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Diurnal changes in percentage loss of hydraulic conductivity (PLC), with recorded values being higher at midday than on the following morning, have been interpreted as evidence for the occurrence of cycles of xylem conduits' embolism and repair. Recent reports have suggested that diurnal PLC changes might arise as a consequence of an experimental artefact, that is, air entry into xylem conduits upon cutting stems, even if under water, while under substantial tension generated by transpiration. Rehydration procedures prior to hydraulic measurements have been recommended to avoid this artefact. In the present study, we show that xylem rehydration prior to hydraulic measurements might favour xylem refilling and embolism repair, thus leading to PLC values erroneously lower than those actually experienced by transpiring plants. When xylem tension relaxation procedures were performed on stems where refilling mechanisms had been previously inhibited by mechanical (girdling) or chemical (orthovanadate) treatment, PLC values measured in stems cut under native tension were the same as those measured after sample rehydration/relaxation. Our data call for renewed attention to the procedures of sample collection in the field and transport to the laboratory, and suggest that girdling might be a recommendable treatment prior to sample collection for PLC measurements.

Water is transported from roots to foliage through the xylem under negative pressure (=tension). Under this metastable status, water is prone to sudden phase change to water vapor (cavitation). In plants, air can also be aspirated into functioning xylem conduits through inter-conduit pit membranes, and the resulting embolism blocks water transport through the conduit and reduces plant hydraulic conductance and productivity. Xylem embolism and hydraulic failure are major factors contributing to tree mortality and forest decline under global-change-type droughts. However, some plants can tolerate even high embolism levels under drought, recovering hydraulic functionality upon partial or total rehydration via embolism repair and xylem refilling. Here, we review current evidence of embolism repair, highlighting possible physiological mechanisms and suggesting some functional and anatomical determinants making embolism reversal a feasible and successful drought resilience mechanism in some plants.

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.

The impact of leaf vein cavitation and embolism on stomatal response and leaf hydraulic conductance was studied in potted plants of sunflower subjected to water limitation. Plant dehydration was achieved either by cuttingwell-watered plants near their base and leaving them dehydrating in air or by depriving intact plants of irrigation. The vein cavitation threshold (Ψ C A V ) was estimated in terms of ultrasound acoustic emissions (UAE) from the leaf blade versus leaf water potential (Y L ). This was found to be the same (Ψ C A V -0.6 MPa) for leaves of both cut and intact plants where stomata began to close in coincidence with starting vein cavitation. Vein embolism was detected by infiltrating leaves at different Ψ L with 0.7 mM fluorescein and measuring the percentage fluorescent area as percentage of total leaf surface area. A distinct loss of vein functionality (up to 50%) was found to occur in leaves at progressively decreasing Ψ L , starting when leaves reached Ψ C A V . A linear positive relationship with high statistical significance was found to exist between g L and percentage leaf fluorescent area, thus indicating that stomata were sensitive to vein embolism. The hydraulic conductance (K L ) of the leaf was affected by leaf dehydration less than expected (K L decreased by about 20% between near full turgor and Ψ L = -1.3 MPa). When the extravascular leaf compartment was excluded either by killing cells by immersing leaves in 70% ethanol or by cutting the main leaf venous system through to allow flow to bypass it, K L turned out to increase 5.5 times, thus suggesting that the high dominance of the hydraulic resistance of the extravascular leaf compartment over the total leaf resistance might buffer or mask possibly large local changes in K L inducing stomatal closure.

