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Summary Drought-induced xylem embolism is a serious threat to plant survival under future climate scenarios. Hence, accurate quantification of species-specific vulnerability to xylem embolism is a key to predict the impact of climate change on vegetation. Low-cost hydraulic measurements of embolism rate have been suggested to be prone to artefacts, thus requiring validation by direct visualization of the functional status of xylem conduits using nondestructive imaging techniques, such as X-ray microtomography (microCT). We measured the percentage loss of conductance (PLC) of excised stems of Laurus nobilis (laurel) dehydrated to different xylem pressures, and compared results with direct observation of gas-filled vs water-filled conduits at a synchrotron-based microCT facility using a phase contrast imaging modality. Theoretical PLC calculated on the basis of microCT observations in stems of laurel dehydrated to different xylem pressures overall were in agreement with hydraulic measurements, revealing that this species suffers a 50% loss of xylem hydraulic conductance at xylem pressures averaging −3.5 MPa. Our data support the validity of estimates of xylem vulnerability to embolism based on classical hydraulic techniques. We discuss possible causes of discrepancies between data gathered in this study and those of recent independent reports on laurel hydraulics.

This study reports experimental evidence on the effect of short term potassium fertilization on potassium uptake, tissue concentration and hydraulic conductance of pot-grown laurel plants. Potassium uptake and loading into the xylem of laurel seedlings increased within 24 h from fertilization. Potassium was not accumulated in roots and leaves, but the [K+] of xylem sap was 80% higher in fertilized plants (+K) than in potassium starved plants (-K), likely as a result of recirculation between xylem and phloem. This increase in xylem sap [K+] resulted in a 45% increase in transpiration rate, a 30% increase in plant hydraulic conductance (Kplant) and a 120% increase in leaf specific conductivity of the shoot (kshoot ). We suggest that this increase was due to ion-mediated up-regulation of xylem hydraulic conductance, possibly caused by the interaction of potassium ions with the pectic matrix of intervessel pits. This enhancement of hydraulic conductance following short-term potassium fertilization is a phenomenon that can be of advantage to plants for maintaining cell turgor, stomatal aperture and gas exchange rates under moderate drought stress and these data provide additional support to the important role of potassium nutrition in agriculture and forestry.

The ion-mediated enhancement of xylem hydraulic conductivity in angiosperms is thought to be controlled by the pectin chemistry of intervessel pit membranes. However, there is little or no direct evidence on the ultrastructure and chemical nature of pit membranes in species that show an ‘ionic effect’. The potential link between the magnitude of the ionic effect and pectin composition in intervessel pit membranes of four Lauraceae species (Laurus nobilis, Lindera megaphylla, Litsea sericea and Umbellularia californica) that show rather similar vessel and pit dimensions was studied using transmission electron microscopy (TEM). The TEM observations confirmed the presence of a pectic matrix associated with intervessel pit mem branes, indicating that the relative abundance of acidic versus methylesterified pectins was closely related to the ionic effect. The two species examined with a high ionic effect (~20%, i.e. Laurus nobilis and Umbellularia californica) showed relatively high levels of acidic pectins, whereas methylesterified pectins were abundant in Lindera megaphylla and Litsea sericea, which showed a low ionic effect (~10%). Variation in the ionic effect is strongly associated with the chemical nature of pit membrane pectins in the species studied. Our findings support the current interpretation of the ionic effect due to dynamic swelling and shrinking behaviour of pit membrane pectins.

Permanent xylem blockage is a common result of attacks by herbivores and fungi. The mitosporic fungus Phoma tracheiphila (Petri) Kantschaveli et Gikachvili, is the agent of a Citrus tracheomycosis (“malsecco disease”) causing xylem impairment and leading to leaf shedding and plant dieback. In the present study, this pathogen was used for monitoring the effects of increasing levels of stem hydraulic resistance (R stem) on leaf water status and gas exchange. In this view, measurements are reported of changes in the hydraulic resistance of infected stems (R stem) of C. aurantium (sour orange) during progressive and irreversible xylem blockage with parallel measurements of leaf water potential and conductance to water vapour. Leaves were highly responsive to increasing R stem as due to fungal infection, with substantial stomatal closure and drop in water potential.

