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Invasive alien species (IAS) threaten the biodiversity richness of the Mediterranean basin, a drought-prone region. However, our knowledge on the adaptive strategies of IAS for facing Mediterranean drought summers is still incomplete. The aim of the present study is to compare the water relations and the critical relative water content (RWC) values leading to loss of cell rehydration capacity of two Mediterranean basin IAS (i.e., Ailanthus altissima (Mill.) Swingle and Robinia pseudoacacia L.) versus two co-occurring native species (i.e., Fraxinus ornus L. and Quercus pubescens Willd.). Study IAS showed higher values of water potential at turgor loss point and osmotic potential at full turgor, lower values of modulus of elasticity and leaf mass area but higher photosynthesis rate, even during the summer, with respect to the Mediterranean native species. These findings supported the hypothesis that IAS are characterized by a resource acquisitive strategy coupled with a safety-efficiency trade-off, compared with Mediterranean native species. However, similar leaf RWC thresholds leading to loss of cell rehydration capacity were recorded in the two groups of species. Moreover, IAS showed higher saturated water content and capacitance values compared with the co-occurring species. Overall, our results suggest that the success of Mediterranean IAS is driven by their ability to delay dehydration damage of mesophyll cells during Mediterranean summer drought, thereby supporting their distinctive high carbon assimilation rate.

Plant water content is a simple and promising parameter for monitoring drought-driven plant mortality risk. However, critical water content thresholds leading to cell damage and plant failure are still unknown. Moreover, it is unclear whether whole-plant or a specific organ water content is the most reliable indicator of mortality risk. We assessed differences in dehydration thresholds in leaf, stem and root samples, hampering the organ-specific rehydration capacity and increasing the mortality risk. We also tested eventual differences between a fast experimental dehydration of uprooted plants, compared to long-term water stress induced by withholding irrigation in potted plants. We investigated three species with different growth forms and leaf habits i.e., Helianthus annuus (herbaceous), Populus nigra (deciduous tree) and Quercus ilex (evergreen tree). Results obtained by the two dehydration treatments largely overlapped, thus validating bench dehydration as a fast but reliable method to assess species-specific critical water content thresholds. Regardless of the organ considered, a relative water content value of 60% induced significant cell membrane damage and loss of rehydration capacity, thus leading to irreversible plant failure and death.

Increase in severity and frequency of drought events is altering plant community composition, exposing biomes to a higher risk of biodiversity losses. This is exacerbated in the most fragile areas as Mediterranean biome. Thus, identifying plant traits for forecasting species with a high risk of drought-driven mortality is particularly urgent. In the present study, we investigated the drought resistance strategy of two Mediterranean native species: Salvia ceratophylloides Ard. (Sc) and Salvia officinalis L. (So) by considering the impact of drought-driven water content decline on plant hydraulics. Well-watered samples of Sc displayed higher leaf and stemsaturated water content and lower shoot biomass than So samples, but similar root biomass. In response to drought, Sc showed a conservative water use strategy, as the prompt stomatal closure and leaves shedding suggested. A drought-tolerant mechanism was confirmed in So samples. Nevertheless, Sc and So showed similar drought-driven plant hydraulic conductance (Kplant) recover ability. Root hydraulic traits played a key role to reach this goal. Relative water content as well as loss of cell rehydration capability and membrane damages, especially of stem and root, were good proxies of drought-driven Kplant decline.

Understanding which structural and functional traits are linked to species' vulnerability to embolism formation (P50) may provide fundamental knowledge on plant strategies to maintain an efficient water transport. We measured P50, wood density (WD), mean conduit area, conduit density, percentage areas occupied by vessels, parenchyma cells (PATOT) and fibers (FA) on branches of angiosperm and gymnosperm species. Moreover, we compiled a dataset of published hydraulic and anatomical data to be compared with our results. Species more vulnerable to embolism had lower WD. In angiosperms, the variability in WD was better explained by PATOT and FA, which were highly correlated. Angiosperms with a higher P50 (less negative) had a higher amount of PATOT and total amount of nonstructural carbohydrates. Instead, in gymnosperms, P50 vs PATOT was not significant. The correlation between PATOT and P50 might have a biological meaning and also suggests that the causality of the commonly observed relationship of WD vs P50 is indirect and dependent on the parenchyma fraction. Our study suggests that angiosperms have a potential active embolism reversal capacity in which parenchyma has an important role, while in gymnosperms this might not be the case.

Stem photosynthesis has been suggested to play relevant roles to cope with different biotic and abiotic stress factors, including drought. In the present study, we performed measurements of stem hydraulic conductance and non-structural carbohydrate content in the evergreen Laurus nobilis L. and the deciduous Populusalba L., subjected to inhibition of stem photosynthesis and successive exposure to a drought-recovery cycle in order to check if stem photosynthesis may be involved in allowing hydraulic recovery after drought stress relief. Stem shading affected the growth of L. nobilis but not of P. alba saplings. By contrast, inhibition of stem photosynthesis was coupled to inhibition of hydraulic recovery following embolism build-up under drought in P. alba but not in L. nobilis. The two study species showed a different content and behavior of nonstructural carbohydrates (NSCs). The differences in NSCs’ trend and embolism reversal ability led to a significant relationship between starch content and the corresponding hydraulic conductance values in L. nobilis but not in P. alba. Our findings suggest that stem photosynthesis plays a key role in the maintenance of hydraulic functioning during drought especially in the deciduous species. This, in turn, may increase their vulnerability under current global climate change scenarios.

