Plant hydraulics at the heart of plant, crops and ecosystem functions in the face of climate change.

Plant hydraulics is crucial for assessing the plants' capacity to extract and transport water from the soil up to their aerial organs. Along with their capacity to exchange water between plant compartments and regulate evaporation, hydraulic properties determine plant water relations, water status and susceptibility to pathogen attacks. Consequently, any variation in the hydraulic characteristics of plants is likely to significantly impact various mechanisms and processes related to plant growth, survival and production, as well as the risk of biotic attacks and forest fire behaviour. However, the integration of hydraulic traits into disciplines such as plant pathology, entomology, fire ecology or agriculture can be significantly improved. This review examines how plant hydraulics can provide new insights into our understanding of these processes, including modelling processes of vegetation dynamics, illuminating numerous perspectives for assessing the consequences of climate change on forest and agronomic systems, and addressing unanswered questions across multiple areas of knowledge.

Mechanisms of grapevine resilience to a vascular disease: investigating stem radial growth, xylem development and physiological acclimation.

BACKGROUND AND AIMS: Plant vascular diseases significantly impact crop yield worldwide. Esca is a vascular disease of grapevine found globally in vineyards which causes a loss of hydraulic conductance due to the occlusion of xylem vessels by tyloses. However, the integrated response of plant radial growth and physiology in maintaining xylem integrity in grapevine expressing esca symptoms remains poorly understood. METHODS: We investigated the interplay between variation in stem diameter, xylem anatomy, plant physiological response and hydraulic traits in two widespread esca-susceptible cultivars, 'Sauvignon blanc' and 'Cabernet Sauvignon'. We used an original experimental design using naturally infected mature vines which were uprooted and transplanted into pots allowing for their study in a mini-lysimeter glasshouse phenotyping platform. KEY RESULTS: Esca significantly altered the timing and sequence of stem growth periods in both cultivars, particularly the shrinkage phase following radial expansion. Symptomatic plants had a significantly higher density of occluded vessels and lower leaf and whole-plant gas exchange. Esca-symptomatic vines showed compensation mechanisms, producing numerous small functional xylem vessels later in development suggesting a maintenance of stem vascular cambium activity. Stabilization or late recovery of whole-plant stomatal conductance coincided with new healthy shoots at the top of the plant after esca symptoms plateaued. CONCLUSIONS: Modified cropping practices, such as avoiding late-season topping, may enhance resilience in esca-symptomatic plants. These results highlight that integrating dendrometers, xylem anatomy and gas exchange provides insights into vascular pathogenesis and its effects on plant physiology.

Evolutionary relationships between drought-related traits and climate shape large hydraulic safety margins in western North American oaks.

Quantitative knowledge of xylem physical tolerance limits to dehydration is essential to understanding plant drought tolerance but is lacking in many long-vessel angiosperms. We examine the hypothesis that a fundamental association between sustained xylem water transport and downstream tissue function should select for xylem that avoids embolism in long-vessel trees by quantifying xylem capacity to withstand air entry of western North American oaks (Quercus spp.). Optical visualization showed that 50% of embolism occurs at water potentials below -2.7 MPa in all 19 species, and -6.6 MPa in the most resistant species. By mapping the evolution of xylem vulnerability to embolism onto a fossil-dated phylogeny of the western North American oaks, we found large differences between clades (sections) while closely related species within each clade vary little in their capacity to withstand air entry. Phylogenetic conservatism in xylem physical tolerance, together with a significant correlation between species distributions along rainfall gradients and their dehydration tolerance, suggests that closely related species occupy similar climatic niches and that species' geographic ranges may have shifted along aridity gradients in accordance with their physical tolerance. Such trends, coupled with evolutionary associations between capacity to withstand xylem embolism and other hydraulic-related traits, yield wide margins of safety against embolism in oaks from diverse habitats. Evolved responses of the vascular system to aridity support the embolism avoidance hypothesis and reveal the importance of quantifying plant capacity to withstand xylem embolism for understanding function and biogeography of some of the Northern Hemisphere's most ecologically and economically important plants.

Model-assisted ideotyping reveals trait syndromes to adapt viticulture to a drier climate.

