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Drought-induced tree mortality frequently occurs in patches with different spatial and temporal distributions, which is only partly explained by inter- and intraspecific variation in drought tolerance. We investigated whether bedrock properties, with special reference to rock water storage capacity, affects tree water status and drought response in a rock-dominated landscape. We measured primary porosity and available water content of breccia (B) and dolostone (D) rocks. Saplings of Fraxinus ornus were grown in pots filled with soil or soil mixed with B and D rocks, and subjected to an experimental drought. Finally, we measured seasonal changes in water status of trees in field sites overlying B or D bedrock. B rocks were more porous and stored more available water than D rocks. Potted saplings grown with D rocks had less biomass and suffered more severe water stress than those with B rocks. Trees in sites with B bedrock had more favourable water status than those on D bedrock which also suffered drought-induced canopy dieback. Bedrock represents an important water source for plants under drought. Different bedrock features translate into contrasting below-ground water availability, leading to landscape-level heterogeneity of the impact of drought on tree water status and dieback.

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.

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.

Global warming is exposing plants to increased risks of drought-driven mortality. Recent advances suggest that hydraulic failure is a key process leading to plant death, and the identification of simple and reliable proxies of species-specific risk of irreversible hydraulic damage is urgently required. We assessed the predictive power of leaf water content and shrinkage for monitoring leaf hydraulic failure in two Mediterranean native species, Salvia ceratophylloides (Sc) and S. officinalis (So). The study species showed significant differences in relative water content (RWC) thresholds inducing loss of rehydration capacity, as well as leaf hydraulic conductance (KL) impairment. Sc turned out to be more resistant to drought than So. However, Sc and So showed different leaf saturated water content values, so that different RWC values actually corresponded to similar absolute leaf water content. Our findings suggest that absolute leaf water content and leaf water potential, but not RWC, are reliable parameters for predicting the risk of leaf hydraulic impairment of two Salvia species, and their potential risk of irreversible damage under severe drought. Moreover, the lack of any KL decline until the turgor loss point in Sc, coupled to consistent leaf shrinkage, rejects the hypothesis to use leaf shrinkage as a proxy to predict KL vulnerability, at least in species with high leaf capacitance. Robust linear correlations between KL decline and electrolyte leakage measurements suggested a role of membrane damage in driving leaf hydraulic collapse.

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.

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.

Embolism repair ability has been documented in numerous species. Although the actual mechanism driving this phenomenon is still debated, experimental findings suggest that non-structural carbohydrates (NSC) stored in wood parenchyma would provide the osmotic forces to drive the refilling of embolized conduits. We selected 12 broadleaved species differing in vulnerability to xylem embolism (P-50) and amount of wood parenchyma in order to check direct evidence about the possible link(s) between parenchyma cells abundance, NSC availability and species-specific capacity to reverse xylem embolism. Branches were dehydrated until similar to 50% loss of hydraulic conductivity was recorded (PLC similar to 50%). Hydraulic recovery (Delta PLC) and NSC content was, then, assessed after 111 of rehydration. Species showed a different ability to recover their hydraulic conductivity from PLC similar to 50%. Removing the bark in the species showing hydraulic recovery inhibited the embolism reversal. Strong correlations between the Delta PLC and: a) the amount of parenchyma cells (mainly driven by the pith area), b) the consumption of soluble NSC have been recorded. Our results support the hypothesis that refilling of embolized vessels is mediated by the mobilization of soluble NSC and it is mainly recorded in species with a higher percentage of parenchyma cells that may be important in the hydraulic recovery mechanism as a source of carbohydrates and/or as a source of water.

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.

The xylem is a complex system that includes a network of dead conduits ensuring long-distance water transport in plants. Under ongoing climate changes, xylem embolism is a major and recurrent cause of drought-induced tree mortality. Non-structural carbohydrates (NSC) play key roles in plant responses to drought and frost stress, and several studies putatively suggest their involvement in the regulation of xylem water transport. However, a clear picture on the roles of NSCs in plant hydraulics has not been drawn to date. We summarize the current knowledge on the involvement of NSCs during embolism formation and subsequent hydraulic recovery. Under drought, sugars are generally accumulated in xylem parenchyma and in xylem sap. At drought-relief, xylem functionality is putatively restored in an osmotically driven process involving wood parenchyma, xylem sap and phloem compartments. By analyzing the published data on stem hydraulics and NSC contents under drought/frost stress and subsequent stress relief, we found that embolism build-up positively correlated to stem NSC depletion, and that the magnitude of post-stress hydraulic recovery positively correlated to consumption of soluble sugars. These findings suggest a close relationship between hydraulics and carbohydrate dynamics. We call for more experiments on hydraulic and NSC dynamics in controlled and field conditions.

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.

The relation between water status and expression profiles of desiccation -related genes has been studied in the desiccation tolerant (DT) aeroterrestrial green microalga Trebouxia gelatinosa, a common lichen photobiont. Algal colonies were desiccated in controlled conditions and during desiccation water content (WC) and water potential (Ψ) were measured to find the turgor loss point (Ψtlp). Quantitative real-time PCR was performed to measure the expression of ten genes related to photosynthesis, antioxidant defense, expansins, heat shock proteins (HSPs), and desiccation related proteins in algal colonies collected during desiccation when still at full turgor (WC > 6 g H2O g−1 dry weight), immediately before and after Ψtlp (−4 MPa; WC∼1 g H2O g−1 dry weight) and before and after complete desiccation (WC < 0.01 g H2O g−1 dry weight), quantifying the HSP70 protein levels by immunodetection. Our analysis showed that the expression of eight out of ten genes changed immediately before and after Ψtlp. Interestingly, the expression of five out of ten genes changed also before complete desiccation, i.e. between 0.2 and 0.01 g H2O g−1 dry weight. However, the HSP70 protein levels were not affected by changes in water status. The study provides new evidences of the link between the loss of turgor and the expression of genes related to the desiccation tolerance of T. gelatinosa, suggesting the former as a signal triggering inducible mechanisms.

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.

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.