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Efficient and safe water transport in plants depends on specific wood anatomical structures. Comparing the hydraulic vulnerability among tree species and individuals can be biased by differences in the sampling procedure resulting in unjustified conclusions. In fact, water-transporting conduits of the xylem widen from the tree-tip toward the base potentially influencing xylem vulnerability to embolism formation.The resistance to cavitation and embolism formation (hydraulic vulnerability curves) was empirically tested at different positions from the tree-tip to the trunk base in a mature Norway spruce and a silver fir tree. Anatomical analyses (conduit diameter, pit traits) were performed on the same material used for hydraulic measurements.Substantial intra-plant variation in xylem anatomy and vulnerability to embolism was observed in both species scaling with the distance from the apex. The P50 values (i.e. the water potential resulting in 50% loss of hydraulic conductivity) were about 3 MPa more negative at the tip than at the trunk base. Tracheid diameters and pit dimensions increased from the apex downwards, confirming the tip-to-base conduit widening. The highest variability of both anatomical and physiological traits was observed within the first 3 m from the apex. We propose a method for the prediction of drought vulnerability based on hydraulically weighed conduit diameter distribution.A clear pattern of anatomical traits’ variation along the longitudinal axes of the tree was observed, likely playing a role in the increase of vulnerability to embolism. In comparative studies, we thus recommend to standardize the sampling of material according to the distance from the apex.

Leaves grown at different light intensities exhibit considerable differences in physiology, morphology and anatomy. Because plant leaves develop over three dimensions, analyses of the leaf structure should account for differences in lengths, surfaces, as well as volumes. In this manuscript, we set out to disentangle the mesophyll surface area available for diffusion per leaf area (Sm,LA) into underlying one-, two- and three-dimensional components. This allowed us to estimate the contribution of each component to Sm,LA, a whole-leaf trait known to link structure and function. We introduce the novel concept of a ‘stomatal vaporshed,’ i.e. the intercellular airspace unit most closely connected to a single stoma, and use it to describe the stomata-to-diffusive-surface pathway. To illustrate our new theoretical framework, we grew two cultivars of Vitis vinifera L. under high and low light, imaged 3D leaf anatomy using microcomputed tomography (microCT) and measured leaf gas exchange. Leaves grown under high light were less porous and thicker. Our analysis showed that these two traits and the lower Sm per mesophyll cell volume (Sm,Vcl) in sun leaves could almost completely explain the difference in Sm,LA. Further, the studied cultivars exhibited different responses in carbon assimilation per photosynthesizing cell volume (AVcl). While Cabernet Sauvignon maintained AVcl constant between sun and shade leaves, it was lower in Blaufränkisch sun leaves. This difference may be related to genotype-specific strategies in building the stomata-to-diffusive-surface pathway.

Plant stress experiments are commonly performed with plants grown in containers to better control environmental conditions. Nevertheless, the container can constrain plant growth and development, and this confounding effect is generally ignored, particularly in studies on woody species. Here, we evaluate the effect of the container volume in drought experiments using grapevine as a model plant. Grapevines grown in small (7 L, S) or large (20 L, L) containers were subjected to drought stress and rewatering treatments. We monitored plant stomatal conductance (gs ), midday stem water potential (Ψs ), and photosynthetic rate (AN ) throughout the experiment. The effect of the container volume on the stem and petiole xylem anatomy, as well as on the total leaf area (LA), was assessed before drought imposition. The results showed that LA did not differ between plants in L or S containers, but S vines exhibited a higher theoretical hydraulic conductance at the petiole level. Under drought L and S similarly reduced gs and AN , but plants in S containers reached lower Ψs than those in L. Nevertheless, upon rewatering droughted plants in S containers exhibited a faster stomata re-opening than those in L, probably as a consequence of the differences in the stress degree experienced and the biochemical adjustment at the leaf level. Therefore, a suitable experimental design should consider the container volume used in relation to the desired traits to be studied for unbiased results.

Environmental sustainability of viticulture is negatively affected by prolonged droughts. In limestone dominated regions, there is limited knowledge on grapevine water status and on methods for accurate evaluation of actual water demand, necessary to appropriately manage irrigation. During a dry vintage, we monitored plant and soil water relations in old and young vines of Istrian Malvasia on Karst red soil. The vineyard with young vines was additionally subdivided into two areas, based on their soil type, 1) karst silty-clay loam, and 2) mixture of crushed rocks and karst silty-clay loam (stony soil). Seasonal changes in exploited water resources were estimated via analysis of oxygen isotope composition (δ18O) of rainfall, deep soil water, and xylem sap. We hypothesized that plants are able to thrive during drought thanks to the water stored in deep soil layers, while they rely less on superficial soil horizons. Our results show that vines growing on karstic substrates have deep roots securing the use of stable water sources during summer, with consequent favourable plant water status. In fact, both young and mature vines approached the threshold of severe water stress, but never surpassed it, as midday leaf water potentials were >−1.3 MPa in all study sites. Vines roots showed flexible water uptake, i.e. the ability to absorb water from deep or shallow soil horizons during drought and after late-summer thunderstorms, which was particularly evident in vines growing on the stony soil. In fact, precipitations of 20 mm were enough for plant water status recovery, due to fast infiltration. On the other hand, at least 50 mm of rainfall were necessary to induce water status recovery in more compact soil (karst silty-clay loam). Our findings provide new knowledge on the rooting depth and water needs of vines growing on shallow soils overlying fractured limestone bedrock.

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.

Phylloxera is one of the most invasive and widespread insects in viticulture. An increase in populations feeding on leaves and/or roots of formeR + Ly resistant grapevines has been observed, but information on leaf and whole plant phylloxera infestation effects is lacking. We monitored the water and carbon metabolism of vines (one rootstock x scion combination) inoculated with insects’ eggs on leaves (L) or both leaves and roots (R+L). Nonstructural carbohydrates (NSC) in infested and noninfested tissue of different organs and plant biomass were measured at the end of the experiment. At the peak of the biotic stress treatment, the plants reduced transpiration by about 30% compared to control, while photosynthesis remained unaffected. Lower soluble NSC were measured in infested than in the nearby noninfested tissue of both L and R+L groups, suggesting sugar consumption by the insect, while infested roots increased starch content by fivefold. NSC were depleted in noninfested roots of R+L plants as well, giving strength to the hypothesis of intense metabolites translocation in favor of the insect. A more distinct physiological depression in R+L vines compared to L was highlighted, even if the total biomass reduction was more marked in L plants. Our preliminary results suggest that the insect reprograms plant metabolism stimulating a more conservative water use, while competing with the host plant for carbon resources. Further studies should validate current results and quantify the NSC invested in the plant’s defense against the pest.