International audience; Three woody species typically encroaching the Karstic region of Trieste (Northeastern Italy) i.e. Cotinus coggygria L., Prunus mahaleb L. and Fraxinus ornus L., have been measured for diurnal and seasonal time courses of leaf conductance to water vapour, transpiration and water potential ($\Psi _{\rm L}$), as well as whole-plant, shoot and root hydraulic resistance (R$_{{\rm plant}}$, R$_{{\rm shoot}}$ and R$_{{\rm root}}$, respectively) in view of investigating the impact of plant hydraulics on water stress resistance. F. ornus suffered the highest $\Psi _{\rm L}$ drop in July and September due to the high Rplant measured in this species. Positive significant relations were found of R$_{{\rm plant}}$ and $\Psi _{\rm L}$ to R$_{{\rm root}}$ in all three species studied, thus suggesting that root hydraulics is a major determinant of plant hydraulics and water stress resistance. The increasing R$_{{\rm root}}$ (and R$_{{\rm plant}}$) from C. coggygria to P. mahaleb to F. ornus provided a possible explanation for the typical temporal sequence of expansion of the three species into degraded areas.; Relations hydriques et caractéristiques hydrauliques de trois espèces ligneuses présentes dans le même habitat. Trois espèces ligneuses, typiquement en cours d'extension dans la région Karstique de Trieste (Italie du nord) i.e. Cotinus coggygria L., Prunus mahaleb L. and Fraxinus ornus L., ont été suivies aux échelles journalières et saisonnières en termes de conductance foliaire pour la vapeur d'eau, de transpiration et de potentiel hydrique ($\Psi _{\rm L}$), ainsi que de résistance hydraulique totale, tige et racinaire (R$_{{\rm plant}}$, R$_{{\rm shoot}}$ and R$_{{\rm root}}$, respectivement) dans l'objectif d'analyser l'impact des paramètres hydrauliques des plantes sur leur résistance au stress hydrique. F. ornus a montré la plus grande chute de $\Psi _{\rm L}$ en Juillet et Septembre en raison de la grande valeur de R$_{{\rm plant}}$ mesurée chez cette espèce. Des relations positives et significatives ont été trouvées entre R$_{{\rm plant}}$, $\Psi _{\rm L}$ et R$_{{\rm root}}$ pour les trois espèces étudiées, ce qui suggère que les propriétés hydrauliques des racines sont un déterminant majeur des propriétés hydrauliques de la plante et de leur résistance au stress hydrique. Les valeurs croissantes de R$_{{\rm root}}$ (et R$_{{\rm plant}}$) de C. coggygria à P. mahaleb et F. ornus fournissent une explication possible pour la séquence temporelle d'expansion de ces trois espèces dans les zones dégradées.

Mediterranean native shrubs can be very useful for green roofs in hot and arid regions. Our data show that both Arbutus unedo L. and Salvia officinalis L. could be successfully utilized, although the choice of species should be based on the drought-resistant strategy relative to the desired technical performance of the green roof. Moreover, substrate selection was found to have a crucial role in the success of green roof installations in the Mediterranean area., Recent studies have highlighted the ecological, economic and social benefits assured by green roof technology to urban areas. However, green roofs are very hostile environments for plant growth because of shallow substrate depths, high temperatures and irradiance and wind exposure. This study provides experimental evidence for the importance of accurate selection of plant species and substrates for implementing green roofs in hot and arid regions, like the Mediterranean area. Experiments were performed on two shrub species (Arbutus unedo L. and Salvia officinalis L.) grown in green roof experimental modules with two substrates slightly differing in their water retention properties, as derived from moisture release curves. Physiological measurements were performed on both well-watered and drought-stressed plants. Gas exchange, leaf and xylem water potential and also plant hydraulic conductance were measured at different time intervals following the last irrigation. The substrate type significantly affected water status. Arbutus unedo and S. officinalis showed different hydraulic responses to drought stress, with the former species being substantially isohydric and the latter one anisohydric. Both A. unedo and S. officinalis were found to be suitable species for green roofs in the Mediterranean area. However, our data suggest that appropriate choice of substrate is key to the success of green roof installations in arid environments, especially if anisohydric species are employed.

Recovery of hydraulic conductivity after the induction of embolisms was studied in woody stems of laurel (Laurus nobilis). Previous experiments confirming the recovery of hydraulic conductivity when xylem pressure potential was less than −1 MPa were repeated, and new experiments were done to investigate the changes in solute composition in xylem vessels during refilling. Xylem sap collected by perfusion of excised stem segments showed elevated levels of several ions during refilling. Stem segments were frozen in liquid N2 to view refilling vessels using cryoscanning electron microscopy. Vessels could be found in all three states of presumed refilling: (a) mostly water with a little air, (b) mostly air with a little water, or (c) water droplets extruding from vessel pits adjacent to living cells. Radiographic probe microanalysis of refilling vessels revealed nondetectable levels of dissolved solutes. Results are discussed in terms of proposed mechanisms of refilling in vessels while surrounding vessels were at a xylem pressure potential of less than −1 MPa. We have concluded that none of the existing paradigms explains the results.

