hydraulic conductance (K) of plant organs is widely recognized to be very variable, not only among species (Tyree et al., 1998; Comstock, 2000; Brodribb et al., 2002) but also in one species when growing in different environments (Nardini et al., 2000; Mencuccini, 2003) and in one plant on a seasonal and diurnal basis (Henzler et al., 1999; Nardini and Pitt, 1999; Tsuda and Tyree, 2000) as well as a consequence of different biotic and abiotic stresses (for a detailed review, see Tyree and Zimmermann, 2002). Xylem dysfunction as a result of cavitation-induced embolism is one of the factors determining the large changes in K recorded in stems (Cochard et al., 1992; Jaquish and Ewers, 2001) and roots (Sperry and Ikeda, 1997; Linton and Nobel, 1999) and, ultimately, in whole plants (Magnani and Borghetti, 1995; Brodribb and Hill, 2000) under water stress. The ‘safety margins’ at which plants operate in the field are such that xylem cavitation is likely to be a common occurrence in a plant’s life (Tyree and Sperry, 1988; Nardini and Salleo, 2000). This implies two operational possibilities for plants to counteract this potentially lethal phenomenon, i.e. embolism repair and cavitation avoidance. Embolism repair has been reported to occur in several species, developing root pressure just prior to sprouting (Sperry et al., 1987; Ewers et al., 2001; Améglio et al., 2002) or at low transpiration (Salleo et al., 1996; Zwieniecki and Holbrook, 1998; Holbrook et al., 2001). Yang and Tyree (1992) have convincingly shown that, according to Henry’s law, previously embolized stems of maple required PX > –2
/r (where PX is xylem pressure, is the surface tension of water in embolized vessels and r their radius) for xylem K to be restored via gas bubble dissolution. An interesting anomaly is represented by stems of Laurus nobilis L. where embolism repair has been reported by Salleo et al. (1996) and by Tyree et al. (1999) to occur within 20 min from plant rehydration and at a PX of about –1.0 MPa, i.e. well below the PX expected. More recently, Hacke and Sperry (2003) have argued that because embolism of L. nobilis was induced through air injection of stems, the vessel-refilling rate might be overestimated with respect to what happens in droughted plants. These authors found that stems of L. nobilis refilled while still at PX near –0.3 MPa which is below the PX threshold predicted by Henry’s law for gas dissolution. The time required for refilling, however, was 24–48 h after plants were rehydrated. Zwieniecki and Holbrook (2000) proposed a nice interpretation of the possible role played by bordered pits in hydraulically isolating embolized conduits, thus allowing them to refill with water that would otherwise be preferentially diverted towards the surrounding functioning conduits where PXs are more negative.
Recently, leaves of several species have been shown to undergo vein cavitation at leaf water potentials (L) between –0.25 MPa (in Ilex aquifolium L.) and –1.5 MPa (in Ceratonia siliqua L.; Kikuta et al., 1997). In some cases, PCAV (i.e. the xylem pressure at which cavitation is triggered) for leaf veins was less negative than that for stems (Salleo et al., 2000, 2001), thus suggesting that leaves might cavitate earlier than stems in the field. Evidence for leaf vein embolism has been provided on the basis of counting of ultrasound acoustic emissions from the leaf blade (Kikuta et al., 1997), from infiltrating leaves with fluorescein (Salleo et al., 2001) and Phloxine B (Nardini et al., 2003) and from hydraulic measurements (Linton and Nobel, 2001; Cochard, 2002; Lo Gullo et al., 2003). The impact of leaf vein embolism on the overall leaf hydraulic conductance (Kleaf) appears to be very variable among species. Very low impact of vein embolism on Kleaf (less than 8% for leaves at PX= –1.2 MPa) has been reported for Prunus laurocerasus L. by Nardini et al. (2001), but a more than 40% drop in Kleaf with concurrent vein embolism has been reported to occur in leaves of C. siliqua (Lo Gullo et al., 2003) and Cercis siliquastrum L. (Nardini et al., 2003).
