An increase in xylem embolism resistance of grapevine leaves during the growing season is coordinated with stomatal regulation, turgor loss point and intervessel pit membranes

Implications of root morphology and anatomy for water deficit tolerance and recovery of grapevine rootstocks

The intensification of drought conditions due to climate change poses a major challenge to sustainable grape production. Rootstocks are essential in supporting grapevine water uptake and drought resilience; however, their physiological responses to water stress are not fully understood. Under the hypothesis that root morphology and anatomy may be key traits in grapevine tolerance to water deficit, this study aimed to investigate these traits across diverse rootstocks under progressive water deficit and recovery phases. Thirteen genotypes, including commercial rootstocks and recently bred RG-series and RM2, were evaluated over two seasons in controlled pot-based conditions. Plants were subjected to five distinct watering stages, from well-watered to severe drought. Root traits, such as length, density, and xylem anatomical features, were analyzed alongside stem water potential (Ψstem) to gauge plant water status. Results showed significant genotype-specific differences in root morphology and anatomy, impacting drought tolerance and recovery. Rootstocks with higher root length density (RLD) and a larger proportion of fine roots maintained Ψstem more effectively under severe drought. Additionally, smaller xylem vessel diameters and reduced xylem area relative to root cross-sectional area correlated with improved water transport efficiency and faster recovery post-drought. A trade-off emerged wherein increased root density enhanced water uptake capacity but came at the cost of reduced transport efficiency. Notably, rootstocks 420A, 41B, RM2, and Fercal displayed superior drought resilience, while the RG-series did not outperform established genotypes like 13-5 Evex, 110 Richter, and 140 Ruggeri. These results underscore the role of root morphology and anatomy in grapevine drought tolerance, suggesting that these traits could be incorporated as criteria for future rootstocks breeding programs. Nevertheless, field-testing under non-limiting soil conditions is essential to validate these findings.

Bimodal pattern of allometric scaling along grapevine shoots

BACKGROUND AND AIMS

Each branch internode, with the organs growing on it, can be seen as a single morphological phytomer subunit made of structurally and functionally interrelated components. However, allometric relationships between the anatomy and morphology of these subunits remain unclear, particularly in the axial context. The aim of this study was to address this knowledge gap by measuring morpho-anatomical parameters and their allometric relationships along grapevine shoots.

METHODS

To facilitate comparison, shoot length was normalized, and a relative position index was calculated for each internode, ranging from zero at the base to one at the apex. Scaling relationships between morpho-anatomical parameters along the axis were developed and validated by statistical modelling.

KEY RESULTS

Most morpho-anatomical parameters displayed an axial behaviour of increasing then decreasing in size from base to apex, with the exception of shoot diameter and shoot vessel density. A relative position index of 0.2 acted as the data turning point for most variables analysed. During the first phase (relative position index of <0.2), the traits of the phytomer organs are uncoupled and show weak allometric correlation, and during the second phase the traits exhibit strong allometric relationships.

CONCLUSIONS

Our findings suggest that allometric relationships along grapevine shoots are not constant; they exhibit a bimodal pattern, possibly influenced by seasonal temperatures. This work could aid in managing productivity shifts in agricultural and natural systems under global climate change and adds to basic knowledge of differentiation and development of growth units in plants.

Rewatering after drought: Unravelling the drought thresholds and function recovery-limiting factors in maize leaves

Drought and subsequent rewatering are common in agriculture, where recovery from mild droughts is easier than from severe ones. The specific drought threshold and factors limiting recovery are under-researched. This study subjected maize plants to varying drought degrees before rewatering, and measuring plant water status, gas exchange, hydraulic conductance, hormone levels, and cellular damage throughout. We discovered that stomatal reopening in plants was inhibited with leaf water potentials below about -1.7 MPa, hindering postdrought photosynthetic recovery. Neither hydraulic loss nor abscisic acid (ABA) content was the factor inhibited stomatal reopening on the second day following moderate drought stress and rewatering. But stomatal reopening was significantly correlated to the interaction between hydraulic signals and ABA content under severe drought. Extended drought led to leaf death at about -2.8 MPa or 57% relative water content, influenced by reduced rehydration capacity, not hydraulic failure. The lethal threshold remained relatively constant across leaf stages, but the recoverable safety margin (RSM), that is, the water potential difference between stomatal closure and recovery capacity loss, significantly decreased with leaf aging due to delayed stomatal closure during drought. Our findings indicate hydraulic failure alone does not cause maize leaf death, highlighting the importance of RSM in future research.

A high-throughput approach for quantifying turgor loss point in grapevine

Quantifying drought tolerance in crops is critical for agriculture management under environmental change, and drought response traits in grape vine have long been the focus of viticultural research. Turgor loss point (πtlp) is gaining attention as an indicator of drought tolerance in plants, though estimating πtlp often requires the construction and analysis of pressure-volume (P-V) curves which are very time consuming. While P-V curves remain a valuable tool for assessing πtlp and related traits, there is considerable interest in developing high-throughput methods for rapidly estimating πtlp, especially in the context of crop screening. We tested the ability of a dewpoint hygrometer to quantify variation in πtlp across and within 12 clones of grape vine (Vitis vinifera subsp. vinifera) and one wild relative (Vitis riparia), and compared these results to those derived from P-V curves. At the leaf-level, methodology explained only 4-5% of the variation in πtlp while clone/species identity accounted for 39% of the variation, indicating that both methods are sensitive to detecting intraspecific πtlp variation in grape vine. Also at the leaf level, πtlp measured using a dewpoint hygrometer approximated πtlp values (r2 = 0.254) and conserved πtlp rankings from P-V curves (Spearman's ρ = 0.459). While the leaf-level datasets differed statistically from one another (paired t-test p = 0.01), average difference in πtlp for a given pair of leaves was small (0.1 ± 0.2 MPa (s.d.)). At the species/clone level, estimates of πtlp measured by the two methods were also statistically correlated (r2 = 0.304), did not deviate statistically from a 1:1 relationship, and conserved πtlp rankings across clones (Spearman's ρ = 0.692). The dewpoint hygrometer (taking ∼ 10-15 min on average per measurement) captures fine-scale intraspecific variation in πtlp, with results that approximate those from P-V curves (taking 2-3 h on average per measurement). The dewpoint hygrometer represents a viable method for rapidly estimating intraspecific variation in πtlp, and potentially greatly increasing replication when estimating this drought tolerance trait in grape vine and other crops.