Quasi-steady-state measurements of root hydraulic conductance (KR) of Olea oleaster Hoffmgg. et Link potted seedlings were performed using a pressure chamber with the aim of: (a) measuring the impact of different water-stress levels on a KR; (b) measuring the kinetics of KR recovery several days after soil rewetting; (c) relating changes in KR to changes in root anatomy and morphology. Increasing water-stress was applied in terms of ratio of leaf water potential (ΨL) measured at midday to that at zero turgor (ΨTLP), i.e. ΨL/ΨTLP=0·5, 1·0, 1·2, 1·6; KR was measured initially and at 24, 48, 72, 96 h after irrigation. Values of KR in seedlings stressed to ΨL/ΨTLP=1·2 increased for 48 h after irrigation from 0·23 to 0·97×10−5 kg s−1 m−2 MPa−1 i.e. from 16% to 66% of that measured in unstressed seedlings. A marked shift of the x-axis intercept of the straight line relating flow to pressure (zero flow at non-zero pressure) was recorded initially after irrigation and persisted up to 48 h. Recovery of KR occurred within 24 h after irrigation in seedlings at ΨL/ΨTLP=0·5 and 48 h later in those at ΨL/ΨTLP=1·0. Severe drought stress (ΨL/ΨTLP=1·6) caused anatomical changes to roots which formed a two-layered exodermis with thicker suberized walls and a three- to four-layered endodermis with completely suberized tangential walls. Recovery of KR in these roots required resumed growth of root tips and emergence of new lateral roots.

Gas exchange and hydraulic features were measured in leaves of three different Citrus species (Citrus aurantium L., Citrus limon L., Citrus × paradisii Macfad) infested by Phyllocnistis citrella Staiton, with the aim to quantify the impact of this pest on leaf hydraulics and, ultimately, on plant fitness. Infested leaves were characterized by the presence on the leaf blade of typical snake-shaped mines and, in some cases, of a crumpled leaf blade. Light microscopy showed that leaf crumpling was induced by damage to the cuticular layer. In all three Citrus species examined: (a) the degree of infestation did not exceed 10% of the total surface area of infested plants; (b) control and infested leaves showed similar values of minimum diurnal leaf water potential, leaf hydraulic conductance and functional vein density; and (c) maximum diurnal values of stomatal conductance to water vapour, transpiration rate and photosynthetic rate (An) were similar in both control leaves and the green areas of infested leaves. A strong reduction of An was recorded only in mined leaf areas. Our data suggest that infestation with P. citrella does not cause conspicuous plant productivity reductions in young Citrus plants, at least not in the three Citrus species studied here.

Measurements of xylem conduit length and width and the distribution of xylem conduit ends were made in inter-nodes (I), nodes (N) and twig junctions (J) of 1-, 2- and 3-year-old twigs of plants of Quercus cerris L. Parallel measurements were also made of the loss of hydraulic conductivity of twigs subjected to pressure differentials across conduit pit membranes, equalling the leaf water potential at the turgor loss point. The loss of theoretical hydraulic conductivity was calculated as the ratio of i esivr4 (where r is the conduit radius) of the non-conducting conduits to that of all the conduits in the outermost wood ring of I, N and J. Stem zones such as 1-year-old nodes and junctions were localized with narrower and shorter xylem conduits and with higher percentages of conduit ends than internodes. Such ‘constricted zonesrsquo; were less vulnerable to embolism than internodes. Latewood conduits were consistently narrower, shorter and less vulnerable to embolism than earlywood ones. A positive relation therefore existed between conduit diameter and length and vulnerability to embolism. The overall vulnerability to embolism of Q. cerris plants is discussed in terms of xylem conduit width and length and of the distribution of conduit ends.