In drought-stressed plants a coordinated cascade of chemical and transcriptional adjustments occurs at the same time as embolism formation. While these processes do not affect embolism formation during stress, they may prime stems for recovery during rehydration by modifying apoplast pH and increasing sugar concentration in the xylem sap. Here we show that in vivo treatments modifying apoplastic pH (stem infiltration with a pH buffer) or reducing stem metabolic activity (infiltration with sodium vanadate and sodium cyanide; plant exposure to carbon monoxide) can reduce sugar accumulation, thus disrupting or delaying the recovery process. Application of the vanadate treatment (NaVO3, an inhibitor of many ATPases) completely halted recovery from drought-induced embolism for up to 24 h after re-irrigation, while partial recovery was observed in vivo in control plants using X-ray microcomputed tomography. Our results suggest that stem hydraulic recovery in poplar is a biological, energy-dependent process that coincides with accumulation of sugars in the apoplast during stress. Recovery and damage are spatially coordinated, with embolism formation occurring from the inside out and refilling from the outside in. The outside-in pattern highlights the importance of xylem proximity to the sugars within the phloem to the embolism recovery process.

The metabolic regulations of evergreen Mediterranean Quercus ilex trees growing at extra-urban and urban sites with different percentages of surrounding impervious pavements and distance to major traffic roads were investigated at the peak of the summer drought season. Plants from extra-urban site experienced lower water stress due to a fine regulation of reactive oxygen species (ROS, concomitant rise of anion superoxide and hydrogen peroxide content, +55 and +44%), abscisic acid and water soluble carbohydrates (+111 and +9%). The level of ROS was kept under the injury threshold by both enzymatic and metabolic antioxidants. Plants from the sub-urban site followed a different strategy, with a lower production of ROS and the concomitant reduction in phenols and carotenoids. Despite these differences, indications for stress-induced injury were not observed in any of the sites. Our work shows new relations between urban stress conditions and metabolic regulations of trees growing under such harsh environments.

The seedling stage is the most susceptible one during a tree′s life. Water relations may be crucial for seedlings due to their small roots, limited water buffers and the effects of drought on water transport. Despite obvious relevance, studies on seedling xylem hydraulics are scarce as respective methodical approaches are limited. Micro‐CT scans of intact Acer pseudoplatanus and Fagus sylvatica seedlings dehydrated to different water potentials (Ψ) allowed the simultaneous observation of gas‐filled versus water‐filled conduits and the calculation of percentage loss of conductivity (PLC) in stems, roots and leaves (petioles or main veins). Additionally, anatomical analyses were performed and stem PLC measured with hydraulic techniques. In A. pseudoplatanus, petioles showed a higher Ψ at 50% PLC (Ψ50 −1.13MPa) than stems (−2.51 MPa) and roots (−1.78 MPa). The main leaf veins of F. sylvatica had similar Ψ50 values (−2.26 MPa) to stems (−2.74 MPa) and roots (−2.75 MPa). In both species, no difference between root and stems was observed. Hydraulic measurements on stems closely matched the micro‐CT based PLC calculations. Micro‐CT analyses indicated a species‐specific hydraulic architecture. Vulnerability segmentation, enabling a disconnection of the hydraulic pathway upon drought, was observed in A. pseudoplatanus but not in the especially shade‐tolerant F. sylvatica. Hydraulic patterns could partly be related to xylem anatomical traits.

Plant invasive alien species (IAS) are a serious threat to biodiversity. Several studies have compared the functional features of IAS and native species to identify the functional traits, or set of traits, favouring the process of invasion. However, most of these studies analysed traits only related to carbon and nutrients, and the inclusion of traits related to water use and acquisition might be useful to describe the functions underlying plant invasion. Here we present an analysis of cost-related, hydraulic and leaf vein traits measured on a large assemblage of woody and herbaceous native and invasive species (93 species in total, 78 natives and 15 IAS), that co-occur in site types in the Mediterranean area with different water availability (i.e. xeric, mesic and hydric sites). IAS shared lower leaf construction costs and drought resistance, but potential higher efficiency in water transport (i.e. higher values of vein length per unit area/mass) than native species. Moreover, IAS and native species separated along the trade-offs drawn by the measured traits, suggesting that hydraulic and vein traits could set an important axis of variation between IAS and native species. At last, IAS tended to occupy the fast-growth region of the functional space, independently of growth form and site type. Hydraulic and vein traits provide stronger mechanistic linkages between construction costs and photosynthetic and growth rates, thus possibly playing a central role in determining the invasive potential of IAS. IAS could reduce costs associated with leaf construction and resistance to drought stress having, at the same time, high efficiency of water transport and photosynthetic rates by developing a denser venation network, translating to higher growth rates than native and more conservative species.

The choice of the best sampling strategy to capture mean values of functional traits for a species/population, while maintaining information about traits’ variability and minimizing the sampling size and effort, is an open issue in functional trait ecology. Intraspecific variability (ITV) of functional traits strongly influences sampling size and effort. However, while adequate information is available about intraspecific variability between individuals (ITVBI) and among populations (ITVPOP), relatively few studies have analyzed intraspecific variability within individuals (ITVWI). Here, we provide an analysis of ITVWI of two foliar traits, namely specific leaf area (SLA) and osmotic potential (π), in a population of Quercus ilex L. We assessed the baseline ITVWI level of variation between the two traits and provided the minimum and optimal sampling size in order to take into account ITVWI, comparing sampling optimization outputs with those previously proposed in the literature. Different factors accounted for different amount of variance of the two traits. SLA variance was mostly spread within individuals (43.4% of the total variance), while π variance was mainly spread between individuals (43.2%). Strategies that did not account for all the canopy strata produced mean values not representative of the sampled population. The minimum size to adequately capture the studied functional traits corresponded to 5 leaves taken randomly from 5 individuals, while the most accurate and feasible sampling size was 4 leaves taken randomly from 10 individuals. We demonstrate that the spatial structure of the canopy could significantly affect traits variability. Moreover, different strategies for different traits could be implemented during sampling surveys. We partially confirm sampling sizes previously proposed in the recent literature and encourage future analysis involving different traits.