Integrating gene expression analysis and ecophysiological responses to water deficit in leaves of tomato plants

Soil water deficit (WD) significantly impacts plant survival and crop yields. Many gaps remain in our understanding of the synergistic coordination between molecular and ecophysiological responses delaying substantial drought-induced effects on plant growth. To investigate this synergism in tomato leaves, we combined molecular, ecophysiological, and anatomical methods to examine gene expression patterns and physio-anatomical characteristics during a progressing WD experiment. Four sampling points were selected for transcriptomic analysis based on the key ecophysiological responses of the tomato leaves: 4 and 5 days after WD (d-WD), corresponding to 10% and 90% decrease in leaf stomatal conductance; 8 d-WD, the leaf wilting point; and 10 d-WD, when air embolism blocks 12% of the leaf xylem water transport. At 4 d-WD, upregulated genes were mostly linked to ABA-independent responses, with larger-scale ABA-dependent responses occurring at 5 d-WD. At 8 d-WD, we observed an upregulation of heat shock transcription factors, and two days later (10 d-WD), we found a strong upregulation of oxidative stress transcription factors. Finally, we found that young leaves present a stronger dehydration tolerance than mature leaves at the same drought intensity level, presumably because young leaves upregulate genes related to increased callose deposition resulting in limiting water loss to the phloem, and related to increased cell rigidity by modifying cell wall structures. This combined dataset will serve as a framework for future studies that aim to obtain a more holistic WD plant response at the molecular, ecophysiological and anatomical level.

Maize yield is associated with abscisic acid and water potential under reduced soil water supply but with indoleacetic acid in genotypic renewal

Irrigation and breeding are important practices for improving yield and water use efficiency of maize (Zea mays L.) in arid regions. However, the physiological mechanisms of yield under varying water supplies and genotypes remain unclear. Here, we examine the different physiological mechanisms underlying maize yield responses to varying soil water supplies and three genotypes (MC670, ZD958, and ZD2#) cultivated in northwestern China over the past five decades. The declining water supply significantly reduced maize leaf hydraulic transport, stomatal conductance (gs), net photosynthetic rate (A), yield, kernel number, biomass, and evapotranspiration (ET). Conversely, it led to an increase in abscisic acid (ABA), hydrogen peroxide, intrinsic water use efficiency, and water productivity. Interestingly, there was no significant impact on indoleacetic acid (IAA), thousand kernel weight, or harvest index (HI). Breeding efforts increased leaf IAA levels, biomass, thousand kernel weight, yield, HI, and water productivity without altering physiological traits or ET. The superior yield of MC670 could be attributed to a simultaneous enhancement in both kernel number and thousand kernel weight, while ZD958 exhibited greater yield stability. ABA and hydraulic traits (predawn leaf water potential, leaf water potential, and whole-plant hydraulic conductance) coordinated gs under reduced soil water supply, while ABA and predawn leaf water potential regulated yield by modulating gs to affect both A and ET. Breeding for yield gains was associated with IAA-induced enhancements in biomass and HI, independent of key physiological traits (e.g., gs and A) and ET. The observed increase in water productivity primarily stemmed from notable yield improvements rather than alterations in ET. Hence, the selection of high-yielding genotypes under water-limited and well-watered conditions requires consideration of water-related physiological traits and IAA levels, respectively.

Seasonal variation of vessel pits in sapwood: microscopical analyses of the morphology and chemical components of pit membrane encrustations in Fraxinus mandschurica

BACKGROUND AND AIMS

Pit pairs and their filter-like partition, i.e. pit membranes, play important roles as water pathways, barriers and regulators in the water-conducting system of angiosperms. In Fraxinus species, the intervessel and vessel-parenchyma pit membranes in sapwood are normally encrusted during winter. Although these encrustations inevitably influence the performance of pits, their properties and functions remain unclear. This study aimed to reveal the morphological and chemical characteristics of encrustations in F. mandshurica in order to deepen understanding of the seasonal encrustation of pit membranes.

METHODS

Seasonal and positional variations in the presence and morphology of encrustations were examined by field-emission scanning electron microscopy (FE-SEM). Cryo-FE-SEM for freeze-fixed greenwood samples was conducted to clarify whether encrustations were present in living trees. Chemical components were examined by histochemical staining using light and electron microscopy, immunofluorescence labelling and ultraviolet microspectroscopy.

KEY RESULTS

Encrustations began to deposit in autumn before leaf senescence and disappeared in spring before bud flushing. They infiltrated within the pit membranes, which suggested that they severely limit the permeation of pits. The encrustations differed in morphology among positions: they entirely filled the pit chambers in latewood, while they covered the pit membranes in earlywood. The encrustations were similarly observed in the samples that were freeze-fixed immediately after collection, indicating that they are present in living trees. The encrustations contained polysaccharides, including xyloglucan and homogalacturonan, and phenolic compounds, possibly including flavonoids and coumarins. These chemical components were also detected in droplets found in the latewood vessels with the encrustations, suggesting that the materials constituting encrustations were supplied through the vessel lumens.

CONCLUSIONS

Encrustations undoubtedly cover the pit membranes in living F. mandshurica trees in winter and their morphology and chemical composition indicate that they are impermeable, have positional differences in function and are characterized by elaborate deposition/removal processes.