Climate change is challenging the resilience of grapevine (Vitis), one of the most important crops worldwide. Adapting viticulture to a hotter and drier future will require a multifaceted approach including the breeding of more drought-tolerant genotypes. In this study, we focused on plant hydraulics as a multi-trait system that allows the plant to maintain hydraulic integrity and gas exchange rates longer under drought. We quantified a broad range of drought-related traits within and across Vitis species, created in silico libraries of trait combinations, and then identified drought tolerant trait syndromes. By modeling the maintenance of hydraulic integrity of current cultivars and the drought tolerant trait syndromes, we identified elite ideotypes that increased the amount of time they could experience drought without leaf hydraulic failure. Generally, elites exhibited a trait syndrome with lower stomatal conductance, earlier stomatal closure, and a larger hydraulic safety margin. We demonstrated that, when compared with current cultivars, elite ideotypes have the potential to decrease the risk of hydraulic failure across wine regions under future climate scenarios. This study reveals the syndrome of traits that can be leveraged to protect grapevine from experiencing hydraulic failure under drought and increase drought tolerance.

Drought survival in conifer species is related to the time required to cross the stomatal safety margin.

The regulation of water loss and the spread of xylem embolism have mostly been considered separately. The development of an integrated approach taking into account the temporal dynamics and relative contributions of these mechanisms to plant drought responses is urgently needed. Do conifer species native to mesic and xeric environments display different hydraulic strategies and temporal sequences under drought? A dry-down experiment was performed on seedlings of four conifer species differing in embolism resistance, from drought-sensitive to extremely drought-resistant species. A set of traits related to drought survival was measured, including turgor loss point, stomatal closure, minimum leaf conductance, and xylem embolism resistance. All species reached full stomatal closure before the onset of embolism, with all but the most drought-sensitive species presenting large stomatal safety margins, demonstrating that highly drought-resistant species do not keep their stomata open under drought conditions. Plant dry-down time to death was significantly influenced by the xylem embolism threshold, stomatal safety margin, and minimum leaf conductance, and was best explained by the newly introduced stomatal margin retention index (SMRIΨ50) which reflects the time required to cross the stomatal safety margin. The SMRIΨ50 may become a key tool for the characterization of interspecific drought survival variability in trees.

Is a seasonally reduced growth potential a convergent strategy to survive drought and frost in plants?

BACKGROUND: Plants have adapted to survive seasonal life-threatening frost and drought. However, the timing and frequency of such events are impacted by climate change, jeopardizing plant survival. Understanding better the strategies of survival to dehydration stress is therefore timely and can be enhanced by the cross-fertilization of research between disciplines (ecology, physiology), models (woody, herbaceous species) and types of stress (drought, frost). SCOPE: We build upon the 'growth-stress survival' trade-off, which underpins the identification of global plant strategies across environments along a 'fast-slow' economics spectrum. Although phenological adaptations such as dormancy are crucial to survive stress, plant global strategies along the fast-slow economic spectrum rarely integrate growth variations across seasons. We argue that the growth-stress survival trade-off can be a useful framework to identify convergent plant ecophysiological strategies to survive both frost and drought. We review evidence that reduced physiological activity, embolism resistance and dehydration tolerance of meristematic tissues are interdependent strategies that determine thresholds of mortality among plants under severe frost and drought. We show that complete dormancy, i.e. programmed growth cessation, before stress occurrence, minimizes water flows and maximizes dehydration tolerance during seasonal life-threatening stresses. We propose that incomplete dormancy, i.e. the programmed reduction of growth potential during the harshest seasons, could be an overlooked but major adaptation across plants. Quantifying stress survival in a range of non-dormant versus winter- or summer-dormant plants, should reveal to what extent incomplete to complete dormancy could represent a proxy for dehydration tolerance and stress survival. CONCLUSIONS: Our review of the strategies involved in dehydration stress survival suggests that winter and summer dormancy are insufficiently acknowledged as plant ecological strategies. Incorporating a seasonal fast-slow economics spectrum into global plant strategies improves our understanding of plant resilience to seasonal stress and refines our prevision of plant adaptation to extreme climatic events.

Photosynthesis, leaf hydraulic conductance and embolism dynamics in the resurrection plant Barbacenia purpurea.