Xylem cavitation is a common occurrence in drought-stressed plants. Cavitation-induced embolism reduces xylem hydraulic conductivity (kxylem) and may lead to stomatal closure and reduction of photosynthetic rates. Recent studies have suggested that plants may compensate for kxylem loss through ion-mediated enhancement of the residual water transport capacity of functioning conduits. To test this hypothesis, field-grown laurel (Laurus nobilis L.) plants were subjected to mild drought stress by suspending irrigation. Drought treatment induced a significant increase in xylem embolism compared with control (well watered) plants. Xylem sap potassium concentration ([K+]) increased during the day both in control and water stressed plants. Midday values of sap [K+] were significantly higher in water stressed plants. The recorded increase in sap potassium concentration induced significant enhancement of residual kxylem when solutions with different [K+] were perfused through excised stems sampled in the field and measured in the laboratory. In planta measurements of stem hydraulic conductance revealed no change between water stressed plants and controls. Our data suggest that ion-mediated enhancement of residual kxylem buffered the actual loss of hydraulic conductance suffered by plants during the warmest hours of the day as well as under mild drought stress conditions.

Long-distance water transport in plants relies on negative pressures established in continuous water columns in xylem conduits. Water under tension is in a metastable state and is prone to cavitation and embolism, which leads to loss of hydraulic conductance, reduced productivity and eventually plant death. Experimental evidence suggests that plants can repair embolized xylem by pushing water from living vessel-associated cells into the gas-filled conduit lumina. Most surprisingly, embolism refilling is known to occur even when the bulk of still functioning xylem is under tension, a finding that is in seemingly contradiction to basic principles of thermodynamics. This review summarizes our current understanding of xylem refilling processes and speculates that embolism repair under tension can be envisioned as a particular case of phloem unloading, as suggested by several events and components of embolism repair, typically involved in phloem unloading mechanisms. Far from being a challenge to irreversible thermodynamics, embolism refilling is emerging as a finely regulated vital process essential for plant functioning under different environmental stresses.

The possible role of water expelled from cavitated xylem conduits in the rehydration of water-stressed leaves has been studied in one-year-old twigs of populus deltoides Bartr. Twigs were dehydrated in air. At desired values of leaf water potential (Ψl) (between near full turgor and -1.62 MPa), twigs were placed in black plastic bags for 1–2h. Leaf water content was measured every 3–5 min before bagging and every 10 min in the dark. Hydraulic conductivity and xylem cavitation were measured both in the open and in the dark. Cavitation was monitored as ultrasound acoustic emissions (AE). A critical Ψl value of -0.96 MPa was found, at which AE increased significantly while the leaf water deficit decreased by gain of water. Since the twigs were no longer attached to roots, it was concluded that water expelled from cavitated xylem conduits was transported to the leaves, thus contributing to their rehydration. Xylem cavitation is discussed in terms of a ‘leaf water deficit buffer mechanism’, under not very severe water stress conditions.

The ability of stems of Laurus nobilis (L.) to refill embolised xylem conduits was studied in plants both at optimal water supply (W) and under conditions of soil drought inducing xylem pressures (Px) of –1.54 (S1) and –2.35 MPa (S2). Starch depolymerisation in wood parenchyma was measured as percentage of cells ‘with high starch content’ (HSC-cells) counted under a microscope. HSC-cells decreased during embolism and increased again in refilled stems. A direct relationship was found between percentage of HSC-cells and Px, with HSC-cells between 65 and 75% of the total at Px ≥ –0.6 MPa, at which recovery from PLC was recorded. At low transpiration, starch re-appeared in wood parenchyma cells but only in plants that showed diurnal stomatal opening (W- and S1-plants). In S2-plants showing diurnal stomatal closure and nocturnal opening with Px between –1.2 to –2.4 MPa, HSC-cells were only 25% and plants did not recover from PLC. This finding suggests that (i) the Px threshold for embolism repair was ≥ –0.6 MPa, and (ii) impeded phloem loading limits starch content in wood parenchyma and embolism repair. We conclude that starch depolymerisation acts as a signal to phloem unloading sugars to embolised conduits thus generating the necessary osmotic gradients driving refilling.