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.

The hydraulic architecture and water relations of two olive genotypes, ‘Leccino Dwarf’ (LD) and ‘Leccino Minerva’ (LM) growing at two irradiance levels i.e. full sunlight irradiance (HI) and 50% sunlight irradiance (LI) were studied. The two clones showed similar plant hydraulic conductances (Kplant) and similar conductance of roots and leaves (Kroot and Kleaf) when growing at equal irradiance levels. However, both Kplant and Kroot were significantly lower in LI plants than in HI ones. On the contrary, Kleaf was unaffected by the light regime. One-year-old twigs of LI plants produced longer xylem conduits but lower average diameter of conduits and less conduits per unit xylem cross-sectional area compared to HI plants. As a consequence total conductive cross-sectional area of twigs was computed to be about 16% smaller in LI individuals than in HI ones. The LM genotype resulted potentially more vulnerable to cavitation than the LD one, although shading did not influence this variable. Shading influenced root biomass negatively with stronger reduction in LM genotype than in LD one. Although transpiration rates were substantially lower in shaded than in HI plants minimum diurnal leaf water potential was about −1.2 MPa for both clones regardless the irradiance regime. Our conclusion is that the hydraulic efficiency of both olive clones was adjusted to meet the evaporative demand imposed by the irradiance regime with consequently similar equal hydraulic sufficiency.

Xylem recovery from embolism was studied in Laurus nobilis L. stems that were induced to cavitate by combining negative xylem pressure potentials (PX = −1.1 MPa) with positive air pressures (PC) applied using a pressure collar. Xylem refilling was measured by recording the percentage loss of hydraulic conductance (PLC) with respect to the maximum 2 min, 20 min and 15 h after pressure release. Sodium orthovanadate (an inhibitor of many ATP-ases) strongly inhibited xylem refilling while fusicoccin (a stimulator of the plasma membrane H+-ATPase) promoted complete embolism reversal. So, the refilling process was interpreted to result from energy-dependent mechanisms. Stem girdling induced progressively larger inhibition to refilling the nearer to the embolized stem segment phloem was removed. The starch content of wood parenchyma was estimated as percentages of ray and vasicentric cells with high starch content with respect to the total, before and after stem embolism was induced. A closely linear positive relationship was found to exist between recovery from PLC and starch hydrolysis. This, was especially evident in vasicentric cells. A mechanism for xylem refilling based upon starch to sugar conversion and transport into embolized conduits, assisted by phloem pressure-driven radial mass flow is proposed.

Drought resistance of Ailanthus altissima (Mill.) Swingle is a major factor underlying the impressively wide expansion of this species in Europe and North America. We studied the specific mechanism used by A. altissima to withstand drought by subjecting potted seedlings to four irrigation regimes. At the end of the 13-week treatment period, soil water potential was -0.05 MPa for well-watered control seedlings (W) and -0.4, -0.8 and -1.7 MPa for drought-stressed seedlings (S) in irrigation regimes S1, S2 and S3, respectively. Root and shoot biomass production did not differ significantly among the four groups. A progressively marked stomatal closure was observed in drought-stressed seedlings, leading to homeostasis of leaf water potential, which was maintained well above the turgor loss point. Root and shoot hydraulics were measured with a high-pressure flow meter. When scaled by leaf surface area, shoot hydraulic conductance did not differ among the treated seedlings, whereas root hydraulic conductance decreased by about 20% in S1 and S2 seedlings and by about 70% in S3 seedlings, with respect to the well-watered control value. Similar differences were observed when root hydraulic conductance was scaled by root surface area, suggesting that roots had become less permeable to water. Anatomical observations of root cross sections revealed that S3 seedlings had shrunken cortical cells and a multilayer endodermal-like tissue that probably impaired soil-to-root stele water transport. We conclude that A. altissima seedlings are able to withstand drought by employing a highly effective water-saving mechanism that involves reduced water loss by leaves and reduced root hydraulic conductance. This water-saving mechanism helps explain how A. altissima successfully competes with native vegetation.

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.

The leaf hydraulic conductance (KL) was measured in Prunus laurocerasus L. and Juglans regia L. in which previous measurements had revealed different impacts of dehydration on KL. Leaves of P. laurocerasus lost 8% of their KL at water potentials (ΨL) of −2.0 MPa. Leaflets of J. regia showed KL losses of 40% at ΨL = −1.0 MPa. When major veins were blocked using cyanoacrylate to simulate their embolism, the KL of P. laurocerasus was reduced by 57% but that of J. regia leaflets was reduced by 80%. Such differences were hypothesized to be due to different axial-to-radial permeabilites of major veins. Infiltration of leaves with Phoxine B revealed that P. laurocerasus major veins were largely leaky in the radial direction whereas those of J. regia leaflets showed prevailing axial water transport. Differences between species in terms of axial-to-radial water permeability were confirmed by measurements of changes of hydraulic resistance along the midrib. The two hydraulic models are discussed in terms of leaf vulnerability to embolism and plant adaptation to dry habitats.