If leaf veins undergo large embolization as reported by Salleo et al. (2001) for C. siliqua and L. nobilis, where an estimated loss of functional veins was measured as 29% and 35%, respectively, it seems likely that embolism is repaired over the short term (e.g. during the night) because the venous network is generally recognized to be an important (although not the sole, Tyree et al., 1981; Hüve et al., 2002; Nardini and Salleo, 2003; Sack et al., 2003) pathway for water distribution across the leaf blade (Roth-Nebelsick et al., 2001; Zwieniecki et al., 2002). Canny (1997), using the Cryo-SEM method for detecting vessel embolism, has reported that embolized conduits of sunflower petioles refilled during active transpiration at substantially negative L values. Cochard et al. (2000) and Richter (2001), however, have argued that spurious vessel embolism is likely to be produced when samples are frozen while under tension. Thus, the question of refilling of embolized conduits in the leaf is still largely unresolved. As far as is known, there have been no studies in the literature describing the time-course of vein refilling in rehydrating leaves or the relationship eventually existing between vein embolism repair and the restoration of Kleaf. Because leaves are one of the sites where the major hydraulic resistance (R=1/K) resides (Yang and Tyree, 1994; Nardini and Tyree, 1999) in a plant (the other one is the radial soil–root pathway, Frensch and Steudle, 1989; Nardini et al., 2002), the drop in Kleaf and its prompt restoration may have a significant impact on stomatal behaviour and, hence, on plant growth and productivity. In the present study, measurements are reported of leaf hydraulic conductance, water potential and functional vein density during dehydration and rehydration of leaves of Helianthus annuus L. with the aim of investigating (a) the extent and the kinetics of vein refilling and (b) the significance of changes in the functional integrity of the leaf venous system in determining changes in the overall leaf hydraulics.
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Plant material and growth conditions
All experiments were conducted on 70 plants of Helianthus annuus L. cv. Margot at 6–9-weeks-old. This cultivar is described by the factory (Maisadour SA, France) as adapted to water stress developing during the early period from emergence (seeds were provided by the courtesy of Maisadour Sementi Italia SpA). Seeds were planted in 3.0 l pots filled with a mixture (1:1, v:v) of peat and sand (one seed per pot). Ten days after germination, each plant received 7.5 g of fertilizer (Nitrophoska Top, BASF Italia SpA, 15% N, 10% P2O5, 15% K2O, 2% MgO, 12% SO3, 0.02% B, 0.01% Zn). Plants were grown in a room where air temperature was adjusted to vary between 25 °C and 16 °C (during day and night, respectively), relative humidity was set at 50±5% and light was provided by lamps (HQI-T 1000 W/D, Osram) with a Photosynthetically Active Radiation (PAR) of 250±25 µmol m–2 s–1. The photoperiod was set at 12 h (lights were turned on at 06.00 h and turned off at 18.00 h). Plants were well irrigated and each plant received about 200 ml of water every 2 d. Plants used for experiments were 0.95±0.26 m tall, with a stem diameter of 10.2±0.7 mm as measured 30 mm above the soil level and were bearing six to ten pairs of leaves.
Estimating leaf vein embolism and leaf hydraulic conductance
Preliminary measurements were made to determine the leaf turgor loss point (TLP) in order to pre-establish a reference dehydration level in terms of decreasing L values. At least seven pressure–volume curves were made of mature leaves from different plants using a pressure chamber (Tyree and Hammel, 1972; Salleo, 1983). TLP turned out to be –1.02±0.12 MPa. Leaf vein embolism and leaf hydraulic conductance were measured (see below) of both leaves from well-hydrated plants and of leaves progressively dehydrated to induce vein cavitation. Plants were irrigated and covered with black plastic bags the evening before the experiments; under these conditions, the water potential (L) of leaves as measured using a pressure chamber (Soilmoisture mod. 3005), was about –0.15 MPa. After measuring the base L, plants were cut near their base and air-dehydrated until L reached pre-established decreasing values up to the turgor loss point (see below). One to two leaves per plant were measured for the amount of vein embolism as well as for their hydraulic conductance.