The sound of lichens: ultrasonic acoustic emissions during desiccation question cavitation events in the hyphae

Lichens are a mutualistic symbiosis between a fungus and one or more photosynthetic partners. They are photosynthetically active during desiccation down to relative water contents (RWCs) as low as 30% (on dry mass). Experimental evidence suggests that during desiccation, the photobionts have a higher hydration level than the surrounding fungal pseudo-tissues. Explosive cavitation events in the hyphae might cause water movements towards the photobionts. This hypothesis was tested in two foliose lichens by measurements of ultrasonic acoustic emissions (UAEs), a method commonly used in vascular plants but never in lichens, and by measurements of PSII efficiency, water potential, and RWC. Thallus structural changes were characterized by low-temperature scanning electron microscopy. The thalli were silent between 380% and 30% RWCs, when explosive cavitation events should cause movements of liquid water. Nevertheless, the thalli emitted UAEs at ~5% RWC. Accordingly, the medullary hyphae were partially shrunken at ~15% RWC, whereas they were completely shrunken at <5% RWC. These results do not support the hypothesis of hyphal cavitation and suggest that the UAEs originate from structural changes at hyphal level. The shrinking of hyphae is proposed as an adaptation to avoid cell damage at very low RWCs.

Seasonality in embolism resistance and hydraulic capacitance jointly mediate hydraulic safety in branches and leaves of oriental cork oak (Quercus variabilis Bl.)

Seasonality in temperate regions is prominent during the era of increased climatic variability. A hydraulic trait that can adjust to seasonally changing climatic conditions is crucial for tree safety. However, little attention has been paid to the intraspecific seasonality of drought-related traits and hydraulic safety of keystone forest trees. We examined seasonal variations in the key morphological and physiological traits as well as multiple hydraulic safety margins (SMs) at the branch and leaf levels in oriental cork oak (Quercus variabilis Bl.), which is predominant in Chinese temperate forests. Pneumatic measurements indicated that, as seasons progressed, the water potential at which 50% of branch embolisms occur (P50_branch) decreased from -3.34 to -4.23 MPa, with a coefficient of variation (CV) of 9.08%. Sapwood capacitance ranged from 48.19 to 248.08 kg m-3 MPa-1, peaking in autumn and reaching minimum in winter (CV 60.58%). Rehydration kinetics confirmed higher leaf embolism vulnerability (P50_leaf) in spring and autumn than those in summer, with values ranging from -1.06 to -3.02 MPa (CV 39.85%). All leaf pressure-volume (PV) traits shifted with growth, with CVs ranging from 6.95% to 46.69%. Sapwood density had significant negative correlations with P50_branch and hydraulic capacitance for elastic water storage, whereas leaf mass per area was linearly associated with PV traits but not with P50_leaf. Furthermore, the branch typical SMs (difference between branch midday water potential and P50_branch) were consistently >1.84 MPa, and vulnerability segmentation was prevalent throughout, implying a plausible hydraulic foundation for the dominance of Q. variabilis. Diverse hydraulic response patterns existed across seasons, leading to positive SMs mediated by the aforementioned physiological traits. Although Q. variabilis exhibits a high level of hydraulic safety, its susceptibility to sudden summer droughts may increase due to global climate change.

Coordination between water relations strategy and carbon investment in leaf and stem in six fruit tree species

The water relation strategy is a key issue in climate change. Given the difficulty of determining water relations strategy, there is a need for simple traits with a solid theoretical basis to estimate it. Traits associated with resource allocation patterns along a 'fast-slow' plant economics spectrum are particularly compelling, reflecting trade-offs between growth rate and carbon allocation. Avocado (Persea americana ), fig tree (Ficus carica ), mandarin (Citrus reticulata ), olive (Olea europaea ), pomegranate (Punica granatum ), and grapevine (Vitis vinifera ) were characterised in terms of iso-anisohydric strategy through stomatal behaviour, water potential at the turgor loss point (TLP), and hydroscape area. Additionally, the association of these metrics with leaf mass per area (LMA) and wood density (WDen) was explored. We observed high coordination between LMA and WDen, and both traits were related to metrics of water relation strategy. More anisohydric species tended to invest more carbon per unit leaf area or unit stem volume, which has implications for hydraulic efficiency and water stress tolerance. WDen and TLP were the most powerful traits in estimating the water relation strategy for six fruit species. These traits are easy to measure, time-cost efficient, and appear central to coordinating multiple traits and behaviours along the water relations strategies.

Rootstocks affect the vulnerability to embolism and pit membrane thickness in Citrus scions

Embolism resistance of xylem tissue varies among species and is an important trait related to drought resistance, with anatomical attributes like pit membrane thickness playing an important role in avoiding embolism spread. Grafted Citrus trees are commonly grown in orchards, with the rootstock being able to affect the drought resistance of the whole plant. Here, we evaluated how rootstocks affect the vulnerability to embolism resistance of the scion using several rootstock/scion combinations. Scions of 'Tahiti' acid lime, 'Hamlin', 'Pera' and 'Valencia' oranges grafted on a 'Rangpur' lime rootstock exhibit similar vulnerability to embolism. In field-grown trees, measurements of leaf water potential did not suggest significant embolism formation during the dry season, while stomata of Citrus trees presented an isohydric response to declining water availability. When 'Valencia' orange scions were grafted on 'Rangpur' lime, 'IAC 1710' citrandarin, 'Sunki Tropical' mandarin or 'Swingle' citrumelo rootstocks, variation in intervessel pit membrane thickness of the scion was found. The 'Rangpur' lime rootstock, which is known for its drought resistance, induced thicker pit membranes in the scion, resulting in higher embolism resistance than the other rootstocks. Similarly, the rootstock 'IAC 1710' citrandarin generated increased embolism resistance of the scion, which is highly relevant for citriculture.