Background Intensity of drought stress and pest attacks is forecasted to increase in the near future posing a serious threat to natural and agricultural ecosystems. Knowledge on potential effects of a combined abiotic-biotic stress on whole-plant physiology is lacking. We monitored the water status and carbon metabolism of a vine rootstock with or without scion subjected to water shortening and/or infestation with the sucking insect phylloxera (Daktulosphaira vitifoliae Fitch). We measured non-structural carbohydrates and biomass of different plant organs to assess the stress-induced responses at the root, stem, and leaf level. Effects of watering on root infestation were also addressed. Results Higher root infestation was observed in drought-stressed plants compared to well-watered. The drought had a significant impact on most of the measured functional traits. Phylloxera further influenced vines water and carbon metabolism and enforced the sink strength of the roots by stimulating photosynthates translocation. The insect induced carbon depletion, reprogramed vine development, while preventing biomass compensation. A synergic effect of biotic-abiotic stress could be detected in several physiological and morphological traits. Conclusions Our results indicate that events of water shortage favour insects’ feeding damage and increase the abundance of root nodosities. Root phylloxera infestation imposes a considerable stress to the plants which might exacerbate the negative effects of drought.

Drought compromises plant's ability to replace transpired water vapor with water absorbed from the soil, leading to extensive xylem dysfunction and causing plant desiccation and death. Short-term plant responses to drought rely on stomatal closure, and on the plant's ability to recover hydraulic functioning after drought relief. We hypothesize a key role for abscisic acid (ABA) not only in the control of stomatal aperture, but also in hydraulic recovery. Young plants of Populus nigra L. were used to investigate possible relationships among ABA, non-structural carbohydrates (NSC) and xylem hydraulic function under drought and after re-watering. In Populus nigra L. plants subjected to drought, water transport efficiency and hydraulic recovery after re-watering were monitored by measuring the percentage loss of hydraulic conductivity (PLC) and stem specific hydraulic conductivity (Kstem). In the same plants ABA and NSC were quantified in wood and bark. Drought severely reduced stomatal conductance (gL) and markedly increased the PLC. Leaf and stem water potential, and stem hydraulic efficiency fully recovered within 24 h after re-watering, but gL values remained low. After re-watering, we found significant correlations between changes in ABA content and hexoses concentration both in wood and bark. Our findings suggest a role for ABA in the regulation of stem carbohydrate metabolism and starch mobilization upon drought relief, possibly promoting the restoration of xylem transport capacity.

Drought tolerance shapes the distribution of plant species, and it is mainly determined by the osmotic potential at full turgor (π0) and the water potential at turgor loss point (Ψtlp). We provide a simplified framework for π0 and Ψtlp measurements based on osmometer determination of π0 (π0_osm). Specifically, we ran regression models to i) improve the predictive power of the estimation of π0 from π0_osm and morpho-anatomical traits; ii) obtain the most accurate model to predict Ψtlp on the basis of the global relationship between π0 and Ψtlp. The inclusion of the leaf dry matter content (LDMC), an easy-to-measure trait, in the regression model improved the predictive power of the estimation of π0 from π0_osm. When π0_osm was used as a simple predictor of Ψtlp, discrepancies arose in comparison with global relationship between π0 and Ψtlp. Ψtlp values calculated as a function of the π0 derived from π0_osm and LDMC (π0_fit) were consistent with the global relationship between π0 and Ψtlp. The simplified framework provided here could encourage the inclusion of mechanistically sound drought tolerance traits in ecological studies.

Ongoing climate change is apparently increasing tree mortality rates, and understanding mechanisms of drought-induced tree decline can improve mortality projections. Differential drought impact on conspecific individuals within a population has been reported, but no clear mechanistic explanation for this pattern has emerged. Following a severe drought (summer 2012), we monitored over a 3-year period healthy (H) and declining (D) Pinus nigra trees co-occurring in a karstic woodland to highlight eventual individual-specific physiological differences underlying differential canopy dieback. We investigated differences in water and carbon metabolism, and xylem anatomy as a function of crown health status, as well as eventual genotypic basis of contrasting drought responses. H and D trees exploited the same water pools and relied on similar hydraulic strategies to cope with drought stress. Genetic analyses did not highlight differences between groups in terms of geographical provenance. Hydraulic and anatomical analyses showed conflicting results. The hydraulic tracheid diameter and theoretical hydraulic conductivity were similar, but D trees were characterized by lower water transport efficiency, greater vulnerability to xylem conduit implosion and reduced carbohydrate stores. Our results suggest that extreme drought events can have different impacts on conspecific individuals, with differential vulnerability to xylem embolism likely playing a major role in setting the fate of trees under climate change.

Xylem resistance to embolism formation determines the species-specific drought tolerance and the survival prospects of plants under extreme climatic conditions. Fourier Transform-Infrared (FTIR) spectroscopy is a cost-effective and rapid analytical tool with potential beyond its current use in plant physiology. We tested the use of FTIR spectroscopy as a method for estimating wood density (WD) and xylem resistance to embolism formation (P50) in 24 angiosperm species. Higher WD was associated with more negative P50 (r2 = 0.41). Partial least squares regression was applied to establish models of FTIR spectra and the reference data. They showed a high predictive quality for WD (r2 = 0.73), whereas the prediction of P50 was weaker (r2 = 0.49). By including WD in the model as an additional factor influencing P50, its predictive power significantly increased (r2 = 0.59). The spectral range in the model elaboration has been also narrowed (bands of lignin, cellulose, hemicellulose), but this did not influence the model descriptors, suggesting that for P50 prediction broad spectral range is more informative than narrow band regions reflecting main wood constituents. In conclusion, FTIR spectroscopy associated with WD measurements has proven to be a promising alternative to traditional methods for screening of individual- or species-specific resistance to embolism in angiosperms.