Leaf hydraulic conductance (K leaf ) is known to be an important determinant of plant gas exchange and photosynthesis. Little is known about the long-term impact of different environmental factors on the hydraulic construction of leaves and its eventual consequences on leaf gas exchange. In this study, we investigate the impact of soil water availability on K leaf of Fraxinus ornus L. as well as the influence of K leaf on gas exchange rates and plant water status. With this aim, K leaf , leaf conductance to water vapour (g L ), leaf water potential (Ψ leaf ) and leaf mass per area (LMA) were measured in F. ornus trees, growing in 21 different sites with contrasting water availability. Plants growing in arid sites had lower K leaf , g L and Ψ leaf than those growing in sites with higher water availability. On the contrary, LMA was similar in the two groups. The K leaf values recorded in sites with two different levels of soil water availability were constantly different from each other regardless of the amount of precipitation recorded over 20 days before measurements. Moreover, K leaf was correlated with g L values. Our data suggest that down-regulation of K leaf is a component of adaptation of plants to drought-prone habitats. Low K leaf implies reduced gas exchange which may, in turn, influence the climatic conditions on a local/regional scale. It is concluded that leaf hydraulics and its changes in response to resource availability should receive greater attention in studies aimed at modelling biosphere-atmosphere interactions.

Xylem hydraulic conductivity (K s) in stems of tobacco (Nicotiana tabacum) wild-type SR1 was compared to that of PG7 and PG16, two transgenic lines with increased levels of expression of the gene encoding the Aspergillus niger endopolygalacturonase (AnPGII). Activity of AnPGII removes in planta blocks of homogalacturonan (HG) with deesterified carboxyls, thus increasing the degree of neutrality of pectins. The effect of K+ was tested in increasing stem K s using model plants with more neutral polysaccharides in primary walls and, hence, in intervessel pit membranes. K s measured with deionized water was compared to that with KCl solutions at increasing concentrations (ΔK s, %). Plants transformed for HG degree of neutrality showed a dwarfed phenotype, but ΔK s did not differ among the three experimental groups. The ion-mediated hydraulic effect saturated at a KCl concentration of 25 mm in SR1 plants. All the three tobacco lines showed ΔK s of around +12.5% and +17.0% when perfused with 10 and 25 mm KCl, respectively. Because modification of HG content did not influence ion-mediated hydraulic enhancement, we suggest that pectin components other than HG, like rhamnogalacturonan-I and/or rhamnogalacturonan-II, might play important roles in the hydrogel behavior of pit membranes.

The mining of leaves of Aesculus hippocastanum caused by the larvae of Cameraria ohridella leads to precocious defoliation of trees. Damage to plant productivity was estimated in terms of the photosynthetic performance as well as of leaf water relations and hydraulics of increasingly mined leaves from infested plants in comparison with the same variables measured in non-mined leaves (controls). Electron microscopy and photosynthesis measurements revealed that chloroplasts within the green portions of mined leaves were still intact and photosynthesis of these areas was close to that of non-mined leaves, i.e. damage to functional integrity of the photosynthetic system did not extend beyond the mines. Stomata below the mines were functional as they maintained their physiological kinetics but most chloroplasts in the spongy parenchyma below the mines were degraded so that a 1:1 relationship existed between photosynthesis loss and loss of leaf green areas. Leaf conductance to water vapour and transpiration rate were 60% lower in mined leaf areas but equal to controls in green portions of mined leaves. Leaf water potential was insensitive to the amount of mined leaf area and so was leaf hydraulic conductance. Anatomical observations of leaf minor veins revealed that they were structurally and functionally intact even in leaves with 90% mined surface area. Our conclusion was that the actual damage to A. hippocastanum plants in terms of loss of photosynthates and water and nutrient transport was less than that visually estimated in recent studies.