The main parameters determining photosynthesis are stomatal and mesophyll conductance and electron transport rate, and for hydraulic dynamics they are leaf hydraulic conductance and the spread of embolism. These parameters have scarcely been studied in desiccation-tolerant (resurrection) plants exposed to drought. Here, we characterized photosynthesis and hydraulics during desiccation and rehydration in a poikilochlorophyllous resurrection plant, Barbacenia purpurea (Velloziaceae). Gas exchange, chlorophyll fluorescence, and leaf water status were monitored along the whole dehydration-rehydration cycle. Simultaneously, embolism formation and hydraulic functioning recovery were measured at leaf level using micro-computed tomography imaging. Photosynthesis and leaf hydraulic conductance ceased at relatively high water potential (-1.28 and -1.54 MPa, respectively), whereas the onset of leaf embolism occurred after stomatal closure and photosynthesis cessation (<-1.61 MPa). This sequence of physiological processes during water stress may be associated with the need to delay dehydration, to prepare the molecular changes required in the desiccated state. Complete rehydration occurred rapidly in the mesophyll, whereas partial xylem refilling, and subsequent recovery of photosynthesis, occurred at later stages after rewatering. These results highlight the importance of stomata as safety valves to protect the vascular system from embolism, even in a plant able to fully recover after complete embolism.

Cross-validation of the high-capacity tensiometer and thermocouple psychrometer for continuous monitoring of xylem water potential in saplings.

The pressure chamber, the most popular method used to measure xylem water potential, is a discontinuous and destructive technique and is therefore not suitable for automated monitoring. Continuous non-destructive monitoring could until very recently be achieved only by use of the thermocouple psychrometer (TP). Here we present the high-capacity tensiometer (HCT) as an alternative method for continuous non-destructive monitoring. This provided us with a unique chance to cross-validate the two instruments by installing them simultaneously on the same sapling stem. The HCT and the TP showed excellent agreement for xylem water potential less than -0.5 MPa. Response to day/night cycles and watering was remarkably in phase, indicating excellent response time of both instruments despite substantially different working principles. For xylem water potential greater than -0.5 MPa, the discrepancies sometimes observed between the HCT and TP were mainly attributed to the kaolin paste used to establish contact between the xylem and the HCT, which becomes hydraulically poorly conductive in this range of water potential once dried beyond its air-entry value and subsequently re-wetted. Notwithstanding this limitation, which can be overcome by selecting a clay paste with higher air-entry value, the HCT has been shown to represent a valid alternative to the TP.

Nighttime transpiration represents a negligible part of water loss and does not increase the risk of water stress in grapevine.

Nighttime transpiration has been previously reported as a significant source of water loss in many species; however, there is a need to determine if this trait plays a key role in the response to drought. This study aimed to determine the magnitude, regulation and relative contribution to whole plant water-use, of nighttime stomatal conductance (gnight ) and transpiration (Enight ) in grapevine (Vitis vinifera L.). Our results showed that nighttime water loss was relatively low compared to daytime transpiration, and that decreases in soil and plant water potentials were mainly explained by daytime stomatal conductance (gday ) and transpiration (Eday ). Contrary to Eday , Enight did not respond to VPD and possible effects of an innate circadian regulation were observed. Plants with higher gnight also exhibited higher daytime transpiration and carbon assimilation at midday, and total leaf area, suggesting that increased gnight may be linked with daytime behaviors that promote productivity. Modeling simulations indicated that gnight was not a significant factor in reaching critical hydraulic thresholds under scenarios of either extreme drought, or time to 20% of soil relative water content. Overall, this study suggests that gnight is not significant in exacerbating the risk of water stress and hydraulic failure in grapevine.

Seasonal and long-term consequences of esca grapevine disease on stem xylem integrity.

Hydraulic failure has been extensively studied during drought-induced plant dieback, but its role in plant-pathogen interactions is under debate. During esca, a grapevine (Vitis vinifera) disease, symptomatic leaves are prone to irreversible hydraulic dysfunctions but little is known about the hydraulic integrity of perennial organs over the short- and long-term. We investigated the effects of esca on stem hydraulic integrity in naturally infected plants within a single season and across season(s). We coupled direct (ks) and indirect (kth) hydraulic conductivity measurements, and tylose and vascular pathogen detection with in vivo X-ray microtomography visualizations. Xylem occlusions (tyloses) and subsequent loss of stem hydraulic conductivity (ks) occurred in all shoots with severe symptoms (apoplexy) and in more than 60% of shoots with moderate symptoms (tiger-stripe), with no tyloses in asymptomatic shoots. In vivo stem observations demonstrated that tyloses occurred only when leaf symptoms appeared, and resulted in more than 50% loss of hydraulic conductance in 40% of symptomatic stems, unrelated to symptom age. The impact of esca on xylem integrity was only seasonal, with no long-term impact of disease history. Our study demonstrated how and to what extent a vascular disease such as esca, affecting xylem integrity, could amplify plant mortality through hydraulic failure.