Questions: 1) Are there differences in abundance-weighted functional trait values between native and alien species in coastal plant communities? 2) Which functional traits are associated with a higher level of invasion in these communities? 3) Do functional diversity patterns differ between native and alien species? 4) Is alien species occurrence linked to small-scale functional homogenization effects on the resident native species? Location: N-Adriatic coastal ecosystems (Marano and Grado lagoon, Friuli Venezia Giulia region, Italy) Methods: We sampled coastal vegetation within two habitats (foredunes and saltmarshes) along 9 belt transects in two sampling sites. Plant species richness and abundance were assessed in 128 plots along with a suite of plant functional traits. We tested for differences in CWMs between native and alien species within the two habitats, and a Linear Mixed Model (LMMs) provided insights on traits fostering the invasion success among alien species. To check for potential functional homogenization driven by alien species invasion, we explored functional diversity patterns of native and alien species (alpha and beta functional diversity) and the relationship between alpha functional diversity and alien cover. Results: Alien species had lower functional diversity than natives and were characterized by lower leaf construction costs coupled with lower drought resistance and higher water transport efficiency. The most abundant aliens were the ones minimizing carbon investment for leaf construction. In addition, we also found evidence for small-scale functional homogenization driven by alien invasion. Conclusions: Our results suggested that native species adopt a resource conservative strategy whereas alien species are characterized by a higher resource acquisition capacity (i.e. acquisitive strategy). Our data also confirmed that alien species are less functionally diverse than natives, Functional diversity of coastal plant communities potentially driving the community towards small-scale functional homogenization, resulting in a loss of species and a reduction in the functional space.

The maintenance of hydraulic function during and after a drought event is crucial for tree survival, but the importance of non-structural carbohydrates (NSCs) in the recovery phase is still debated. We tested whether higher NSC availability facilitates post-drought hydraulic recovery, by applying a short-term drought (Sdr) and a long-term drought combined with shading (Ldr+sh) in Fraxinus ornus and Ostrya carpinifolia. Plants were then re-irrigated and recovery was checked 24 h later, by measuring water potential, stem percentage loss of hydraulic conductance (PLC) and NSC content. The relative magnitude of hydraulic and carbon constraints was also assessed in desiccated plants. During drought, PLC increased only in F. ornus, while it was maintained almost constant in O. carpinifolia due to tighter stomatal control of xylem pressure (i.e. more isohydric). In F. ornus, only Sdr plants maintained high NSC contents at the end of drought and, when re-irrigated, recovered PLC to control values. Whereas hydraulic failure was ubiquitous, only F. ornus depleted NSC reserves at mortality. Our results suggest that preserving higher NSC content at the end of a drought can be important for the hydraulic resilience of trees.

Extreme summer droughts induced differential impacts in a population of Pinus nigra trees. We analyzed eventual physiological differences between desiccated and healthy trees. Desiccated trees were more vulnerable to drought-induced xylem embolism, and were also suffering from depletion of carbohydrate reserves. Our data suggest that genotypic or phenotypic intraspecific variability can play fundamental roles in defining individual drought responses/impacts, dictating the fate of single trees under global-change-type droughts., Ongoing climate change is apparently increasing tree mortality rates, and understanding mechanisms of drought-induced tree decline can improve mortality projections. Differential drought impact on conspecific individuals within a population has been reported, but no clear mechanistic explanation for this pattern has emerged. Following a severe drought (summer 2012), we monitored over a 3-year period healthy (H) and declining (D) Pinus nigra trees co-occurring in a karstic woodland to highlight eventual individual-specific physiological differences underlying differential canopy dieback. We investigated differences in water and carbon metabolism, and xylem anatomy as a function of crown health status, as well as eventual genotypic basis of contrasting drought responses. H and D trees exploited the same water pools and relied on similar hydraulic strategies to cope with drought stress. Genetic analyses did not highlight differences between groups in terms of geographical provenance. Hydraulic and anatomical analyses showed conflicting results. The hydraulic tracheid diameter and theoretical hydraulic conductivity were similar, but D trees were characterized by lower water transport efficiency, greater vulnerability to xylem conduit implosion and reduced carbohydrate stores. Our results suggest that extreme drought events can have different impacts on conspecific individuals, with differential vulnerability to xylem embolism likely playing a major role in setting the fate of trees under climate change.

Green roofs are gaining momentum in the arid and semi-arid regions due to their multiple benefits as compared with conventional roofs. One of the most critical steps in green roof installation is the selection of drought and heat tolerant species that can thrive under extreme microclimate conditions. We monitored the water status, growth and survival of 11 drought-adapted shrub species grown on shallow green roof modules (10 and 13cm deep substrate) and analyzed traits enabling plants to cope with drought (symplastic and apoplastic resistance) and heat stress (root membrane stability). The physiological traits conferring efficiency/safety to the water transport system under severe drought influenced plant water status and represent good predictors of both plant water use and growth rates over green roofs. Moreover, our data suggest that high substrate temperature represents a stress factor affecting plant survival to a larger extent than drought per se. In fact, the major cause influencing seedling survival on shallow substrates was the species-specific root resistance to heat, a single and easy measurable trait that should be integrated into the methodological framework for screening and selection of suitable shrub species for roof greening in the Mediterranean.

In 2012, an extreme summer drought induced species-specific die-back in woody species in NE-Italy. Quercus pubescens and Ostrya carpinifolia were heavily impacted, while Prunus mahaleb was largely unaffected. By comparing seasonal changes in isotopic composition of xylem sap, rainfall and deep soil samples, we show that P. mahaleb has a deeper root system than the other two species. This morphological trait allowed P. mahaleb to maintain higher water potential (Ψ), gas exchange rates and non-structural carbohydrates content (NSC) throughout the summer, when compared to the other species. More favourable water and carbon states allowed relatively stable maintenance of stem hydraulic conductivity (k) throughout the growing season. In contrast, in Q. pubescens and O. carpinifolia decreasing Ψ and NSC were associated with significant hydraulic failure, with spring-to-summer k loss averaging 60%. Our data support to the hypothesis that drought-induced tree decline is a complex phenomenon that cannot be modelled on the basis of single predictors of tree status like hydraulic efficiency, vulnerability, and carbohydrate content. Our data highlight the role of rooting depth in seasonal progression of water status, gas exchange and NSC, with possible consequences for energy-demanding mechanisms involved in the maintenance of vascular integrity. This article is protected by copyright. All rights reserved.