Competition for water availability was studied in a mixed natural stand of Quercus suber L. and Quercus cerris L. growing in Sicily by measuring diurnal changes of leaf conductance to water vapour (g L ), water potential (Ψ L ) and relative water content (RWC) in April, July and October 1997 as well as the seasonal changes in root hydraulic conductance per unit leaf surface area (K RL ). Quercus cerris behaved as a drought-tolerant species, with strong reductions of K RL , Ψ L , and RWC in the summer. By contrast, Q. suber appeared to withstand summer drought by an avoidance strategy based on reducing g L , maintaining Ψ L and RWC high and K RL at the same level as that measured in the spring. A 'conductance ratio' (CR) was calculated in terms of the ratio of g L to K RL . Seasonal changes of this ratio contrasted in the two species, thus suggesting that Q. suber and Q. cerris did not really compete for available water. In the summer, when Q. suber was extracting water from the soil to maintain high leaf hydration, Q. cerris had restricted water absorption, thus suffering drought but tolerating its effects. The possibility that cohabitation of drought-tolerant with drought-avoiding species can be generalized is also discussed.

Xylem hydraulic conductance varies in response to changes in sap solute content, and in particular of potassium (K(+)) ion concentration. This phenomenon, known as the 'ionic effect', is enhanced in embolized stems, where it can compensate for cavitation-induced loss of hydraulic conductance. Previous studies have shown that in well-watered laurel plants (Laurus nobilis L.), potassium concentration of the xylem sap and plant hydraulic conductance increased 24 h after fertilization with KCl. The aim of this work was to test whether water-stressed laurel plants, grown under low potassium availability, could recover earlier from stress when irrigated with a KCl solution instead of potassium-free water. Two-year-old potted laurel seedlings were subjected to water stress by suspending irrigation until leaf conductance to water vapour (g(L)) dropped to ∼30% of its initial value and leaf water potential (ψ(L)) reached the turgor loss point (ψ(TLP)). Plants were then irrigated either with water or with 25 mM KCl and monitored for water status, gas exchange and plant hydraulics recovery at 3, 6 and 24 h after irrigation. No significant differences were found between the two experimental groups in terms of ψ(L), g(L), plant transpiration, plant hydraulic conductance or leaf-specific shoot hydraulic conductivity. Analysis of xylem sap potassium concentration showed that there were no significant differences between treatments, and potassium levels were similar to those of potassium-starved but well-watered plants. In conclusion, potassium uptake from the soil solution and/or potassium release to the xylem appeared to be impaired in water-stressed plants, at least up to 24 h after relief from water stress, so that fertilization after the onset of stress did not result in any short-term advantage for recovery from drought.

• The hydraulic conductance of angiosperm xylem has been suggested to vary with changes in sap solute concentrations because of intervessel pit properties. • The magnitude of the ‘ionic effect’ was linked with vessel and pit dimensions in 20 angiosperm species covering 13 families including six Lauraceae species. • A positive correlation was found between ionic effect and vessel grouping parameters, especially the portion of vessel walls in contact with neighbouring vessels. Species with intervessel contact fraction (FC) values < 0.1 showed an ionic effect between 2% and 17%, while species with FC values > 0.1 exhibited a response between 10% and 32%. The ionic effect increased linearly with the mean fraction of the total vessel wall area occupied by intervessel pits as well as with the intervessel contact length. However, no significant correlation occurred between the ionic effect and total intervessel pit membrane area per vessel, vessel diameter, vessel length, vessel wall area, and intervessel pit membrane thickness. • Quantitative vessel and pit characters are suggested to contribute to interspecific variation of the ionic effect, whereas chemical properties of intervessel pit membranes are likely to play an additional role.

Major restrictions to the hydraulic conductance of xylem (K(XYL)) in vascular plants have traditionally been attributed to anatomical constraints. More recently, changes in the cationic concentration of xylem sap have been suggested to be responsible for short-term changes in K(XYL) based on data for 35 dicot species, and very few gymnosperms and ferns, indicating that xylem water transport may no longer be considered as an entirely passive process. Recent studies have revealed that this so-called ionic effect: (i) varies from little or no increase to30%, (ii) is species specific, (iii) changes on a seasonal basis, (iv) depends on the cationic concentration, (v) is enhanced in embolized stems, and (vi) is positively correlated with vessel grouping. Furthermore, the ionic effect has been suggested to play functional roles in planta with respect to: (i) phloem-mediated control of xylem hydraulic properties, (ii) compensation of cavitation-induced loss of hydraulic conductance, with the result of optimizing light and water utilization, and (iii) differential regulation of water delivery to branches exposed to different levels of light. Pits are likely to play a key role in the ionic effect, which has largely been explained as a consequence of the poly-electrolytic nature and hydrogel properties of the pectic matrix of interconduit pit membranes, despite little evidence that pit membrane pectins remain present after cell hydrolysis. More research is needed to address the ionic effect in more species, physico-chemical properties of pit membranes, and how the ionic effect may increase xylem hydraulic conductance 'on demand'.