When and where soil is important to modify the carbon and water economy of leaves.

Photosynthetic 'least-cost' theory posits that the optimal trait combination for a given environment is that where the summed costs of photosynthetic water and nutrient acquisition/use are minimised. The effects of soil water and nutrient availability on photosynthesis should be stronger as climate-related costs for both resources increase. Two independent datasets of photosynthetic traits, Globamax (1509 species, 288 sites) and Glob13C (3645 species, 594 sites), were used to quantify biophysical and biochemical limitations of photosynthesis and the key variable Ci /Ca (CO2 drawdown during photosynthesis). Climate and soil variables were associated with both datasets. The biochemical photosynthetic capacity was higher on alkaline soils. This effect was strongest at more arid sites, where water unit-costs are presumably higher. Higher values of soil silt and depth increased Ci /Ca , likely by providing greater H2 O supply, alleviating biophysical photosynthetic limitation when soil water is scarce. Climate is important in controlling the optimal balance of H2 O and N costs for photosynthesis, but soil properties change these costs, both directly and indirectly. In total, soil properties modify the climate-demand driven predictions of Ci /Ca by up to 30% at a global scale.

Drought-induced lacuna formation in the stem causes hydraulic conductance to decline before xylem embolism in Selaginella.

Lycophytes are the earliest diverging extant lineage of vascular plants, sister to all other vascular plants. Given that most species are adapted to ever-wet environments, it has been hypothesized that lycophytes, and by extension the common ancestor of all vascular plants, have few adaptations to drought. We investigated the responses to drought of key fitness-related traits such as stomatal regulation, shoot hydraulic conductance (Kshoot ) and stem xylem embolism resistance in Selaginella haematodes and S. pulcherrima, both native to tropical understory. During drought stomata in both species were found to close before declines in Kshoot , with a 50% loss of Kshoot occurring at -1.7 and -2.5 MPa in S. haematodes and S. pulcherrima, respectively. Direct observational methods revealed that the xylem of both species was resistant to embolism formation, with 50% of embolized xylem area occurring at -3.0 and -4.6 MPa in S. haematodes and S. pulcherrima, respectively. X-ray microcomputed tomography images of stems revealed that the decline in Kshoot occurred with the formation of an air-filled lacuna, disconnecting the central vascular cylinder from the cortex. We propose that embolism-resistant xylem and large capacitance, provided by collapsing inner cortical cells, is essential for Selaginella survival during water deficit.

Exploring the Hydraulic Failure Hypothesis of Esca Leaf Symptom Formation.

Vascular pathogens cause disease in a large spectrum of perennial plants, with leaf scorch being one of the most conspicuous symptoms. Esca in grapevine (Vitis vinifera) is a vascular disease with huge negative effects on grape yield and the wine industry. One prominent hypothesis suggests that vascular disease leaf scorch is caused by fungal pathogen-derived elicitors and toxins. Another hypothesis suggests that leaf scorch is caused by hydraulic failure due to air embolism, the pathogen itself, and/or plant-derived tyloses and gels. In this study, we transplanted mature, naturally infected esca symptomatic vines from the field into pots, allowing us to explore xylem integrity in leaves (i.e. leaf midveins and petioles) using synchrotron-based in vivo x-ray microcomputed tomography and light microscopy. Our results demonstrated that symptomatic leaves are not associated with air embolism. In contrast, symptomatic leaves presented significantly more nonfunctional vessels resulting from the presence of nongaseous embolisms (i.e. tyloses and gels) than control leaves, but there was no significant correlation with disease severity. Using quantitative PCR, we determined that two vascular pathogen species associated with esca necrosis in the trunk were not found in leaves where occlusions were observed. Together, these results demonstrate that symptom development is associated with the disruption of vessel integrity and suggest that symptoms are elicited at a distance from the trunk where fungal infections occur. These findings open new perspectives on esca symptom expression where the hydraulic failure and elicitor/toxin hypotheses are not necessarily mutually exclusive.

Neither xylem collapse, cavitation, or changing leaf conductance drive stomatal closure in wheat.