Seasonality drives leaf solute potential independently from drought in grapevines. A commentary on 'Grape cultivars adapted to hotter, drier growing regions exhibit greater photosynthesis in hot conditions despite less drought-resistant leaves'

This article comments on:

Gabriela Sinclair, Erin R. Galarneau, Josh F. Hnizdor, Andrew J. McElrone, Michael Andrew Walker and Megan K. Bartlett, Grape cultivars adapted to hotter, drier growing regions exhibit greater photosynthesis in hot conditions despite less drought-resistant leaves, Annals of Botany, Volume 134, Issue 2, 1 August 2024, Pages 205–217 https://doi.org/10.1093/aob/mcae032

Grape cultivars adapted to hotter, drier growing regions exhibit greater photosynthesis in hot conditions despite less drought-resistant leaves

BACKGROUND AND AIMS

Many agricultural areas are expected to face hotter, drier conditions from climate change. Understanding the mechanisms that crops use to mitigate these stresses can guide breeding for more tolerant plant material. We tested relationships between traits, physiological function in hot conditions and historical climate associations to evaluate these mechanisms for winegrapes. We expected a more negative leaf osmotic potential at full hydration (πo), which reduces leaf turgor loss during drought, and either a metabolically cheaper or more osmoprotectant leaf chemical composition, to allow cultivars associated with hot, dry regions to maintain greater gas exchange in hot growing conditions.

METHODS

We measured πo, gas exchange and leaf chemistry for seven commercially important winegrape cultivars that vary widely in historical climate associations. Vines were grown in common-garden field conditions in a hot wine-growing region (Davis, CA, USA) and measured over the hottest period of the growing season (July-September).

KEY RESULTS

The value of πo varied significantly between cultivars, and all cultivars significantly reduced πo (osmotically adjusted) over the study period, although osmotic adjustment did not vary across cultivars. The value of πo was correlated with gas exchange and climate associations, but in the direction opposite to expected. Photosynthesis and πo were higher in the cultivars associated with hotter, less humid regions. Leaf chemical composition varied between cultivars but was not related to climate associations.

CONCLUSIONS

These findings suggest that maintenance of leaf turgor is not a primary limitation on grapevine adaptation to hot or atmospherically dry growing conditions. Thus, selecting for a more negative πo or greater osmotic adjustment is not a promising strategy to develop more climate-resilient grape varieties, contrary to findings for other crops. Future work is needed to identify the mechanisms increasing photosynthesis in the cultivars associated with hot, dry regions.

Seasonal plasticity of stem embolism resistance and its potential driving factors in six temperate woody species

The seasonal plasticity of resistance to xylem embolism has been demonstrated in leaves of some tree species, but is controversial in stems. In this study, we investigated the seasonality of stem xylem resistance to embolism in six temperate woody species (four deciduous and two evergreen tree species) that were grown at the same site. The xylem conduit anatomy, the concentrations, and ratios of the main cation in the xylem sap, as well as the content of nonstructural carbohydrates (including soluble sugars and starch) were measured in each species under each season to reveal the potential mechanisms of seasonal change in embolism resistance. The stem of all species showed increasing resistance to embolism as seasons progressed, with more vulnerable xylem in spring, but no significant adjustment in the other three seasons. The seasonal plasticity of stem embolism resistance was greater in deciduous species than in evergreen. On a seasonal scale, conduit diameter and conduit implosion resistance, the ratios of K+/Ca2+ and K+/Na+, and starch content were generally not correlated with embolism resistance, suggesting that these are probably not the main drivers of seasonal plasticity of stem embolism resistance. The seasonality of embolism resistance provides critical information for better understanding plant hydraulics in response to seasonal environments, especially under climate change.

Spectral ecophysiology: hyperspectral pressure-volume curves to estimate leaf turgor loss

Turgor loss point (TLP) is an important proxy for plant drought tolerance, species habitat suitability, and drought-induced plant mortality risk. Thus, TLP serves as a critical tool for evaluating climate change impacts on plants, making it imperative to develop high-throughput and in situ methods to measure TLP. We developed hyperspectral pressure-volume curves (PV curves) to estimate TLP using leaf spectral reflectance. We used partial least square regression models to estimate water potential (Ψ) and relative water content (RWC) for two species, Frangula caroliniana and Magnolia grandiflora. RWC and Ψ's model for each species had R2 ≥ 0.7 and %RMSE = 7-10. We constructed PV curves with model estimates and compared the accuracy of directly measured and spectra-predicted TLP. Our findings indicate that leaf spectral measurements are an alternative method for estimating TLP. F. caroliniana TLP's values were -1.62 ± 0.15 (means ± SD) and -1.62 ± 0.34 MPa for observed and reflectance predicted, respectively (P > 0.05), while M. grandiflora were -1.78 ± 0.34 and -1.66 ± 0.41 MPa (P > 0.05). The estimation of TLP through leaf reflectance-based PV curves opens a broad range of possibilities for future research aimed at understanding and monitoring plant water relations on a large scale with spectral ecophysiology.

Gold perfusion experiments support the multi-layered, mesoporous nature of intervessel pit membranes in angiosperm xylem

Fluid transport across intervessel pit membranes of angiosperm xylem plays a major role in plant transpiration, with transport resistance largely depending on pore constriction sizes. Traditionally, fluid particles traversing pit membranes are assumed to cross a single instead of multiple pore constrictions. We tested a multi-layered pit membrane model in xylem of eight angiosperm species by estimating the size frequency of pore constrictions in relation to pit membrane thickness and compared modelled data with perfusion characteristics of nanoscale gold particles based on transmission electron microscopy. The size frequency of modelled pore constrictions showed similar patterns to the measured number of perfused particle sizes inside pit membranes, although frequency values measured were 10-50 times below modelled data. Small particles enter pit membranes most easily, especially when injected in thin pit membranes. The trapping of gold particles by pore constrictions becomes more likely with increasing pore constriction number and pit membrane thickness. While quantitative differences between modelled and experimental data are due to various practical limitations, their qualitative agreement supports a multi-layered pit membrane model with multiple pore constrictions. Pore constrictions between 5 and 50 nm are realistic, and confirm the mesoporous nature of pit membranes.