This study reports experimental evidence for light-mediated changes of stem hydraulic conductance (K(stem)) in field-grown laurel plants. Field measurements based on the evaporative flux method revealed that sun-exposed branches had 60% higher K(stem) with respect to shade-exposed branches. Xylem sap potassium concentration was approximately 3 mM as recorded in shaded branches and up to 12 mM in illuminated ones. Such a difference in [K(+)] proved to induce significant increase in xylem hydraulic conductance of excised twigs when artificially perfused with various solutions, as a likely consequence of the interaction of cations with the pectic matrix of pit membranes (the so-called 'ionic effect'). We propose that this mechanism provides plants with a large potential for fine regulation of water flow towards different parts of the canopy exposed to different environmental conditions with the result of optimizing light and water utilization.

Leaf hydraulics of Aesculus hippocastanum L. were measured over the growing season and during extensive leaf mining by the larvae of an invasive moth (Cameraria ohridella Deschka et Dimic) that specifically destroy the palisade tissue. Leaves showed seasonal changes in hydraulic resistance (R(lamina)) which were related to ontogeny. After leaf expansion was complete, the hydraulic resistance of leaves and the partitioning of resistances between vascular and extra-vascular compartments remained unchanged despite extensive disruption of the palisade by leafminers (up to 50%). This finding suggests that water flow from the petiole to the evaporation sites might not directly involve the palisade cells. The analysis of the temperature dependence of R(lamina) in terms of Q(10) revealed that at least one transmembrane step was involved in water transport outside the leaf vasculature. Anatomical analysis suggested that this symplastic step may be located at the bundle sheath where the apoplast is interrupted by hydrophobic thickening of cell walls. Our findings offer some support to the view of a compartmentalization of leaves into well-organized water pools so that the transpiration stream would involve veins, bundle sheath and spongy parenchyma, while the palisade tissue would be largely by-passed with the possible advantage of protecting cells from short-term fluctuations in water status.

Spatial heterogeneity of gas exchange rates in the leaves of Nicotiana tabacum L. (tobacco) was investigated. Leaf conductance to water vapour was higher (by about 18%) at the apical regions of leaves than at the basal ones. Local, small-scale measurements of pressure-volume (PV) parameters and water status (performed with a dewpoint hygrometer) revealed that bulk leaf water potential, osmotic potential, turgor pressure and bulk modulus of elasticity were not significantly different in the leaf apex or base. Hydraulic measurements showed that the apical regions of the leaf blade were about 30% more conductive than the basal regions. Such differences were explained by analogous differences in terms of venation patterns. In fact, vein density turned out to be higher (by about 13%) near the leaf apex with respect to the leaf base. On the contrary, stomatal density was the same both in the apical and basal leaf portions. Our data suggest that spatial stomatal heterogeneity may arise from heterogenous distribution of local hydraulic resistances and would be addressed to maintaining local water potential above critical values, possibly triggering vein cavitation.