Invasion of natural habitats by alien trees is a threat to forest conservation. Our understanding of fundamental ecophysiological mechanisms promoting plant invasions is still limited, and hydraulic and water relation traits have been only seldom included in studies comparing native and invasive trees. We compared several leaf and wood functional and mechanistic traits in co-occurring Ailanthus altissima (Mill.) Swingle (Aa) and Fraxinus ornus L. (Fo). Aa is one of the most invasive woody species in Europe and North America, currently outcompeting several native trees including Fo. We aimed at quantifying inter-specific differences in terms of: (i) performance in resource use and acquisition; (ii) hydraulic efficiency and safety; (iii) carbon costs associated to leaf and wood construction; and (iv) plasticity of functional and mechanistic traits in response to light availability. Traits related to leaf and wood construction and drought resistance significantly differed between the two species. Fo sustained higher structural costs than Aa, but was more resistant to drought. The lower resistance to drought stress of Aa was counterbalanced by higher water transport efficiency, but possibly required mechanisms of resilience to drought-induced hydraulic damage. Larger phenotypic plasticity of Aa in response to light availability could also promote the invasive potential of the species.

Drought tolerance was greater in the whole lichen than in its isolated photobiont. Cell turgor state has an influence on the functionality of photosynthetic process in lichens. Irreversible thermodynamics is widely used to describe the water relations of vascular plants. However, poikilohydrous organisms like lichens and aeroterrestrial microalgae have seldom been studied using this approach. Water relations of lichens are generally addressed without separate analysis of the mycobiont and photobiont, and only few studies have correlated changes in photosynthetic efficiency of dehydrating lichens to accurate measurements of their water potential components. We measured water potential isotherms and chlorophyll a fluorescence in the lichen Flavoparmelia caperata harvested in different seasons, as well as in its isolated photobiont, the green alga Trebouxia gelatinosa, either exposed to water stress cycles or fully hydrated. No significant seasonal trends were observed in lichen water relations parameters. Turgor loss point and osmotic potential of the whole thallus were significantly lower than those measured in the photobiont, while differences between the water stressed photobiont and controls were not significant. Dehydration-induced drop of F v/F m was correlated with turgor loss, revealing that the photosynthetic activity of lichens partly depends on their turgor level. We provided one of the first quantitative evidences of the influence that turgor status could exert on the functionality of photosynthetic processes in lichens.

Synchrotron X-ray computed micro-tomography (microCT) has emerged as a promising noninvasive technique for in vivo monitoring of xylem function, including embolism build-up under drought and hydraulic recovery following re-irrigation. Yet, the possible harmful effects of ionizing radiation on plant tissues have never been quantified. We specifically investigated the eventual damage suffered by stem living cells of three different species exposed to repeated microCT scans. Stem samples exposed to one, two or three scans were used to measure cell membrane and RNA integrity, and compared to controls never exposed to X-rays. Samples exposed to microCT scans suffered serious alterations to cell membranes, as revealed by marked increase in relative electrolyte leakage, and also underwent severe damage to RNA integrity. The negative effects of X-rays were apparent in all species tested, but the magnitude of damage and the minimum number of scans inducing negative effects were species-specific. Our data show that multiple microCT scans lead to disruption of fundamental cellular functions and processes. Hence, microCT investigation of phenomena that depend on physiological activity of living cells may produce erroneous results and lead to incorrect conclusions.

Novel features related to turgor loss adjustment and higher ability to modulate leaf-construction costs could improve A. altissima fitness in its invaded range. Phenotypic plasticity has been recently proposed to play an important role in invasion process. More precisely, phenotypic plasticity of alien invasive species could be higher in their exotic rather than native habitat, probably because of the release from biotic or abiotic constraints. In this paper, several plant functional traits were analysed on Ailanthus altissima (Mill.) Swingle, a highly invasive species in Europe, aiming at providing a comparison among key functional traits measured on individuals growing in their native habitat and to test if values of functional traits and their related plasticity are higher in exotic habitats. Our analysis pinpointed that variability of functional traits in the native habitat was mainly driven by different irradiance, temperature and evaporative demand of the studied sites, in accordance to the traits trade-offs in the Leaf Economic Spectrum. Physiological traits related to drought tolerance were different between native and exotic habitats. In the native one, A. altissima relied on osmoregulation processes to adjust leaf turgor loss point (Ψtlp), as the osmotic potential at full turgor (π0) were lower in drier sites. In the exotic habitat, individuals in drier sites had similar π0 but lower wall elasticity (e), suggesting that leaves had thinner cell walls. Moreover, plasticity in specific leaf area and e were higher in the exotic habitat. The novel features related to Ψtlp adjustment and the higher plasticity in traits related to leaf-construction costs could increase A. altissima fitness in response to different environmental conditions in its invasive range.

Ongoing variations in rainfall and temperature regimes affect the physiology and productivity of grapevines, calling for irrigation in drought-prone areas. During vintage 2015, we monitored plants water status and indirectly assessed rooting depth and exploited water sources (oxygen isotope analyses) in a mature Vitis vinifera cv. Malvasia Istriana vineyard on red soils ("terra rossa") developed on highly permeable carbonate rocks. We also investigated effects of topsoil irrigation or late summer rains on plant water status and yield. Under the harsh summer environmental conditions of 2015, the plant water status was overall favorable (moderate water deficit) and never reached critical levels, suggesting that irrigation was not mandatory. Leaf conductance to water vapor (g(L)) measured in July decreased by about 70% compared to spring, while minimum leaf water potential (Psi(min)) dropped by only 16%, suggesting an isohydric behavior of the cultivar (strict stomatal control of transpiration). Both Psi(min) and g(L) reached a minimum in July (peak of drought), and returned to pre-drought values in late summer. Rainfalls or supplemental irrigation (about 40 mm) promoted prompt recovery of plant water status. Irrigation treatments or occasional summer rainfalls can influence the water status of plants, although roots have access to deep water sources. In fact, the isotopic composition of xylem sap was similar to that of soil water sampled in a nearby deep cave, supporting the hypothesis that deep soil is the main water source for grapevines in karstic areas during summertime. Deficit irrigation, based on careful evaluation of physiological indicators of plant water status, might be an effective strategy for promoting sustainable viticulture, and a rationale use of water resources in karstic ecosystems.