Whole-leaf hydraulic conductance (Kleaf) was measured using the vacuum chamber technique introduced by Kolb et al. (1996) for stems and first used for leaf blades by Nardini et al. (2001). Further validation of the vacuum chamber technique for measuring Kleaf can be found in Sack et al. (2002), Lo Gullo et al. (2003) and Nardini et al. (2003). The vacuum chamber was an 8.0 l PVC flask. The petiole was connected to rigid PEEK tubing using a 5 mm length of 1.5 mm i.d. Tygon tubing. The PEEK tubing passed through the rubber seal of the vacuum flask to a beaker of solution (50 mM KCl) resting on a digital balance (mod. AE220, Sartorius, Goettingen, Germany; accuracy ±0.1 mg). A vacuum pump was used to reduce the pressure in the flask in steps of 20 kPa and at each pressure a computer recorded the weight of the beaker on the digital balance at 20 s intervals to compute flow. All flow readings were made at a temperature of 22±1 °C. At least 10 flow readings were made at each pressure, ranging from atmospheric pressure to depressions of 20 kPa each in four steps starting from 80 kPa below atmospheric pressure and continuing at 60, 40 and 20 kPa. Volume flow rates were recorded until the flow became stable (i.e. the SD of the mean of the last 10 readings was less than 3% of the mean). The flows (F) were plotted versus the absolute pressures applied (P) and Kleaf was computed from the slope of the F-to-P linear relationship. Kleaf was scaled by leaf surface area (AL) as measured at the end of experiments using a leaf area meter (LI-3000A, Li-Cor Inc., Lincoln, Nebraska, USA). All Kleaf measurements were performed at ambient irradiance (PAR <10 µm m–2 s–1). Preliminary experiments were performed consisting of dehydrating leaves to the turgor loss point (L= –1.1 MPa, n=7) and, after measuring Kleaf, immediately measuring L. This procedure was aimed at testing whether rehydration of the samples occurred during Kleaf measurements using the vacuum chamber.
In order to check the amount of vein embolism induced by leaf dehydration, leaves were infiltrated with Phloxine B (a dye commonly used in medical cytology as a substitute for Eosin Y) which has been successfully used to stain the vein network of both fresh and water-stressed leaves (Nardini et al., 2003; Raimondo et al., 2003). After measuring Kleaf, the KCl solution was replaced by a Phloxine B (2% w/v) dissolved in a 50 mM KCl solution. Pressure in the vacuum chamber was decreased to 80 kPa below atmospheric and leaves were infiltrated with the dye for 20 min. Preliminary experiments had shown that this time was sufficient to infiltrate veins of well-hydrated leaves of sunflower completely, without any visible interruption even in the minor veins, as revealed by leaf areas with non-stained veins. After dye perfusion, leaf images were acquired with a scanner connected to a computer. Three leaf regions with a surface area of 400 mm2 each were selected from the middle part of the leaf blade (at both sides of the midrib) and the density of the functional (red) veins was recorded in terms of the total length of stained veins divided by the surface area of the leaf sample (one side only), thus obtaining the total functional vein length per unit leaf surface area (mm mm–2). Images were processed using SigmaScan Pro 5.0 (SPSS Science, Chicago, IL, USA). Only veins higher than the third order were measured, as veins of lower orders are known to contain many xylem vessels. Because not all vessels embolize under water-stress conditions, even a few functioning vessels would result in the major veins being stained, and this might lead to vein embolism being consistently underestimated.
Because Kleaf of air-dehydrated leaves was found to decrease with respect to controls (see below), a lower amount of Phloxine B solution can be expected to enter dehydrated samples with respect to controls at equal perfusion times. This might lead to an overestimation of the amount of vein embolism in the leaf, simply because still functional vessels might not yet have been reached by the dye. Therefore, leaves dehydrated to L= –1.1 MPa (n=7) were infiltrated with the Phloxine B solution until the volume of solution that entered the leaf equalled that previously recorded for control leaves. This, turned out to correspond to about 0.5 g of solution per leaf and required up to 45 min infiltration.
Measuring vein refilling and Kleaf recovery
In order to check whether and to what extent leaf vein embolism and loss of Kleaf (see below) were reversible upon leaf rehydration, the kinetics of vein refilling and Kleaf recovery were measured in leaves previously dehydrated to L= –1.1 MPa. In fact, experiments had shown that this water stress level induced a marked drop both in Kleaf and in functional vein density. Plants were first cut near their base and then air-dehydrated as described above. Two leaves per plant were cut under 50 mM KCl solution and maintained in the dark with their petiole in contact with the KCl solution for 2.5, 5, 10, 15, and 90 min. Such short rehydration times were established on the basis of empirical observations, during which sunflower leaves dehydrated to apparent wilting were found to recover their turgor in a matter of a few minutes. After leaves were rehydrated for the pre-established time interval, one leaf was measured for L while the other one was first measured for Kleaf in the vacuum chamber, then infiltrated with the Phloxine B solution as described above and measured for the number of red stained veins per unit leaf surface area (see above).