Recent studies have shown that, in some species, xylem hydraulic conductivity (K(h)) increases with increasing cation concentration of xylem sap. Evidence indicates that K(h) increases as a result of the de-swelling of pit membrane pectins caused by cation neutralization of polygalacturonanes. We tested whether this ionic effect partly compensates for the embolism-induced loss of stem hydraulic conductivity (PLC) by increasing K(h) of functioning conduits. We report changes in PLC, leaf water status and potassium concentration ([K(+)]) of xylem sap measured in April and July in two evergreens (Ceratonia siliqua L. and Phytolacca dioica L.) and one deciduous tree (Platanus orientalis L.) growing in the field in Sicily. In summer, Ceratonia siliqua and Phytolacca dioica showed similar native embolism (PLC = 30-40%) and [K(+)] of xylem sap (14 to 17 mM), and K(h) of stems perfused with 10 to 25 mM KCl increased by 15 to 18% compared with K(h) of stems perfused with a low concentration of a multi-ionic solution. In contrast, native [K(+)] of sap of Platanus orientalis was 50% of that in the two evergreens in summer, with a parallel lack of detectable changes in PLC that was below 10% in both spring and summer. The ionic effect was PLC-dependent: the enhancement of K(h) induced by 10 to 25 mM KCl changed from 15% for fully hydrated stems to 50-75% for stems with PLC = 50%. In Ceratonia siliqua, PLC was less than 10% in spring and about 40% in summer; concurrently, xylem sap [K(+)] increased from 3 to about 15 mM. This [K(+)] at the recorded PLC would cause an increase in residual K(h) of about 30%. Hence, the actual reduction in water transport capacity of Ceratonia siliqua stems in summer is about 20%. Similar calculations for Phytolacca dioica suggest that the actual loss of hydraulic conductivity in stems of this species in summer would be only about 10%, and not 30% as suggested by hydraulic measurements performed in the laboratory. We conclude that an increase in [K(+)] of xylem sap might be involved in the up-regulation of residual K(h), thus substantially alleviating the embolism-induced reduction in leaf water supply.

The physiological significance of ion-mediated enhancement of xylem hydraulic conductivity (K h ) in planta has recently been questioned. The phenomenon has been suggested to be an artefact caused by the use of deionized water as a reference fluid during measurements of the impact of different ions on K h . In the present study, ion-mediated changes in K h were measured in twigs of five woody species during perfusion with 25 mM KCl compared with different reference fluids like deionized water, a commercial mineral water containing different ions (including 0.5 mM Ca 2+ ), and a 1 mM CaCl 2 solution. Both fully hydrated twigs and twigs with about 50% loss of hydraulic conductivity due to cavitation-induced embolism were tested. Adding 25 mM KCl to the three reference fluids caused K h to increase by about 20%. The KCl-mediated increase of K h was even larger (up to 100%) in embolized twigs. The presence of Ca 2+ in the reference solution decreased, but not suppressed, the KCl-mediated enhancement of K h in fully hydrated twigs of three species, but not in the other two species tested. Ca 2+ did not affect the K h response to KCl in embolized twigs. These data suggest that the recently reported suppression of the ‘ionic effect’ by the presence of calcium in the xylem sap is not a general phenomenon and that ion-mediated changes of K h may play a role in planta partially to compensate for cavitation-induced loss of xylem hydraulic conductivity.

Two clones of Olea europaea L. were studied for their potential impact on hydraulic architecture and vulnerability to xylem cavitation, when used as rootstocks. The clones used were “Leccino Minerva” (LM), showing vigorous growth and “Leccino Dwarf” (LD) with strongly reduced growth. Self-rooted LM and LD plants as well as their grafting combinations were compared, namely, LM/LD (Leccino Minerva grafted onto Leccino Dwarf rootstock) and LD/LM (Leccino Dwarf grafted onto Leccino Minerva rootstocks). Plants with LD roots (LD and LM/LD) showed significantly reduced leaf surface area compared with plants with LM roots. Xylem conduits of LD shoots were 25% more numerous than in LM shoots. When grafted onto LM rootstocks, however, LD shoots produced consistently wider and longer vessels than measured in LD self-rooted plants. This caused LD/LM plants to increase stem vulnerability to cavitation with threshold pressures for cavitation (P c) of less than 0.5 MPa compared with LD self-rooted plants that had P c of over 2.0 MPa. By contrast, although LD rootstocks caused some reduction of vessel diameter and length of LM scions, their influence on LM hydraulic architecture was too small to reduce vulnerability to cavitation of LM scions with respect to that measured for LM self-rooted plants. Our conclusion is that although dwarfing rootstocks effectively reduce grafted plant size, they do not necessarily confer higher resistance to xylem cavitation to scions which would improve plant resistance to drought.