Easily measurable functional traits can be used as proxies in the selection of drought-tolerant saplings for reforestation in Mediterranean ecosystems. Heat and drought events—increasing both in frequency and severity—have led to forest decline, and are a serious threat for the Mediterranean biome. Whereas drought tolerance of adult trees of different Mediterranean species has been widely investigated, this is not the case for saplings and young trees. We analysed correlations and trade-offs among leaf (water potential at the turgor loss point, Ψ tlp, modulus of elasticity, e, osmotic potential at full turgor, π 0, leaf capacitance, C leaf_dw, leaf venation, VLA, leaf mass per area, LMA) and stem (wood capacitance and wood density, C wood and D wood, stem-specific conductivity and water potential inducing 50% loss of hydraulic conductance) functional traits of saplings for 14 woody species of the Mediterranean flora. The results support previously reported correlations among functional traits known to confer drought tolerance to plants. In particular, Ψ tlp was positively correlated to π 0, C leaf_dw and VLA, while negatively correlated to e and LMA. A highly significant correlation was highlighted between C wood and D wood. Overall, we observed surprisingly low symplastic and apoplastic resistance. We identify some easily measurable traits (π 0 and LMA), which evidence seedlings’ ability to cope with drought, and which therefore could be used as proxies in the selection of drought-tolerant saplings for reforestation in Mediterranean areas.

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.

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.

Vulnerability curves (VCs) are a useful tool to investigate the susceptibility of plants to drought-induced hydraulic failure, and several experimental techniques have been used for their measurement. The validity of the bench dehydration method coupled to hydraulic measurements, considered as a 'golden standard', has been recently questioned calling for its validation with non-destructive methods. We compared the VCs of a herbaceous crop plant (Helianthus annuus) obtained during whole-plant dehydration followed by i) hydraulic flow measurements in stem segments (classical destructive method) or by ii) in vivo micro-CT observations of stem xylem conduits in intact plants. The interpolated P50 values (xylem water potential inducing 50% loss of hydraulic conductance) were −1.74 MPa and −0.87 MPa for the hydraulic and the micro-CT VC, respectively. Interpolated P20 values were similar, while P50 and P80 were significantly different, as evidenced by non-overlapping 95% confidence intervals. Our results did not support the tension-cutting artefact, as no overestimation of vulnerability was observed when comparing the hydraulic VC to that obtained with in vivo imaging. After one scan, 25% of plants showed signs of x-ray induced damage, while three successive scans caused the formation of a circular brownish scar in all tested plants. Our results support the validity of hydraulic measurements of samples excised under tension provided standard sampling and handling protocols are followed, but also show that caution is needed when investigating vital plant processes with x-ray imaging.

Summary Urban trees help towns to cope with climate warming by cooling both air and surfaces. The challenges imposed by the urban environment, with special reference to low water availability due to the presence of extensive pavements, result in high rates of mortality of street trees, that can be increased by climatic extremes. We investigated the water relations and xylem hydraulic safety/efficiency of Quercus ilex trees growing at urban sites with different percentages of surrounding impervious pavements. Seasonal changes of plant water potential and gas exchange, vulnerability to cavitation and embolism level, and morpho-anatomical traits were measured. We found patterns of increasing water stress and vulnerability to drought at increasing percentages of impervious pavement cover, with a consequent reduction in gas exchange rates, decreased safety margins toward embolism development, and increased vulnerability to cavitation, suggesting the occurrence of stress-induced hydraulic deterioration. The amount of impermeable surface and chronic exposure to water stress influence the site-specific risk of drought-induced dieback of urban trees under extreme drought. Besides providing directions for management of green spaces in towns, our data suggest that xylem hydraulics is key to a full understanding of the responses of urban trees to global change.

Climate features of the Mediterranean area make plant survival over green roofs challenging, thus calling for research work to improve water holding capacities of green roof systems. We assessed the effects of polymer hydrogel amendment on the water holding capacity of a green roof substrate, as well as on water status and growth of Salvia officinalis. Plants were grown in green roof experimental modules containing 8 cm or 12 cm deep substrate (control) or substrate mixed with hydrogel at two different concentrations: 0.3 or 0.6%. Hydrogel significantly increased the substrate's water content at saturation, as well as water available to vegetation. Plants grown in 8 cm deep substrate mixed with 0.6% of hydrogel showed the best performance in terms of water status and membrane integrity under drought stress, associated to the lowest above-ground biomass. Our results provide experimental evidence that polymer hydrogel amendments enhance water supply to vegetation at the establishment phase of a green roof. In particular, the water status of plants is most effectively improved when reduced substrate depths are used to limit the biomass accumulation during early growth stages. A significant loss of water holding capacity of substrate-hydrogel blends was observed after 5 months from establishment of the experimental modules. We suggest that cross-optimization of physical-chemical characteristics of hydrogels and green roof substrates is needed to improve long term effectiveness of polymer-hydrogel blends.

The influence of different green roof layering types on the amount of water available to plants was investigated in the specific climatic context of the Mediterranean region. Water status, productivity and survival rate of Salvia officinalis L. plants growing in experimental green roof modules were monitored between early spring and late summer. Experimental data showed that: (a) substrate and water retention layer retained respectively 34% and 90% in volume of water potentially available to plants; (b) water retention layer had a positive effect on plant water status and survival; (c) the design of the overall green roof system, and in particular the characteristics of the drainage layer, influenced the amount of water transferred between different green roof elements, thus significantly influencing the amount of water available to plants. In particular, significant amounts of water were shown to be transferred from the retention layer into the cavities of the plastic drainage layer in response to temperature fluctuations, leading to day/night cycles of water evaporation/condensation. Targeted modifications of the geometrical and technical features of drainage elements were shown to be potentially useful to improve plant survival during intense and/or prolonged drought events.

Efficient leaf water supply is fundamental for assimilation processes and tree growth. Renovating the architecture of the xylem transport system requires an increasing carbon investment while growing taller, and any deficiency of carbon availability may result in increasing hydraulic constraints to water flow. Therefore, plants need to coordinate carbon assimilation and biomass allocation to guarantee an efficient and safe long-distance transport system. We tested the hypothesis that reduced branch elongation rates together with carbon-saving adjustments of xylem anatomy hydraulically compensate for the reduction in biomass allocation to xylem. We measured leaf biomass, hydraulic and anatomical properties of wood segments along the main axis of branches in 10 slow growing (SG) and 10 fast growing (FG) Fraxinus ornus L. trees. Branches of SG trees had five times slower branch elongation rate (7 vs 35 cm year-1), and produced a higher leaf biomass (P < 0.0001) and thinner xylem rings with fewer but larger vessels (P < 0.0001). On the contrary, we found no differences between SG and FG trees in terms of leaf-specific conductivity (P > 0.05) and xylem safety (φ50 ≈ -3.2 MPa). Slower elongation rate coupled with thinner annual rings and larger vessels allows the reduction of carbon costs associated with growth, while maintaining similar leafspecific conductivity and xylem safety.