Statistics
Data were analysed with the SigmaStat 2.0 (SPSS, Inc.) statistics package. One-way-ANOVA was used to test differences between experimental groups. Post hoc all pairwise comparisons between all means were made using Tukey’s test. Regressions between variables were tested for statistical significance using the Pearson Product Moment Correlation.
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The hydraulic efficiency of sunflower leaves of cv. Margot appeared to be sensitive to dehydration. When leaves were dehydrated beyond their average turgor loss point, Kleaf decreased from about 28.0 to about 15.0x10–5 kg s–1 m–2 MPa–1, i.e. by about 46% with respect to controls (leaves at
L= –0.15 MPa, Fig. 1) with a positive linear relationship between the two variables and high statistical significance (P <0.0001). During rehydration, leaf water potential increased very quickly (Fig. 2B). Only 2.5 min after leaves were put in contact with the KCl solution, L was raised from –1.1 to –0.9 MPa and further to –0.4 MPa, 5 min later. Ten minutes after beginning rehydration, L was about –0.25 MPa which was no longer statistically different from that measured in controls (leaves prior to dehydration). The even less negative L recorded 90 min after leaves were put in contact with water (less than –0.05 MPa) was likely to be due to leaf oversaturation with water.
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The time-course of Kleaf during rehydration (Fig. 2A) showed the tendency of Kleaf to increase, 5 min after beginning rehydration (Kleaf increased from 18 to about 28x10–5 kg s–1 m–2 MPa–1). This new Kleaf value was intermediate (and not statistically different) between the Kleaf value recorded for dehydrated leaves (about 15x10–5 kg s–1 m–2 MPa–1) and that measured 10 min after beginning rehydration. Ten minutes after leaves were put in contact with water, the recovery of Kleaf was completed. It has to be noted that
L measured at the end of vacuum chamber experiments turned out to recover, completely, while Kleaf and functional vein density (see below) were not yet restored.
Infiltration of leaves with Phloxine B gave similar results in terms of functional vein density, both when leaves were perfused with the red dye for the pre-established time of 20 min and when they received equal amounts of Phloxine B solution (0.482 g per leaf, Fig. 3). No statistically significant differences were found to exist between the two perfusion methods both for controls and for leaves dehydrated to L= –1.1 MPa, thus indicating that the decreased Kleaf (Fig. 1) did not cause any underestimation of the number of red-stained veins. Functional vein density expressed as mm of stained veins mm–2 of leaf surface area (Fig. 4), turned out to be about 1.1 mm mm–2 in leaves dehydrated to L= –1.1 MPa versus about 1.9 mm mm–2 measured for controls, i.e. it was reduced by about 42% due to leaf dehydration and remained approximately constant for the first 5 min after beginning leaf rehydration. Five minutes later, however, all veins of dehydrated leaves appeared to be red stained like those of controls, i.e. the functional integrity of the venous system was completely restored. It is worth noting that at this time (10 min after leaves began to rehydrate), L had completely recovered as well (Fig. 2B), but 5 min earlier, the functional vein density had not yet begun to recover although L had recovered consistently (L was at this time about –0.4 MPa versus –1.1 MPa as measured in leaves prior to rehydration).
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Figure 5 shows the relationship between Kleaf and the functional vein density recorded both during leaf dehydration to below the turgor loss point (i.e. to
L= –1.25 MPa, solid circles) and during rehydration (open circles). A positive linear relationship appeared to exist between the two variables in both cases with a combined correlation coefficient (r2) of 0.54 for the Kleaf-to-functional vein density relationship and high statistical significance (P <0.0001).