Identifying the drivers of stomatal closure and leaf damage during stress in grasses is a critical prerequisite for understanding crop resilience. Here, we investigated whether changes in stomatal conductance (gs ) during dehydration were associated with changes in leaf hydraulic conductance (Kleaf ), xylem cavitation, xylem collapse, and leaf cell turgor in wheat (Triticum aestivum). During soil dehydration, the decline of gs was concomitant with declining Kleaf under mild water stress. This early decline of leaf hydraulic conductance was not driven by cavitation, as the first cavitation events in leaf and stem were detected well after Kleaf had declined. Xylem vessel deformation could only account for <5% of the observed decline in leaf hydraulic conductance during dehydration. Thus, we concluded that changes in the hydraulic conductance of tissues outside the xylem were responsible for the majority of Kleaf decline during leaf dehydration in wheat. However, the contribution of leaf resistance to whole plant resistance was less than other tissues (<35% of whole plant resistance), and this proportion remained constant as plants dehydrated, indicating that Kleaf decline during water stress was not a major driver of stomatal closure.

Over-accumulation of abscisic acid in transgenic tomato plants increases the risk of hydraulic failure.

Climate change threatens food security, and plant science researchers have investigated methods of sustaining crop yield under drought. One approach has been to overproduce abscisic acid (ABA) to enhance water use efficiency. However, the concomitant effects of ABA overproduction on plant vascular system functioning are critical as it influences vulnerability to xylem hydraulic failure. We investigated these effects by comparing physiological and hydraulic responses to water deficit between a tomato (Solanum lycopersicum) wild type control (WT) and a transgenic line overproducing ABA (sp12). Under well-watered conditions, the sp12 line displayed similar growth rate and greater water use efficiency by operating at lower maximum stomatal conductance. X-ray microtomography revealed that sp12 was significantly more vulnerable to xylem embolism, resulting in a reduced hydraulic safety margin. We also observed a significant ontogenic effect on vulnerability to xylem embolism for both WT and sp12. This study demonstrates that the greater water use efficiency in the tomato ABA overproducing line is associated with higher vulnerability of the vascular system to embolism and a higher risk of hydraulic failure. Integrating hydraulic traits into breeding programmes represents a critical step for effectively managing a crop's ability to maintain hydraulic conductivity and productivity under water deficit.

The sequence and thresholds of leaf hydraulic traits underlying grapevine varietal differences in drought tolerance.

Adapting agriculture to climate change is driving the need for the selection and breeding of drought-tolerant crops. The aim of this study was to identify key drought tolerance traits and determine the sequence of their water potential thresholds across three grapevine cultivars with contrasting water use behaviors, Grenache, Syrah, and Semillon. We quantified differences in water use between cultivars and combined this with the determination of other leaf-level traits (e.g. leaf turgor loss point, π TLP), leaf vulnerability to embolism (P50), and the hydraulic safety margin (HSM P50). Semillon exhibited the highest maximum transpiration (Emax), and lowest sensitivity of canopy stomatal conductance (Gc) to vapor pressure deficit (VPD), followed by Syrah and Grenache. Increasing Emax was correlated with more negative water potential at which stomata close (Pgs90), π TLP, and P50, suggesting that increasing water use is associated with hydraulic traits allowing gas exchange under more negative water potentials. Nevertheless, all the cultivars closed their stomata prior to leaf embolism formation. Modeling simulations demonstrated that despite a narrower HSM, Grenache takes longer to reach thresholds of hydraulic failure due to its conservative water use. This study demonstrates that the relationships between leaf hydraulic traits are complex and interactive, stressing the importance of integrating multiple traits in characterizing drought tolerance.

Xylem embolism in leaves does not occur with open stomata: evidence from direct observations using the optical visualization technique.

Drought represents a major abiotic constraint to plant growth and survival. On the one hand, plants keep stomata open for efficient carbon assimilation while, on the other hand, they close them to prevent permanent hydraulic impairment from xylem embolism. The order of occurrence of these two processes (stomatal closure and the onset of leaf embolism) during plant dehydration has remained controversial, largely due to methodological limitations. However, the newly developed optical visualization method now allows concurrent monitoring of stomatal behaviour and leaf embolism formation in intact plants. We used this new approach directly by dehydrating intact saplings of three contrasting tree species and indirectly by conducting a literature survey across a greater range of plant taxa. Our results indicate that increasing water stress generates the onset of leaf embolism consistently after stomatal closure, and that the lag time between these processes (i.e. the safety margin) rises with increasing embolism resistance. This suggests that during water stress, embolism-mediated declines in leaf hydraulic conductivity are unlikely to act as a signal for stomatal down-regulation. Instead, these species converge towards a strategy of closing stomata early to prevent water loss and delay catastrophic xylem dysfunction.