International audience; Prescribed burning (PB) is a widespread management technique for wildfire hazard abatement. Understanding PB effects on tree ecophysiology is key to defining burn prescriptions aimed at reducing fire hazard in Mediterranean pine plantations, such as Pinus pinea L. stands. We assessed physiological responses of adult P. pinea trees to PB using a combination of dendroecological, anatomical, hydraulic and isotopic analyses. Tree-ring widths, xylem cell wall thickness, lumen area, hydraulic diameter and tree-ring δ 13 C and δ 18 O were measured in trees on burned and control sites. Vulnerability curves were elaborated to assess tree hydraulic efficiency or safety. Despite the relatively intense thermal treatment (the residence time of temperatures above 50 °C at the stem surface ranged between 242 and 2239 s), burned trees did not suffer mechanical damage to stems, nor significant reduction in radial growth. Moreover, the PB did not affect xylem structure and tree hydraulics. No variations in 13 C-derived water use efficiency were recorded. This confirmed the high resistance of P. pinea to surface fire at the stem base. However, burned trees showed consistently lower δ 18 O values in the PB year, as a likely consequence of reduced competition for water and nutrients due to the understory burning, which increased both photosynthetic activity and stomatal conductance. Our multi-approach analysis offers new perspectives on post-fire survival strategies of P. pinea in an environment where fires are predicted to increase in frequency and severity during the 21st century.

Recent reports on tree mortality associated with anomalous drought and heat have raised interest into processes underlying tree resistance/resilience to water stress. Hydraulic failure and carbon starvation have been proposed as main causes of tree decline, with recent theories treating water and carbon metabolism as interconnected processes. We subjected young plants of two native (Quercus pubescens [Qp] and Prunus mahaleb [Pm]) and two invasive (Robinia pseudoacacia [Rp] and Ailanthus altissima [Aa]) woody angiosperms to a prolonged drought leading to stomatal closure and xylem embolism, to induce carbon starvation and hydraulic failure. At the end of the treatment, plants were measured for embolism rates and NSC content, and re-irrigated to monitor recovery of xylem hydraulics. Data highlight different hydraulic strategies in native vs invasive species under water stress, and provide physiological explanations for species-specific impacts of recent severe droughts. Drought-sensitive species (Qp and Rp) suffered high embolism rates and were unable to completely refill xylem conduits upon restoration of water availability. Species that better survived recent droughts were able to limit embolism build-up (Pm) or efficiently restored hydraulic functionality after irrigation (Aa). Species-specific capacity to reverse xylem embolism correlated to stem-level concentration of soluble carbohydrates, but not to starch content.

Drought stress can impair leaf hydraulic conductance (Kleaf), but the relative contribution of changes in the efficiency of the vein xylem water pathway and in the mesophyll route outside the xylem in driving the decline of Kleaf is still debated. We report direct measurements of dehydration-induced changes in the hydraulic resistance (R=1/K) of whole leaf (Rleaf), as well as of the leaf xylem (Rx) and extra-vascular pathways (Rox) in four Angiosperm species. Rleaf, Rx, and Rox were measured using the vacuum chamber method (VCM). Rleaf values during progressive leaf dehydration were also validated with measurements performed using the rehydration kinetic method (RKM). We analysed correlations between changes in Rx or Rox and Rleaf, as well as between morpho-anatomical traits (including dehydration-induced leaf shrinkage), vulnerability to embolism, and leaf water relation parameters. Measurements revealed that the relative contribution of vascular and extra-vascular hydraulic properties in driving Kleaf decline during dehydration is species-specific. Whilst in two study species the progressive impairment of both vascular and extra-vascular pathways contributed to leaf hydraulic vulnerability, in the other two species the vascular pathway remained substantially unaltered during leaf dehydration, and Kleaf decline was apparently caused only by changes in the hydraulic properties of the extra-vascular compartment.

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 aquaporins (AQPs) are involved in the transport of water and other small solutes across cell membranes, and thus play major roles in the regulation of plant water balance, as well as in growth regulation and response to abiotic stress factors. Limited information is currently available about the presence and role of AQPs in Coffea arabica L., despite the economic importance of the species and its vulnerability to drought stress. We identified candidate AQP genes by screening a proprietary C. arabica transcriptome database, resulting in the identification of nine putative aquaporins. A phylogenetic analysis based on previously characterized AQPs from Arabidopsis thaliana and Solanum tuberosum allowed to assign the putative coffee AQP sequences to the Tonoplast (TIP) and Plasma membrane (PIP) subfamilies. The possible functional role of coffee AQPs was explored by measuring hydraulic conductance and aquaporin gene expression on leaf and root tissues of two-year-old plants ( C. arabica cv. Pacamara) subjected to different experimental conditions. In a first experiment, we tested plants for root and leaf hydraulic conductance both before dawn and at mid-day, to check the eventual impact of light on AQP activity and plant hydraulics. In a second experiment, we measured plant hydraulic responses to different water stress levels as eventually affected by changes in AQPs expression levels. Our results shed light on the possible roles of AQPs in the regulation of C. arabica hydraulics and water balance, opening promising research lines to improve the sustainability of coffee cultivation under global climate change scenarios.

Green roofs are artificial ecosystems providing ecological, economic, and social benefits to urban areas. Recently, the interest in roof greening has increased even in Mediterranean and sub-Mediterranean areas, despite the climatic features and reduced substrate depth expose plants to extreme stress. To limit installation weight and costs, recent green roof research aims to reduce substrate depth, which apparently contrasts with the need to maximize the amount of water available to vegetation. We monitored water status, growth, and evapotranspiration of drought-adapted shrubs (Cotinus coggygria, Prunus mahaleb) growing in experimental green roof modules filled with 10 or 13 cm deep substrate. Experimental data showed that: (a) reduced substrate depth translated into less severe water stress experienced by plants; (b) shallower substrate indirectly promoted lower water consumption by vegetation as a likely consequence of reduced plant biomass; (c) both large and small rainfalls induced better recovery of water content of substrate, drainage, and water retention layers when shallow substrate was used. Evidence was provided for the possibility to install extensive green roofs vegetated with stress-tolerant shrubs in sub-Mediterranean areas using 10 cm deep substrate. Green roofs based on the combination of shallow substrate and drought-tolerant plants may be an optimal solution for solving urban ecological issues.