Xylem embolism repair is preceded by starch depolymerisation in vessel-associated cells (VAC) of Laurus nobilis L. (laurel) twigs, but the primary signal triggering such a process is still unknown. We tested the hypothesis that conduit wall vibrations during cavitation may be sensed by VAC inducing starch-to-sugar conversion. Twigs of laurel from watered or stressed plants were exposed to ultrasound for 60 min to simulate acoustic waves emitted by cavitating conduits. Preliminary tests showed that ultrasound caused no damage to cell membrane integrity nor did they cause xylem embolism. The number of VAC with high starch content (HSC-cells) was estimated microscopically by counting the cells with more than 50% of their lumen filled with starch granules. Sonication had no effect on HSC-cells in twigs from watered plants while it induced a drop in the percentage HSC-cells from 80 to 40% in twigs from stressed plants, at the ultrasound source location. No effect was recorded in these twigs 20 mm from the ultrasound source. Sonication was a good simulator of cavitation in inducing starch depolymerisation which suggests a possible bio- physical nature for the signal initiating embolism repair.

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.

Phoma trachephila (Petri) Kantschaveli et Gikachvili causes dieback of several Citrus species. The impact of this fungus on leaf hydraulics was studied in Citrus aurantium L. (sour orange) with the aim of identifying the primary mechanism of damage to leaves. Leaves inoculated with a conidial suspension were measured for conductance to water vapor (gL) and specific hydraulic conductance (Kleaf) every 3 days after inoculation. The earliest symptom of infection consisted of vein chlorosis. Functional vein density (FVD) was monitored and microscopic observations were made of major vein conduits. Impairment of vein hydraulics started 25 days after inoculation with a losses of Kleaf of 40% and gL of ~60%. Most minor veins within chlorotic areas were no longer functioning and some conduits of the major veins showed digested interconduit pits leading to vein cavitation. The close Kleaf–FVD relationship revealed that vein impairment caused drop of Kleaf and, consequently, of gL at chlorotic areas. Leaf infection was focused to veins that were first forced to embolise and then invaded by fungal hyphae. The vein embolism due to the Phoma amplifies the native dominant hydraulic resistance of leaf veins, and leads ultimately to early shedding of infected leaves.

Phenotypes more or less deficient in photosynthetic pigments show reduced productivity. Not much is known, however, about the influence of pigment-less twigs on the water balance of whole trees. We studied the water relations and hydraulic properties of normal and pigment-less (white) and 1-year-old shoots of 12-year-old Citrus sinensis L. trees. Compared with green leaves, white leaves showed a pronounced deficiency of pigments, higher stomatal density, the absence of chloroplasts in the guard cells and a different organization of leaf parenchyma. Stomatal conductance (gL) and transpiration rate (EL) were higher in white leaves than in green leaves during the hottest hours of the day, especially in July and September. The absence of chloroplasts in the stomatal guard cells seemed to be one of the factors causing insufficient stomatal control. Hydraulic conductance (KL) was higher in white leaves than in green leaves (16.96+/-2.24x10(-5) versus 11.26+/-0.66x10(-5) kg s-1 m-2 MPa). The ratio between the sum of the fourth power of xylem conduit radius (Sigmar4) (which determines theoretical conductance) and the total leaf area supplied by petioles and midribs was higher in white leaves than in green leaves. This was because of a smaller leaf area in white leaves and a statistically different distribution of lumen diameters of the conduits in midribs and petioles. The hydraulic properties of white twigs profoundly disturbed the water balance and physiology of the whole tree.

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.

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 leaf miner Cameraria ohridella causes premature defoliation of Aesculus hippocastanum trees. Repeated defoliation has been reported to cause decrease in radial growth of trees and a progressive decline due to reduced production and allocation of photosynthates. Our study represents an attempt to estimate the impact of C. ohridella on annual wood increments and the hydraulic properties of the wood as well as on the dry mass of seeds. Twenty-two adult horse chestnut trees were selected, four of which had been chemically treated to prevent attack (controls). All other trees were heavily infested. The ground cover (GC) of each tree, measured from monthly hemispherical photographs, revealed that infested trees were completely defoliated in September and the slope of the GC-to-measurement dates relationship (named GC decrease index) was positively related to the number of mines per leaf. Anatomical observations showed that infested trees produced more wood per year than controls through more false rings with wider xylem conduits and, hence, with higher conductive area and theoretical flow. In fact, the theoretical flow was positively related to the defoliation intensity. In contrast, the allocation of photosynthates to seeds was strongly reduced in infested trees with respect to controls (up to 50% less). The hypothesis was advanced that horse chestnut trees reacted to C. ohridella attacks by increasing the hydraulic efficiency of the wood, thus ameliorating the water and nutrient supply to leaves between the spring and mid-summer and, therefore, compensating, at least partly, the reduced leaf lifespan.