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Two main facts appear to emerge from these data. The first one is that the recovery from dehydration was, in leaves of sunflower, surprisingly rapid. In fact, only 10 min after leaves were put in contact with water,
L, Kleaf and the functionality of the leaf venous system were completely restored (Figs 2, 4). The second conclusion is that vein refilling occurred while the average PX was substantially negative (Fig. 2) and far more negative than the PX required for gas bubble dissolution on the basis of the predictions of Henry’s law (see above). The vein conduit diameters were not measured, but if the measurements of vein conduit diameters reported by Canny (1990) for sunflower are taken as more or less similar to sunflower cv. Margot, it can be assumed that conduit radii (r) ranged between 15 µm for conduits of the midrib and major veins to about 4 µm for veins of the fifth order. At a temperature of 22 °C, the pressures required for gas dissolution on the basis of Henry’s equation should be higher than –0.01 and –0.036 MPa for major and fifth order veins, respectively. In this case, veins of sunflower were shown to refill completely at L of about –0.25 MPa. Theoretically, L when measured using the pressure chamber is assumed to be a measure of the average water status within the leaf conduits (Scholander et al., 1964; Tyree and Hammel, 1972) to which surrounding cells would equilibrate. If this is the case, then the PX at which vein refilling was completed in sunflower, was 7–25-fold more negative than that required for spontaneous bubble dissolution. It has to be pointed out, however, that pressure chamber-measured L is unable to resolve possible local changes in PX such as the positive PX that might develop within xylem conduits during refilling and/or in their nearest surroundings.
It is worth noting that in these experiments Kleaf was measured by applying a pressure of –80, –60, –40, and –20 kPa relative to atmospheric pressure. Under such conditions, leaves apparently recovered their turgor completely (as shown by L values which were close to zero at the end of measurements), but neither Kleaf nor functional vein density did recover. In the authors’ opinion, this was the probable result of two facts: (a) during measurements leaves were, on average, under pressures more negative than those required for spontaneous bubble dissolution on the basis of Henry’s law; and (b) bubble expansion under below-atmospheric pressures might inhibit the probable vital mechanisms (De Boer and Volkov, 2003) favouring refilling, as observed in leaves rehydrating at atmospheric pressure. This further validates the vacuum chamber technique as a suitable tool for measuring the cavitation-induced loss of conductance of whole leaves.
Studies by Zwieniecki and Holbrook (2000) have shown that the hydraulic isolation of conduits during refilling is possible and is such that it can sustain the maximum pressure difference that can develop between water in a refilling conduit and gas trapped within the bordered pit chamber without any expansion of water through the pits. This pressure would be as large as between 0.07 and 0.30 MPa. Recently, Vesala et al. (2003) have developed a model showing that the refilling process is physically possible in hydraulically isolated conduits even in the presence of surrounding conduits under negative pressure. This, however, would require osmotic potentials (of the order of –1 MPa) of xylem sap in refilling conduits much larger than those estimated by Tyree et al. (1999) for refilling times of the order of 20 min as reported by Salleo et al. (1996). According to the model, less negative osmotic potentials (about –0.4 MPa) may also act as a driving force for refilling, but, in this case, the refilling time would be of the order of 10 h. This finding by Vesala et al. (2003) is in agreement with Hacke and Sperry (2003) who found similar refilling times (10–48 h) for L. nobilis stems (see above). Vesala and co-workers, however, purposely excluded from their model any contribution of eventual mechanisms of active solute loading into cavitated conduits. De Boer and Volkov (2003) in a wide review of the ‘logistics’ of water and solute transport in plants, attribute great importance to outward rectifying K+ channels and to H+-ATPase located in vessel-associated cells for facilitating solute loading into vessels, as well as to non-symmetrical distribution of aquaporins for determining vectorial cell-to-vessel water flows. All these mechanisms may help explain the short refilling time measured in this work in sunflower leaves. Although leaves rehydrated under very favourable conditions (the cut surface of petioles was directly in contact with a dilute KCl solution), their refilling time was surprisingly shorter than expected although in agreement with simple empirical observations of leaf turgor recovery in this species (see above).