Recent studies have reported correlations between stem sapwood capacitance (C(wood)) and xylem vulnerability to embolism, but it is unclear how C(wood) relates to the eventual ability of plants to reverse embolism. We investigated possible functional links between embolism reversal efficiency, C(wood), wood density (WD), vulnerability to xylem embolism and hydraulic safety margins in nine woody species native to dry sclerophyllous forests with different degrees of iso versus anisohydry. Substantial inter-specific differences in terms of seasonal/diurnal changes of xylem and leaf water potential, maximum diurnal values of transpiration rate and xylem vulnerability to embolism formation were recorded. Significant diurnal changes in percentage loss of hydraulic conductivity (PLC) were recorded for five species. Significant correlations were recorded between diurnal PLC changes and P50 and P88 values (i.e., xylem pressure inducing 50 and 88% PLC, respectively) as well as between diurnal PLC changes and safety margins referenced to P50 and P88. WD was linearly correlated with minimum diurnal leaf water potential, diurnal PLC changes and wood capacitance across all species. In contrast, significant relationships between P50, safety margin values referenced to P50 and WD were recorded only for the isohydric species. Functional links between diurnal changes in PLC, hydraulic strategies and WD and C(wood) are discussed.

Leaf hydraulic conductance (Kleaf) and drought vulnerability in terms of leaf water potential inducing 50% loss of Kleaf (P50), were assessed in four genotypes of Coffea arabica L. We tested three hypotheses: (1) leaf P50 is lower in small leaves with higher vein densities; (2) lower P50 translates into lower Kleaf, limiting gas exchange rates and higher leaf mass per unit area (LMA); (3) P50 values are coordinated with symplastic drought tolerance. We found partial support for Hypotheses 1 and 3, but not for Hypothesis 2. Significant correlations existed among leaf size, vein network and drought resistance. Smaller leaves displayed higher major vein density, higher Kleaf and more negative P50. Kleaf was correlated with leaf gas exchange rates. A negative relationship was observed between Kleaf and LMA, whereas P50 was found to be positively correlated with LMA. Across coffee genotypes, reduced leaf surface area and increased vein density shifts P50 towards more negative values while not translating into higher LMA or lower Kleaf. Breeding crop varieties for both increased safety of the leaf hydraulic system towards drought-induced dysfunction and high gas exchange rates per unit of leaf area is probably a feasible target for future adaptation of crops to climate change scenarios.

Summary Plant water status and hydraulics were measured in six woody angiosperms growing in a karstic woodland, during an extreme summer drought. Our aim was to take advantage of an unusual climatic event to identify key traits related to species-specific drought damage. The damage suffered by different species was assessed in terms of percentage of individuals showing extensive crown desiccation. Stem water potential (Ψstem) and percent loss of hydraulic conductivity (PLC) were measured in healthy and desiccated individuals. Vulnerability to cavitation was assessed in terms of stem water potential inducing 50% PLC (Ψ50). Stem density (ρstem) was also measured. Species-specific percentage of desiccated individuals was correlated to Ψ50 and ρstem. Crown desiccation was more widespread in species with less negative Ψ50 and lower ρstem. Desiccated individuals had lower Ψstem and higher PLC than healthy ones, suggesting that hydraulic failure was an important mechanism driving shoot dieback. Drought-vulnerable species showed lower safety margins (Ψstem − Ψ50) than resistant ones. The Ψ50, safety margins and ρstem values emerge as convenient traits to be used for tentative predictions of differential species-specific impact of extreme drought events on a local scale. The possibility that carbohydrate depletion was also involved in induction of desiccation symptoms is discussed.

Functional coordination between leaf and stem hydraulics has been proposed as a key trait of drought-resistant plants. A balanced water transport efficiency and safety of different plant organs might be of particular importance for plant survival in the Mediterranean climate. We monitored seasonal changes of leaf and stem water relations of Salvia officinalis L. in order to highlight strategies adopted by this species to survive in harsh environmental conditions. During summer drought, the water potential dropped below the turgor loss point thus reducing water loss by transpiration, whereas the photosynthetic efficiency remained relatively high. Leaves lost their water transport efficiency earlier than stems, although in both plant organs P50 (water potential inducing 50% loss of hydraulic conductivity) indicated surprisingly high vulnerability when compared with other drought-tolerant species. The fast recovery of leaf turgor upon restoration of soil water availability suggests that the reduction of leaf hydraulic conductance is not only a consequence of vein embolism, but cell shrinkage and consequent increase of resistance in the extra-xylem pathway may play an important role. We conclude that the drought tolerance of S. officinalis arises at least partly as a consequence of vulnerability segmentation.

More frequently occurring, drought waves call for a deeper understanding of tree hydraulics and fast and easily applicable methods to measure drought stress. The aim of this study was to establish empirical relationships between the percent loss of hydraulic conductivity (PLC) and the relative water loss (RWL) in woody stem axes with different P50, i.e. the water potential (Ψ) that causes 50% conductivity loss. Branches and saplings of temperate conifer (Picea abies, Larix decidua) and angiosperm species (Acer campestre, Fagus sylvatica, Populus x canescens, Populus tremula, Sorbus torminalis) and trunk wood of mature P. abies trees were analyzed. P50 was calculated from hydraulic measurements following bench top dehydration or air injection. RWL and PLC were fitted by linear, quadratic or cubic equations. Speciesor age-specific RWLs at P50 varied between 10 and 25% and P88, the Ψ that causes 88% conductivity loss, between 18 and 44%. P50 was predicted from the relationship between Ψ and the RWL. The predictive quality for P50 across species was almost 1:1 (r2 =0.99). The approach presented allows thus reliable and fast prediction of PLC from RWL. Branches and saplings with high hydraulic vulnerability tended to have lower RWLs at P50 and at P88. The results are discussed with regard to the different water storage capacities in sapwood and survival strategies under drought stress. Potential applications are screening trees for drought sensitivity and a fast interpretation of diurnal, seasonal or drought induced changes in xylem water content upon their impact on conductivity loss. Estación Experimental Agropecuaria Bariloche Fil: Rosner, Sabine. University of Natural Resources and Life Sciences. Institute of Botany; Austria Fil: Heinzeb, Berthold. Austrian Research Centre for Forest. Department of Forest Genetics; Austria Fil: Savia, Tadeja. University of Natural Resources and Life Sciences; Austria Fil: Dalla Salda, Guillermina. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Bariloche. Área Sistemas Forestales. Grupo de Ecología Forestal; Argentina