Temperate woody species across the angiosperm phylogeny acquire tolerance to water deficit stress during the growing season

Understanding the capacity of temperate trees to acclimate to limited soil water has become essential in the face of increasing drought risk due to climate change. We documented seasonal - or phenological - patterns in acclimation to water deficit stress in stems and leaves of tree species spanning the angiosperm phylogeny. Over 3 yr of field observations carried out in two US arboreta, we measured stem vulnerability to embolism (36 individuals of 7 Species) and turgor loss point (119 individuals of 27 species) over the growing season. We also conducted a growth chamber experiment on 20 individuals of one species to assess the mechanistic relationship between soil water restriction and acclimation. In three-quarters of species measured, plants became less vulnerable to embolism and/or loss of turgor over the growing season. We were able to stimulate this acclimatory effect by withholding water in the growth chamber experiment. Temperate angiosperms are capable of acclimation to soil water deficit stress, showing maximum vulnerability to soil water deficits following budbreak and becoming more resilient to damage over the course of the growing season or in response to simulated drought. The species-specific tempo and extent of this acclimatory potential constitutes preadaptive climate change resilience.

The legacy of past droughts induces water-sparingly behaviour in Grüner Veltliner grapevines

Drought is becoming more frequent and severe in numerous wine-growing regions. Nevertheless, limited research has examined the legacy of recurrent droughts, focusing on leaf physiology and anatomy over consecutive seasons. We investigated drought legacies (after 2 years of drought exposure) in potted grapevines, focusing on stomatal behaviour under well-watered conditions during the third year. Vines were subjected for two consecutive years to short- (SD) or long-term (LD) seasonal droughts, or well-watered conditions (WW). In the third year, all plants were grown without water limitation. Water potential and gas exchange were monitored throughout the three seasons, while leaf morpho-anatomical traits were measured at the end of the third year. During droughts (1st and 2nd year), stem water potential of SD and LD plants fell below -1.1 MPa, with a consequent 75% reduction in stomatal conductance (gs ) compared to WW. In the 3rd year, when all vines were daily irrigated to soil capacity (midday stem water potential ~ -0.3 MPa), 45% lower values of gs were observed in the ex-LD group compared to both ex-SD and ex-WW. Reduced midrib vessel diameter, lower leaf theoretical hydraulic conductivity, and smaller stomata were measured in ex-LD leaves compared to ex-SD and ex-WW, likely contributing to the reduced gas exchange. Our findings suggest that grapevines exposed to drought may adopt a more water-conserving strategy in subsequent seasons, irrespective of current soil water availability, with the degree of change influenced by the intensity and duration of past drought events.

Elevated CO2 concentration increases maize growth under water deficit or soil salinity but with a higher risk of hydraulic failure

Climate change presents a challenge for plants to acclimate their water relations under changing environmental conditions, and may increase the risks of hydraulic failure under stress. In this study, maize plants were acclimated to two different CO2 concentrations ([CO2]; 400 ppm and 700 ppm) while under either water stress (WS) or soil salinity (SS) treatments, and their growth and hydraulic traits were examined in detail. Both WS and SS inhibited growth and had significant impacts on hydraulic traits. In particular, the water potential at 50% loss of stem hydraulic conductance (P50) decreased by 1 MPa in both treatments at 400 ppm. When subjected to elevated [CO2], the plants under both WS and SS showed improved growth by 7-23%. Elevated [CO2] also significantly increased xylem vulnerability (measured as loss of conductivity with decreasing xylem pressure), resulting in smaller hydraulic safety margins. According to the plant desiccation model, the critical desiccation degree (time×vapor pressure deficit) that the plants could tolerate under drought was reduced by 43-64% under elevated [CO2]. In addition, sensitivity analysis showed that P50 was the most important trait in determining the critical desiccation degree. Thus, our results demonstrated that whilst elevated [CO2] benefited plant growth under WS or SS, it also interfered with hydraulic acclimation, thereby potentially placing the plants at a higher risk of hydraulic failure and increased mortality.

Assessing inter-intraspecific variability of leaf vulnerability to embolism for 10 alpine Rhododendron species growing in Southwestern China

Alpine Rhododendron species are prominent constituents and renowned ornamental plants in alpine ecosystems. Consequently, evaluating the genetic variation in embolism resistance within the genus Rhododendron and predicting their adaptability to future climate change is important. Nevertheless, the assessment of embolism resistance in Rhododendron species remains limited. This investigation aimed to examine leaf vulnerability to embolism across ten alpine Rhododendron species, which are frequently employed as ornamental species in Rhododendron forests in Southwest China. The study analyzed the correlation between embolism resistance and various morphological traits, while also conducting water control experiments to evaluate the relationship between embolism resistance and drought resistance. The outcomes indicated pronounced variations in leaf vulnerability to embolism among species, as reflected by the water potential at 50% of embolized pixels (P50 ). Furthermore, the leaf P50 exhibited a significant positive correlation with vessel diameter (D) (R2  = 0.44, P = 0.03) and vessel wall span (b) (R2  = 0.64, P = 0.005), while displaying a significant negative correlation with vessel reinforcement ((t/b)2 ) (R2  = 0.67, P = 0.004). These findings underscore the reliability of selecting species based on embolism vulnerability to preserve the diversity of alpine ecosystems and foster resilience to climate change.

Intraspecific variations of leaf hydraulic, economic, and anatomical traits in Cinnamomum camphora along an urban-rural gradient

Urbanization brings numerous benefits to residents, but it also introduces complex, variable, and heterogeneous habitat conditions to urban plants, resulting in an arid and hot urban environment that decreases tree growth and the ecological service capacity of trees. In this study, we evaluated leaf hydraulic, economic, and anatomical traits and their covariations of Cinnamomum camphora along an urban-rural gradient in Hefei, Eastern China. We found that Cinnamomum camphora in urban adopted a conservative hydraulic strategy with low leaf turgor loss point (Tlp), leaf hydraulic conductance (Kleaf), and leaf water potential resulting in 50 % loss of hydraulic conductance (P50), as well as a quick investment-return economic strategy with low unit leaf dry matter content (LMA) and high leaf nitrogen content (Leaf N). P50, Kleaf and LMA were significantly positively correlated with the urban-rural gradient (PC1urban-rural gradient), while Leaf N exhibited a negative correlation with it. The results showed a trade-off between intraspecific safety and efficiency in leaf hydraulic traits along the urban-rural gradient and an intraspecific coordinated variation in leaf hydraulic and economic traits. In addition, based on the analysis of a trait coordination network, it was revealed that leaf mesophyll and stomata were key structures for trait adjustment and coordination. Furthermore, our findings offer a significant theoretical underpinning for the effective management of landscape trees and the strategic planning of urban tree species.