Embolism resistance in petioles and leaflets of palms.

BACKGROUND AND AIMS: Hydraulic studies are currently biased towards conifers and dicotyledonous angiosperms; responses of arborescent monocots to increasing temperature and drought remain poorly known. This study aims to assess xylem resistance to drought-induced embolism in palms. METHODS: We quantified embolism resistance via P50 (xylem pressure inducing 50 % embolism or loss of hydraulic conductivity) in petioles and leaflets of six palm species differing in habitat and phylogenetic relatedness using three techniques: in vivo X-ray-based microcomputed tomography, the in situ flow centrifuge technique and the optical vulnerability method. KEY RESULTS: Our results show that P50 of petioles varies greatly in the palm family, from -2.2 ± 0.4 MPa in Dypsis baronii to -5.8 ± 0.3 MPa in Rhapis excelsa (mean ± s.e.). No difference or weak differences were found between petioles and leaf blades within species. Surprisingly, where differences occurred, leaflets were less vulnerable to embolism than petioles. Embolism resistance was not correlated with conduit size (r = 0.37, P = 0.11). CONCLUSIONS: This study represents the first estimate of drought-induced xylem embolism in palms across biomes and provides the first step towards understanding hydraulic adaptations in long-lived arborescent monocots. It showed an almost 3-fold range of embolism resistance between palm species, as large as that reported in all angiosperms. We found little evidence for hydraulic segmentation between leaflets and petioles in palms, suggesting that when it happens, hydraulic segregation may lack a clear relationship with organ cost or replaceability.

Is xylem of angiosperm leaves less resistant to embolism than branches? Insights from microCT, hydraulics, and anatomy.

According to the hydraulic vulnerability segmentation hypothesis, leaves are more vulnerable to decline of hydraulic conductivity than branches, but whether stem xylem is more embolism resistant than leaves remains unclear. Drought-induced embolism resistance of leaf xylem was investigated based on X-ray microcomputed tomography (microCT) for Betula pendula, Laurus nobilis, and Liriodendron tulipifera, excluding outside-xylem, and compared with hydraulic vulnerability curves for branch xylem. Moreover, bordered pit characters related to embolism resistance were investigated for both organs. Theoretical P50 values (i.e. the xylem pressure corresponding to 50% loss of hydraulic conductance) of leaves were generally within the same range as hydraulic P50 values of branches. P50 values of leaves were similar to branches for L. tulipifera (-2.01 versus -2.10 MPa, respectively), more negative for B. pendula (-2.87 versus -1.80 MPa), and less negative for L. nobilis (-6.4 versus -9.2 MPa). Despite more narrow conduits in leaves than branches, mean interconduit pit membrane thickness was similar in both organs, but significantly higher in leaves of B. pendula than in branches. This case study indicates that xylem shows a largely similar embolism resistance across leaves and branches, although differences both within and across organs may occur, suggesting interspecific variation with regard to the hydraulic vulnerability segmentation hypothesis.

Intraspecific variation in embolism resistance and stem anatomy across four sunflower (Helianthus annuus L.) accessions.

Drought-induced xylem embolism is a key process closely related to plant mortality during extreme drought events. However, this process has been poorly investigated in crop species to date, despite the observed decline of crop productivity under extreme drought conditions. Interspecific variation in hydraulic traits has frequently been reported, but less is known about intraspecific variation in crops. We assessed the intraspecific variability of embolism resistance in four sunflower (Helianthus annuus L.) accessions grown in well-watered conditions. Vulnerability to embolism was determined by the in situ flow-centrifuge method (cavitron), and possible trade-offs between xylem safety, xylem efficiency and growth were assessed. The relationship between stem anatomy and hydraulic traits was also investigated. Mean P50 was -3 MPa, but significant variation was observed between accessions, with values ranging between -2.67 and -3.22 MPa. Embolism resistance was negatively related to growth and positively related to xylem-specific hydraulic conductivity. There is, therefore, a trade-off between hydraulic safety and growth but not between hydraulic safety and efficiency. Finally, we found that a few anatomical traits, such as vessel density and the area of the vessel lumen relative to that of the secondary xylem, were related to embolism resistance, whereas stem tissue lignification was not. Further investigations are now required to investigate the link between the observed variability of embolism resistance and yield, to facilitate the identification of breeding strategies to improve yields in an increasingly arid world.