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.

Drought resistance was examined for 2-year-old saplings of Ceratonia siliqua L., Olea oleaster Hoffmgg. et Link., Quercus suber L. and Q. pubescens Willd. growing in the field in Sicily, with the aim of testing their possible use in the reforestation of degraded areas. To this purpose, leaf conductance to water vapour (gL), transpiration rate (EL), relative water content (RWC) and water potential (ΨL) were measured between pre-dawn and sunset, monthly from May to November. Parallel measurements of loss of hydraulic conductance of twigs of the current year (PLC) were made together with an estimate of whole-plant hydraulic conductance (KPLANT) on the basis of the ratio of maximum EL to (ΨPD-ΨMIN), where ΨPD is pre-dawn ΨL and ΨMIN is the minimum diurnal ΨL. C. siliqua saplings maintained high gL throughout the study period with high RWC (over 90%) and ΨL. They grew rapidly and increased their foliage area (AL) by over 60% from May to winter rest. This was accompanied by low twig PLC (about 20% in September) resulting in high KPLANT all through the study period. In contrast, O. oleaster saplings underwent distinct dehydration in July, i.e. they showed stomatal closure due to a drop in RWC (to 75%) and ΨL (to the turgor-loss point). This was apparently due to twig cavitation, resulting in a strong decrease of KPLANT. Plants, however, tolerated summer drought, and showed growth (AL increased by 15% from May to November). Saplings of Q. suber and Q. pubescens were much more vulnerable to twig cavitation (PLC was 35–48% from June to November) than the other two species, and their average KPLANT was lower. Saplings of Q. suber and Q. pubescens grew only during the wet spring period, and no new foliage was produced thereafter. Changes in twig hydraulic conductance played a dominant role in determining changes in KPLANT in that the two variables were well correlated to each other (r=0.68 for P=0.001). We conclude that C. siliqua is an ideal candidate for reforestation of Sicilian degraded areas as is O. oleaster that, however, requires some additional water supply in the summer, at least during the early years after plantation. In wetter locations of Sicily, to an altitude between 0 and 500 m, Q. suber and Q. pubescens can be used for reforestation with expected higher competitiveness of the latter over the former species.

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.

Plant water relations, xylem anatomy and the hydraulic architecture of 1-year-old twigs of Spartium junceum, both healthy and affected by a phytoplasm disease, were studied. The disease causes twigs to be either shortened (witches broom disease, WBD) or flat (fasciate disease, FD). WBD twigs show a sevenfold increase in total leaf area, smaller and shorter xylem conduits, a higher stomatal conductance (g 1 ) and a decline of minimum leaf water potentials (ψ 1 ) below the turgor loss point. FD twigs had nearly twice the leaf area of the healthy controls as well as high g 1 values and ψ 1 values below the turgor loss point. Moreover, significant differences between healthy and affected twigs in stem stomatal conductance (g s ) and in the total stem area were recorded. Affected twigs die back under drought stress, which is explained by a pronounced loss of hydraulic conductivity of the infected stems (40 and 60%) in FD and WBD as well as by the unfavourable ratio of weighted conduit radius (Σr 4 ) to total surface area (A 1 ), so that the efficiency of the stem in supplying the whole transpiring area with water is strongly reduced.

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.

†Background and Aims Potassium, sulphur and zinc contents of mistletoe leaves are generally higher than in their hosts. This is attributed to the fact that chemical elements which are cycled between xylem and phloem in the process of phloem loading of sugars are trapped in the mistletoe, because these parasites do not feed their hosts. Here it is hypothesized that mutant albino shoots on otherwise green plants should behave similarly, because they lack photosynthesis and thus cannot recycle elements involved in sugar loading. †Methods The mineral nutrition of the mistletoe Scurrula elata was compared with that of albino shoots on Citrus sinensis and Nerium oleander. The potential for selective nutrient uptake by the mistletoe was studied by comparing element contents of host leaves on infected and uninfected branches and by manipulation of the haustorium– shoot ratio in mistletoes. Phloem anatomy of albino leaves was compared with that of green leaves. †Key Results Both mistletoes and albino leaves had higher contents of potassium, sulphur and zinc than hosts or green leaves, respectively. Hypothetical discrimination of nutrient elements during the uptake by the haustorium is not supported by our data. Anatomical studies of albino leaves showed characteristics of release phloem. †Conclusions Both albino shoots and mistletoes are traps for elements normally recycled between xylem and phloem, because retranslocation of phloem mobile elements into the mother plant or the host is low or absent. It can be assumed that the lack of photosynthetic activity in albino shoots and thus of sugars needed in phloem loading is responsible for the accumulation of elements. The absence of phloem loading is reflected in phloem anatomy of these abnormal shoots. In mistletoes the evolution of a parasitic lifestyle has obviously eliminated substantial feeding of the host with photosynthates produced by the mistletoe.

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.

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 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 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.

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.

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.