Hydraulic segmentation explains differences in loss of branch conductance caused by fire

The hydraulic death hypothesis suggests that fires kill trees by damaging the plant's hydraulic continuum in addition to stem cambium. A corollary to this hypothesis is that plants that survive fires possess 'pyrohydraulic' traits that prevent heat-induced embolism formation in the xylem and aid post-fire survival. We examine whether hydraulic segmentation within stem xylem may act as such a trait. To do so, we measured the percentage loss of conductance (PLC) and vulnerability to embolism axially along segments of branches exposed to heat plumes in two differing species, fire-tolerant Eucalyptus cladocalyx F. Muell and fire-sensitive Kiggelaria africana L., testing model predictions that fire-tolerant species would exhibit higher degrees of hydraulic segmentation (greater PLC in the distal parts of the branch than the basal) than fire-intolerant species (similar PLC between segments). Following exposure to a heat plume, K. africana suffered between 73 and 84% loss of conductance in all branch segments, whereas E. cladocalyx had 73% loss of conductance in whole branches, including the distal tips, falling to 29% in the most basal part of the branch. There was no evidence for differences in resistance segmentation between the species, and there was limited evidence for differences in distal vulnerability to embolism across the branches. Hydraulic segmentation in E. cladocalyx may enable it to resprout effectively post-fire with a functional hydraulic system. The lack of hydraulic segmentation in K. africana reveals the need to understand possible trade-offs associated with hydraulic segmentation in long-lived woody species with respect to drought and fire.

Angiosperms follow a convex trade-off to optimize hydraulic safety and efficiency

Intervessel pits are considered to function as valves that avoid embolism spreading and optimize efficient transport of xylem sap across neighbouring vessels. Hydraulic transport between vessels would therefore follow a safety-efficiency trade-off, which is directly related to the total intervessel pit area (Ap ), inversely related to the pit membrane thickness (TPM ) and driven by a pressure difference. To test this hypothesis, we modelled the relative transport rate of gas (ka ) and water (Q) at the intervessel pit level for 23 angiosperm species and correlated these parameters with the water potential at which 50% of embolism occurs (Ψ50 ). We also measured ka for 10 species using pneumatic measurements. The pressure difference across adjacent vessels and estimated values of ka and Q were related to Ψ50 , following a convex safety-efficiency trade-off based on modelled and experimental data. Minor changes in TPM and Ap exponentially affected the pressure difference and flow, respectively. Our results provide clear evidence that a xylem safety-efficiency trade-off is not linear, but convex due to flow across intervessel pit membranes, which represent mesoporous media within microporous conduits. Moreover, the convex nature of long-distance xylem transport may contribute to an adjustable fluid balance of plants, depending on environmental conditions.

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.

Rapid leaf xylem acclimation diminishes the chances of embolism in grapevines

Under most conditions tight stomatal regulation in grapevines (Vitis vinifera) avoids xylem embolism. The current study evaluated grapevine responses to challenging scenarios that might lead to leaf embolism and consequential leaf damage. We hypothesized that embolism would occur if the vines experienced low xylem water potential (Ψx) shortly after bud break or later in the season under a combination of extreme drought and heat. We subjected vines to two potentially dangerous environments: (i) withholding irrigation from a vineyard grown in a heatwave-prone environment, and (ii) subjecting potted vines to terminal drought 1 month after bud break. In the field experiment, a heatwave at the beginning of August resulted in leaf temperatures over 45 °C. However, effective stomatal response maintained the xylem water potential (Ψx) well above the embolism threshold, and no leaf desiccation was observed. In the pot experiment, leaves of well-watered vines in May were relatively vulnerable to embolism with 50% embolism (P50) at -1.8 MPa. However, when exposed to drought, these leaves acclimated their leaf P50 by 0.65 MPa in less than a week and before reaching embolism values. When dried to embolizing Ψx, the leaf damage proportion matched (percentage-wise) the leaf embolism level. Our findings indicate that embolism and leaf damage are usually avoided by the grapevines' efficient stomatal regulation and rapid acclimation of their xylem vulnerability.

Leaf rolling precedes stomatal closure in rice (Oryza sativa) under drought conditions

Leaf rolling is a physiological response to drought that may help to reduce water loss, but its significance as a contribution to drought tolerance is uncertain. We scored the leaf rolling of four rice genotypes along an experimental drought gradient using an improved cryo-microscopy method. Leaf water potential (Ψleaf), gas exchange, chlorophyll fluorescence, leaf hydraulic conductance, rehydration capacity, and the bulk turgor loss point were also analysed. During the drought process, stomatal conductance declined sharply to reduce water loss, and leaves rolled up before the stomata completely closed. The leaf water loss rate of rolled leaves was significantly reduced compared with artificially flattened leaves. The Ψleaf threshold of initial leaf rolling ranged from -1.95 to -1.04 MPa across genotypes. When a leaf rolled so that the leaf edges were touching, photosynthetic rate and stomatal conductance declined more than 80%. Across genotypes, leaf hydraulic conductance declined first, followed by gas exchange and chlorophyll fluorescence parameters. However, the Ψleaf threshold for a given functional trait decline differed significantly among genotypes, with the exception of leaf hydraulic conductance. Our results suggested that leaf rolling was mechanistically linked to drought avoidance and tolerance traits and might serve as a useful phenotypic trait for rice breeding in future drought scenarios.