The Kleaf-to-L relationship showed a 46% loss of conductance at below TLP (Fig. 1). In a previous work (Trifilò et al., 2003), a lower loss of conductance was recorded in a different cultivar of sunflower (about 20%) at the same L (about –1.3 MPa). Although the cultivar used by Trifilò and co-workers in their study was less genetically fixed than that used in the present study, so that the experimental points were rather scattered, thus giving rise to low correlation coefficient between Kleaf and L, the higher loss of conductance measured in the present study seems to suggest that leaf vulnerability to cavitation may be substantially different among different ecotypes or cultivated varieties. Of great interest is that the correlation coefficient calculated for the Kleaf-to-functional vein density relationship was rather high (r2=0.54) for both rehydration and dehydration (Fig. 5). This strongly suggests that leaf hydraulic conductance is related to the functional integrity of the leaf venous system. This is somewhat surprising. In fact, previous studies (Tyree and Cheung, 1977; Tyree et al., 2001) have led to the conclusion that the hydraulic resistance of the non-vascular leaf compartment accounts for the largest fraction of the total leaf hydraulic resistance (Rleaf=1/Kleaf). In particular, the hydraulic resistance of the symplast (Rsymplast) of sunflower leaves (Trifilò et al., 2003) has been estimated on the basis of both ethanol immersion of the leaf and of cross-sectioning the leaf blade between major veins (Tyree et al., 2001). Under the former conditions, membranes of cells are destroyed, while under the latter most non-vascular leaf compartments are bypassed. In both cases, the resistance of the non-vascular compartment represented about 78% of the total. Similar fractions of Rleaf have been measured in other species (Tyree et al., 2001; Salleo et al., 2003; A Nardini and S Salleo, unpublished data). In this case, Kleaf should be not correlated or only weakly correlated to the functional vein density. In the present study, the correlation of Kleaf to functional vein density was not weak. This indicates that about one-half of Kleaf of sunflower leaves is influenced by cavitation-induced vein embolism. On the other hand, it can be noted that over the L range explored (i.e. between –0.15 and –1.27 MPa), the total length of functional veins per unit leaf surface area decreased from 2.7 to 0.6 mm mm–2 (Fig. 5), i.e. by about 78%, thus suggesting that substantial vein embolization had taken place. Within the same L range, Kleaf decreased from 28 to 15x10–5 kg s–1 m–2 MPa–1, i.e. by 46%. In other words, Kleaf decreased less than proportionally to the decrease in the amount of functional veins. Recently, Meinzer (2002) reported a hydraulic model computing how much the loss of stem conductivity can be expected to influence the relative total hydraulic resistance of the shoot. He convincingly showed that for a Rleaf/Rshoot ratio of 0.7 (i.e. for the leaf resistance representing 70% of the total shoot resistance), a loss of stem conductivity of 70% would cause the total plant resistance to increase by only 24%. A similar model may apply to sunflower leaves where Rsymplast/Rleaf has been found to be even higher than 0.7. In other words, the lower hydraulic conductance (or the higher resistance) of the non-vascular leaf compartment with respect to the vascular one may represent an effective hydraulic bottleneck in a leaf blade that would consistently buffer the impact of vein cavitation on Kleaf.
It has to be taken into account that dehydration may intrinsically affect water transport in the leaf through changes in wall properties such as, for instance, the hydrogel status of pectins (Zwieniecki et al., 2001). Moreover, membrane water permeability is known to be a function of aquaporin expression and functionality which might be affected by leaf dehydration. All these factors, together with possible changes in sap pH and ionic composition during dehydration, might provide an alternative explanation for Kleaf changes during dehydration and need to be further investigated in order to validate the results presented in this study.
In conclusion, leaves of sunflower were shown to undergo substantial vein embolism during dehydration that was partially buffered by the low conductance of the non-vascular compartment, so that Kleaf varied less than proportionally with respect to the loss of functional veins. Moreover, leaf hydraulic efficiency apparently recovered from dehydration completely and rapidly and so did the cavitated xylem conduits. Very little is known of the mechanism(s) involved in vessel refilling, but it is thought that the mere physical analysis of the process may be insufficient to explain the refilling rate observed in this and other studies. The correct interpretation of the refilling process probably does not need any diverse paradigm(s) (Tyree et al., 1999) or ‘miracles’ (Holbrook and Zwieniecki, 1999), but only a better understanding of the biological transport systems operating at the interface between xylem conduits and associated cells (De Boer and Volkov, 2003).
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Acknowledgements |
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This study was funded by the University of Trieste (Finanziamenti di Ateneo per Progetti di Ricerca). The visit by A Gascó to the Department of Biology, University of Trieste, was made possible by a grant from Ministerio de Educación, Cultura y Deporte de España. We are very grateful to Maisadour Sementi Italia SpA, for providing seeds.
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References |
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