Changes in hydraulic conductivity (K h ) were measured monthly in stems of Laurus nobilis between July 2005 and June 2006 and in March and June 2006 in stems of Prunus laurocerasus and Phillyrea latifolia. Percent enhancement of K h (ΔK h ) was recorded as KCI injected into stems compared with deionized water. Stems of L. nobilis perfused with increasing [KCl] saturated the hydraulic effect at 50 mM KCI, while P. laurocerasus and Ph. latifolia showed saturation at 17 and less than 5 mM KCI, respectively. Impressive seasonal changes in ΔK h were recorded in L. nobilis, from +120% in February to +20% during the growth period. ΔK h was negatively correlated to minimum air temperatures and was highest during winter frost. Negative correlation also existed between ΔK h and conduit lengths as detected during transition of plants from activity to winter rest and from winter to spring. In the winter, the thermal effect was prevailing. High cavitation-induced K h loss coincided with high ΔK h . Similar but much smaller seasonal effects on ΔK h were recorded in the other two species where, however, also smaller seasonal changes in conduit lengths were recorded. We conclude that (1) the K + -induced effect on K h was because of interference of the cation with pectins of intervessel pit membranes (increasingly crossed by the ionic solution in stems with shorter conduits); (2) winter frost might have an effect on ΔK h and (3) seasonal changes in ΔK h because of possible change in pectic composition may represent a mechanism to regulate water flows in planta.

We investigated the hydraulic architecture of young olive trees either self-rooted or grafted on rootstocks with contrasting size-controlling potential. Clones of Olea europea L. (Olive) cv ''Leccino'' inducing vigorous scion growth (Leccino ''Minerva'', LM) or scion dwarfing (Leccino ''Dwarf'', LD) were studied in different scion/rootstock combinations (LD, LM, LD/LD, LM/LM, LD/LM and LM/LD). Shoots growing on LD root systems developed about 50% less leaf surface area than shoots growing on LM root systems. Root systems accounted for 60-70% of plant hydraulic resistance (R), whereas hydraulic resistance of the graft union was negligible. Hydraulic conductance (K = 1/R) of LD root systems was up to 2.5 times less than that of LM root systems. Total leaf surface area (AL) was closely and positively related to root hydraulic conductance so that whole-plant hydraulic conductance scaled by A L did not differ between experimental groups. Accordingly, maximum transpiration rate and minimum leaf water potential did not differ significantly among experimental groups. We conclude that reduced root hydraulic conductance may explain rootstock-induced dwarfing in olive. © 2006 Heron Publishing.

Changes in hydraulic conductivity (K(h)) were measured in stems of Laurus nobilis L. during perfusion with KCl, NaCl or sucrose solutions. Ionic solutes induced marked increase of K(h) with respect to deionized water but sucrose had no effect. The kinetics of KCl-induced K(h) increase was measured together with changes in [K(+)] of the perfused solution. K(h) increases were paralleled by increases in the [K(+)](out)/[K(+)](in) ratio. Samples of different lengths or with increasing percentage loss of conductivity (PLC) due to xylem cavitation were tested, with the aim of increasing radial flow through intervessel pits. KCl solutions enhanced the K(h) of 12-cm-long samples with a concentration-dependent effect up to 100 mm KCl. DeltaK(h) increased from 3 to 30% in 1.5- and 12-cm-long samples, respectively and remained constant for longer samples. Increasing PLC induced an exponential increase in DeltaK(h). PLC measured with KCl solutions was significantly less than that measured with deionized water, suggesting that measurements of PLC can be affected by the composition of the perfused solution. Experiments support the hypothesis that the 'ionic effect' is mediated by physico-chemical changes of pectins of the pit membranes and raise the possibility that plants might alter the ionic composition of the xylem sap to alleviate the hydraulic impact of cavitation.

We studied changes in the hydraulic conductance of leaves (K(leaf)) between dawn and dusk during the growth period (July) and at midday at the beginning of autumn in four tree species. The main objectives of the study were to check the extent of diurnal and seasonal changes in K(leaf) and the relationships between K(leaf), irradiance and leaf gas exchange. Two evergreen (Aleurites moluccana and Persea americana) and two deciduous trees (Platanus orientalis and Quercus rubra) were studied. Leaf hydraulic conductance was measured every 2 h between 0700 and 1900 h in July and compared with values measured between 0900 and 1300 h in October. Other variables measured were photosynthetically active radiation (PAR), leaf conductance to water vapor (gL) and water potential (psiL). In July, K(leaf) varied by up to 75% in Pe. americana on a diurnal basis and by at least 44% in Q. rubra. The diurnal time course of K(leaf) showed a distinct increase between dawn and late morning (1100 h) and a subsequent decrease in the evening in A. moluccana and Pl. orientalis, whereas in the other two species, K(leaf) was highest just after dawn and lowest in the evening. In October, K(leaf) of all the species studied was lower than in July, with differences of 20 to 28% for A. moluccana and Pl. orientalis and of 66 to over 70% in Pe. americana and Q. rubra, respectively. Significant correlations were found between PAR and K(leaf) (in all species) as well as between gL and K(leaf) (in three out of four species). Leaf habit (evergreen or deciduous) did not influence absolute values of K(leaf) or its diurnal variation.