Changes in leaf hydraulic conductance (K) were measured using the vacuum chamber technique during dehydration and rehydration of potted plants of Ceratonia siliqua. K of whole, compound leaves as well as that of rachides and leaflets decreased by 20-30% at leaf water potentials (Ψ L ) of -1.5 and -2.0 MPa, i.e. at Ψ L values commonly recorded in field-growing plants of the species. Higher K losses (up to 50%) were measured for leaves at Ψ L of -2.5 and -3.0 MPa, i.e. near or beyond the leaf turgor loss point. Leaves of plants rehydrated while in the dark for 30 min, 90 min and 12 h recovered from K loss with characteristic times and to extents inversely proportional to the initial water stress applied. Leaf conductance to water vapour of plants dehydrated to decreasing Ψ L and rehydrated at low transpiration was inversely related to loss of K, thus suggesting that leaf vein embolism and refilling (and related changes in leaf hydraulics) may play a significant role in the stomatal response.

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.

The annual time course of the water relations of six sclerophylls has been studied with the aim of: a) defining strategies adopted for withstanding summer water stress and b) to check whether their common sclerophyllous habitus could represent a case of convergent evolution devoted to a univocal functional role. In particular, Phillyrea latifolia L. showed to behave like a drought tolerant as indicated by deep summer drop in leaf water potential (Ψ1) to near the full turgor loss point (Ψlp) and in leaf relative water content (RWC) as caused by xylem cavitation. Prunus laurocerasus L. and Laurus nobilis L. showed to be drought avoiding water spenders in that their Ψ1 dropped in summer causing prompt recovery in RWC. This was made possible by low cavitation in their twigs. Ilex aquifolium L. was a typical drought avoiding water saver in that both Ψ1 and RWC remained at very high levels throughout the year. Quercus ilex L. behaved like a drought avoiding water spender which switched to drought toler...

Xylem recovery from embolism was studied in stems of Laurus nobilis L. that were induced to cavitate by combining negative xylem pressures with positive air pressures applied with a pressure collar. Xylem refilling was measured 2 and 20 min and 15 h after air pressure release in January, March and June when increasing percentages of wood parenchyma cells with high starch content (HSC-VAC) were counted (from 0% in January to 87.3% in June). In January, no xylem repair was measured. In June, stems refilled by 75% of previous conductivity loss with a parallel decrease of HSC-VAC. Xylem refilling was tested for stems with phloem either intact or excised by 20 and 50% and with phloem inactivated by girdling stems at both sides of the embolised segment. Stems with 50% of the cortex removed showed some recovery 15 h after embolism. Girdled stems did not recover from embolism and no starch depolymerisation was measured. Girdled stems where a radial mechanical pressure was applied for 20 min after embolism refilled in the same way as stems with intact phloem. Our conclusion is that phloem may export some signal for starch depolymerisation and this, in turn, would drive sugar efflux into embolised conduits with consequent osmotic water flows and refilling.

Measurements of Leaf Specific Conductivity (LSC) of the vertical water potential gradients and of vessels diameters in 1-year-old twigs of young olive trees have been made. LSC prooves to vary along the twigs differently from other species we have already studied in that it is significantly lower in the proximal twig's segment. Consequently, the water potential gradients are in this region higher so indicating that a higher resistance to flow exists. The frequency distribution of the vessels diameters shows that changes in the xylem hydraulic conductivity are mainly due to analogous changes in the relative number of large-diameter vessels. Also the nodes appear to behave as constriction zones of the xylem system. The functional significance of such constrictions in terms of a safety system against cavitation is discussed.