24-Epibrassinolide alleviates drought effects in young Carapa guianensis plants, improving the hydraulic safety margin, gas exchange and antioxidant defence

Climate change is increasing the frequency of extreme events such as droughts, limiting plant growth and productivity. Exogenous application of plant growth regulators, such as 24-epibrassinolide (EBR), might be a solution as this molecule is organic, eco-friendly, and biodegradable. This is the first research to examine possible roles of EBR on the hydraulic safety margin, physiological behaviour, and metabolism in Carapa guianensis Aubl. (Meliaceae) exposed to drought. C. guianensis is a widely distributed tree in tropical forests of the Amazon. The objective was to determine whether EBR can improve tolerance to water deficit in young C. guianensis by measuring hydraulic traits, nutritional, biochemical and physiological responses, and biomass. The experiment had four randomized treatments: two water conditions (control and water deficit) and two concentrations of EBR (0 and 100 nM EBR). EBR increased the water potential and hydraulic safety margin, increased CO2 fixation, and improved stomatal performance. EBR also stimulated antioxidant defences (SOD, CAT, APX, and POX). Overall, tretreatment with EBR improved drought tolerance of young C. guianensis plants.

Contrasting Responses of Two Grapevine Cultivars to Drought: The Role of Non-structural Carbohydrates in Xylem Hydraulic Recovery

Xylem embolism is one of the possible outcomes of decreasing xylem pressure when plants face drought. Recent studies have proposed a role for non-structural carbohydrates (NSCs) in osmotic pressure generation, required for refilling embolized conduits. Potted cuttings of grapevine Grenache and Barbera, selected for their adaptation to different climatic conditions, were subjected to a drought stress followed by re-irrigation. Stem embolism rate and its recovery were monitored in vivo by X-ray micro-computed tomography (micro-CT). The same plants were further analyzed for xylem conduit dimension and NSC content. Both cultivars significantly decreased Ψpd in response to drought and recovered from xylem embolism after re-irrigation. However, although the mean vessel diameter was similar between the cultivars, Barbera was more prone to embolism. Surprisingly, vessel diameter was apparently reduced during recovery in this cultivar. Hydraulic recovery was linked to sugar content in both cultivars, showing a positive relationship between soluble NSCs and the degree of xylem embolism. However, when starch and sucrose concentrations were considered separately, the relationships showed cultivar-specific and contrasting trends. We showed that the two cultivars adopted different NSC-use strategies in response to drought, suggesting two possible scenarios driving conduit refilling. In Grenache, sucrose accumulation seems to be directly linked to embolism formation and possibly sustains refilling. In Barbera, maltose/maltodextrins could be involved in a conduit recovery strategy via the formation of cell-wall hydrogels, likely responsible for the reduction of conduit lumen detected by micro-CT.

Best Procedures for Leaf and Stem Water Potential Measurements in Grapevine: Cultivar and Water Status Matter

The pressure chamber is the most used tool for plant water status monitoring. However, species/cultivar and seasonal effects on protocols for reliable water potential determination have not been properly tested. In four grapevine cultivars and two times of the season (early season, Es; late season, Ls, under moderate drought), we assessed the maximum sample storage time before leaf water potential (Ψ_leaf) measurements and the minimum equilibration time for stem water potential (Ψ_stem) determination, taking 24 h leaf cover as control. In 'Pinot gris', Ψ_leaf already decreased after 1 h leaf storage in both campaigns, dropping by 0.4/0.5 MPa after 3 h, while in 'Refosk', it decreased by 0.1 MPa after 1 and 2 h in Es and Ls, respectively. In 'Merlot' and 'Merlot Kanthus', even 3 h storage did not affect Ψ_leaf. In Es, the minimum Ψ_stem equilibration was 1 h for 'Refošk' and 10 min for 'Pinot gris' and 'Merlot'. In Ls, 'Merlot Kanthus' required more than 2 h equilibration, while 1 h to 10 min was sufficient for the other cultivars. The observed cultivar and seasonal differences indicate that the proposed tests should be routinely performed prior to experiments to define ad hoc procedures for water status determination.

GrapevineXL reliably predicts multi-annual dynamics of vine water status, berry growth, and sugar accumulation in vineyards

Climate and water availability greatly affect each season's grape yield and quality. Using models to accurately predict environment impacts on fruit productivity and quality is a huge challenge. We calibrated and validated the functional-structural model, GrapevineXL, with a data set including grapevine seasonal midday stem water potential (Ψ_xylem), berry dry weight (DW), fresh weight (FW), and sugar concentration per volume ([Sugar]) for a wine grape cultivar (Vitis vinifera cv. Cabernet Franc) in field conditions over 13 years in Bordeaux, France. Our results showed that the model could make a fair prediction of seasonal Ψ_xylem and good-to-excellent predictions of berry DW, FW, [Sugar] and leaf gas exchange responses to predawn and midday leaf water potentials under diverse environmental conditions with 14 key parameters. By running virtual experiments to mimic climate change, an advanced veraison (i.e. the onset of ripening) of 14 and 28 days led to significant decreases of berry FW by 2.70% and 3.22%, clear increases of berry [Sugar] by 2.90% and 4.29%, and shortened ripening duration in 8 out of 13 simulated years, respectively. Moreover, the impact of the advanced veraison varied with seasonal patterns of climate and soil water availability. Overall, the results showed that the GrapevineXL model can predict plant water use and berry growth in field conditions and could serve as a valuable tool for designing sustainable vineyard management strategies to cope with climate change.