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 (R(leaf)) before and after making several cuts across minor veins, thus getting the hydraulic resistance of the venation system (R(venation)). R(leaf) was nearly the same in controls and PEG04 plants but it was reduced by about 30% in PEG08 plants. On the contrary, R(venation) 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 R(venation) to the overall R(leaf) markedly increased from controls to PEG08 plants. Leaf conductance to water vapour (g(L)) was highest in controls and significantly lower in PEG04 and PEG08 plants. Moreover, g(L) was correlated to R(venation) and to leaf water potential (psi(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.

The circadian regulation of leaf hydraulic conductance (Kleaf) was investigated in Helianthus annuus L. (sunflower). Kleaf was measured with an high pressure flow meter during the light and dark period from plants growing at a photoperiod of 12 h. Kleaf was 4.0 e-4 kg s -1 m-2 MPa-1 during the light period (LL) and 30-40% less during the dark period (DL). When photoperiod was inverted and leaves were measured for Kleaf at their subjective light or dark periods, Kleaf adjusted to the new conditions requiring 48 h for increasing from dark to light values and 4 d for the opposite transition. Plants put in continuous dark showed Kleaf oscillating from light to dark values in phase with their subjective photoperiod indicating that K leaf changes were induced by the circadian clock. Several cuts through the minor veins reduced leaf hydraulic resistance (Rleaf) of both LL and DL to the same value (1.0 e + 3 MPa m2 s kg-1) that equalled the vascular resistance (Rv). The contribution of the non-vascular leaf resistance (Rnv) to Rleaf was of 71.9% in DL and of 58.4% in LL. The dominant Rnv was shown to be reversibly modulated by mercurials, suggesting that aquaporins play a role in diurnal changes of Kleaf.

This paper examines the dependence of whole leaf hydraulic conductance to liquid water (K(L)) on irradiance when measured with a high pressure flowmeter (HPFM). During HPFM measurements, water is perfused into leaves faster than it evaporates hence water infiltrates leaf air spaces and must pass through stomates in the liquid state. Since stomates open and close under high versus low irradiance, respectively, the possibility exists that K(L) might change with irradiance if stomates close tightly enough to restrict water movement. However, the dependence of K(L) on irradiance could be due to a direct effect of irradiance on the hydraulic properties of other tissues in the leaf. In the present study, K(L) increased with irradiance for 6 of the 11 species tested. Whole leaf conductance to water vapour, g(L), was used as a proxy for stomatal aperture and the time-course of changes in K(L) and g(L) was studied during the transition from low to high irradiance and from high to low irradiance. Experiments showed that in some species K(L) changes were not paralleled by g(L) changes. Measurements were also done after perfusion of leaves with ABA which inhibited the g(L) response to irradiance. These leaves showed the same K(L) response to irradiance as control leaves. These experimental results and theoretical calculations suggest that the irradiance dependence of K(L) is more consistent with an effect on extravascular (and/or vascular) tissues rather than stomatal aperture. Irradiance-mediated stimulation of aquaporins or hydrogel effects in leaf tracheids may be involved.

The hydraulic architecture of sunflower (Helianthus annuus L. cv. Margot) was studied in terms of the partitioning of the hydraulic conductance (Kleaf) of leaves inserted at progressively more apical nodes both in growing plants (GP) and in plants at full anthesis (mature plants, MP). Leaf conductance to water vapour (gL), leaf water potential (PsiL), leaf water potential at zero turgor (Psi tlp), and leaf osmotic potential at full turgor (pi0) were also measured. Sunflower plants showed gL and Kleaf values significantly increasing in the acropetal direction, while PsiL of basal leaves was significantly more negative than that of distal leaves; Psi tlp markedly decreased in the acropetal direction in MP so that leaves of MP retained increasingly more turgor the more apical they were. This hydraulic pattern, already present in very young plants (GP), strongly favours apical leaves. These data suggest that the progressive leaf dieback starting from the stem base, as observed when the inflorescence of sunflower reached maturity, might be due to time-dependent loss of hydraulic conductance. In fact, Kleaf loss was correlated with PsiL drop and stomatal closure. Leaf dehydration was aggravated by solute exportation from the basal towards the apical leaves, as revealed by the acropetal decrease of pi0. Kleaf was shown to be linearly and positively related to the prevailing ambient irradiance during plant growth, thus suggesting that leaf hydraulics is very sensitive to environmental conditions. It was concluded that the pronounced apical dominance of some sunflower cultivars is determined, among other factors, by plant hydraulic architecture.

The leaf miner Cameraria ohridella causes premature defoliation of Aesculus hippocastanum trees. In order to assess the whole-plant loss of productivity caused by the parasite, we monitored seasonal changes of leaf gas exchange and leaf area losses in horse chestnut trees freely infested or chemically treated to prevent moth infestation (controls). Data were integrated in a model and the annual loss of net primary productivity (NPP) was calculated for infested trees with respect to controls. Measurements showed marked vertical stratification of C. ohridella attacks, with lower crown strata being more infested than higher ones. Leaf gas exchange was maximum between May and early June, but it strongly decreased starting from mid-June even in controls. Model calculations showed that NPP loss of infested trees was about 30% on an annual basis (when the first moth attack is recorded at the end of April). Model simulations showed that postponing the start day of attack would have important positive effects on plant’s NPP. For example, if the start day of attack was postponed to 20 May, the annual loss of NPP would be about 15%. Our study suggests that A. hippocastanum trees attacked by C. ohridella are not facing serious risks of decline, especially if methods are adopted to postpone the start day of attack (e.g. removal of fallen leaves in autumn). Our data do not support the view that plants need to be totally protected from the parasite by application of insecticides.

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 (Psi 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 Psi 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 substantial 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.

The possible link between stomatal conductance (gL), leaf water potential (ΨL) and xylem cavitation was studied in leaves and shoots of detached branches as well as of whole plants of Laurus nobilis L. (Laurel). Shoot cavitation induced complete stomatal closure in air-dehydrated detached branches in less than 10 min. By contrast, a fine regulation of gL in whole plants was the consequence of ΨL reaching the cavitation threshold (ΨCAV) for shoots. A pulse of xylem cavitation in the shoots was paralleled by a decrease in gL of about 50%, while ΨL stabilized at values preventing further xylem cavitation. In these experiments, no root signals were likely to be sent to the leaves from the roots in response to soil dryness because branches were either detached or whole plants were growing in constantly wet soil. The stomatal response to increasing evaporative demand appeared therefore to be the result of hydraulic signals generated during shoot cavitation. A negative feedback link is proposed between gL and ΨCAV rather than with ΨL itself.

We tested the hypothesis that hydraulic conductance per unit leaf surface area of plant shoots (KSL) determines the maximum diurnal stomatal conductance (gL) that can be reached by plants growing in the field. A second hypothesis was tested that some xylem cavitation cannot be avoided by transpiring plants and might act as a signal for regulating gL. Eleven woody species were studied, differing from each other with respect to taxonomy, wood anatomy and leaf habit. Maximum diurnal gL, transpiration rate (EL), pre-dawn and minimum diurnal leaf water potential (Ψpd and Ψmin, respectively) were measured in the field. The critical Ψ level at which stem cavitation was triggered (Ψcav) was measured on detached branches, using the acoustic method. A high-pressure flow meter was used to measure maximum KSL of 1-year-old shoots. Both gL and EL were positively related to KSL. The whole-plant hydraulic conductance per unit leaf area (KWL) of all the species studied, calculated as the ratio of EL to ΔΨ (=Ψpd-Ψmin) was closely related to KSL. In every case, Ψmin (ranging between –0.85 and –1.35 MPa in the different species) dropped to the Ψcav range or was