Xylem resistance to cavitation increases during summer in Pinus halepensis

Cavitation resistance has often been viewed as a relatively static trait, especially for stems of forest trees. Meanwhile, other hydraulic traits, such as turgor loss point (Ψ_tlp ) and xylem anatomy, change during the season. In this study, we hypothesized that cavitation resistance is also dynamic, changing in coordination with Ψ_tlp . We began with a comparison of optical vulnerability (OV), microcomputed tomography (µCT) and cavitron methods. All three methods significantly differed in the slope of the curve,Ψ₁₂ and Ψ₈₈ , but not in Ψ₅₀ (xylem pressures that cause 12%, 88%, 50% cavitation, respectively). Thus, we followed the seasonal dynamics (across 2 years) of Ψ₅₀ in Pinus halepensis under Mediterranean climate using the OV method. We found that Ψ₅₀ is a plastic trait with a reduction of approximately 1 MPa from the end of the wet season to the end of the dry season, in coordination with the dynamics of the midday xylem water potential (Ψ_midday ) and the Ψ_tlp . The observed plasticity enabled the trees to maintain a stable positive hydraulic safety margin and avoid cavitation during the long dry season. Seasonal plasticity is vital for understanding the actual risk of cavitation to plants and for modeling species' ability to tolerate harsh environments.

Quantifying the grapevine xylem embolism resistance spectrum to identify varieties and regions at risk in a future dry climate

Maintaining wine production under global warming partly relies on optimizing the choice of plant material for a given viticultural region and developing drought-resistant cultivars. However, progress in these directions is hampered by the lack of understanding of differences in drought resistance among Vitis genotypes. We investigated patterns of xylem embolism vulnerability within and among 30 Vitis species and sub-species (varieties) from different locations and climates, and assessed the risk of drought vulnerability in 329 viticultural regions worldwide. Within a variety, vulnerability to embolism decreased during summer. Among varieties, we have found wide variations in drought resistance of the vascular system in grapevines. This is particularly the case within Vitis vinifera, with varieties distributed across four clusters of embolism vulnerability. Ugni blanc and Chardonnay featured among the most vulnerable, while Pinot noir, Merlot and Cabernet Sauvignon ranked among the most resistant. Regions possibly at greater risk of being vulnerable to drought, such as Poitou-Charentes, France and Marlborough, New Zealand, do not necessarily have arid climates, but rather bear a significant proportion of vulnerable varieties. We demonstrate that grapevine varieties may not respond equally to warmer and drier conditions, and highlight that hydraulic traits are key to improve viticulture suitability under climate change.

Addressing controversies in the xylem embolism resistance-vessel diameter relationship

Although xylem embolism is a key process during drought-induced tree mortality, its relationship to wood anatomy remains debated. While the functional link between bordered pits and embolism resistance is known, there is no direct, mechanistic explanation for the traditional assumption that wider vessels are more vulnerable than narrow ones. We used data from 20 temperate broad-leaved tree species to study the inter- and intraspecific relationship of water potential at 50% loss of conductivity (P₅₀ ) with hydraulically weighted vessel diameter (D_h ) and tested its link to pit membrane thickness (T_PM ) and specific conductivity (K_s ) on species level. Embolism-resistant species had thick pit membranes and narrow vessels. While D_h was weakly associated with T_PM , the P₅₀ -D_h relationship remained highly significant after accounting for T_PM . The interspecific pattern between P₅₀ and D_h was mirrored by a link between P₅₀ and K_s , but there was no evidence for an intraspecific relationship. Our results provide robust evidence for an interspecific P₅₀ -D_h relationship across our species. As a potential cause for the inconsistencies in published P₅₀ -D_h relationships, our analysis suggests differences in the range of trait values covered, and the level of data aggregation (species, tree or sample level) studied.

Hydraulic trade-offs underlie enhanced performance of polyploid trees under soil water deficit

The relationships between aerial organ morpho-anatomy of woody polyploid plants with their functional hydraulics under water stress remain largely understudied. We evaluated growth-associated traits, aerial organ xylem anatomy, and physiological parameters of diploid, triploid, and tetraploid genotypes of atemoyas (Annona cherimola x Annona squamosa), which belong to the woody perennial genus Annona (Annonaceae), testing their performance under long-term soil water reduction. The contrasting phenotypes of vigorous triploids and dwarf tetraploids consistently showed stomatal size-density trade-off. The vessel elements in aerial organs were ∼1.5 times wider in polyploids compared with diploids, and triploids displayed the lowest vessel density. Plant hydraulic conductance was higher in well-irrigated diploids while their tolerance to drought was lower. The phenotypic disparity of atemoya polyploids associated with contrasting leaf and stem xylem porosity traits that coordinate to regulate water balances between the trees and the belowground and aboveground environments. Polyploid trees displayed better performance under soil water scarcity, presenting as more sustainable agricultural and forestry genotypes to cope with water stress.

Phenological physiology: seasonal patterns of plant stress tolerance in a changing climate

The physiological challenges posed by climate change for seasonal, perennial plants include increased risk of heat waves, postbudbreak freezing ('false springs'), and droughts. Although considerable physiological work has shown that the traits conferring tolerance to these stressors - thermotolerance, cold hardiness, and water deficit stress, respectively - are not static in time, they are frequently treated as such. In this review, I synthesize the recent literature on predictable seasonal - and therefore, phenological - patterns of acclimation and deacclimation to heat, cold, and water-deficit stress in perennials, focusing on woody plants native to temperate climates. I highlight promising, high-throughput techniques for quantifying thermotolerance, cold hardiness, and drought tolerance. For each of these forms of stress tolerance, I summarize the current balance of evidence regarding temporal patterns over the course of a year and suggest a characteristic temporal scale in these responses to environmental stress. In doing so, I offer a synthetic framework of 'phenological physiology', in which understanding and leveraging seasonally recurring (phenological) patterns of physiological stress acclimation can facilitate climate change adaptation and mitigation.

Ageing-induced shrinkage of intervessel pit membranes in xylem of Clematis vitalba modifies its mechanical properties as revealed by atomic force microscopy

Bordered pit membranes of angiosperm xylem are anisotropic, mesoporous media between neighbouring conduits, with a key role in long distance water transport. Yet, their mechanical properties are poorly understood. Here, we aim to quantify the stiffness of intervessel pit membranes over various growing seasons. By applying an AFM-based indentation technique "Quantitative Imaging" we measured the effective elastic modulus (E effective) of intervessel pit membranes of Clematis vitalba in dependence of size, age, and hydration state. The indentation-deformation behaviour was analysed with a non-linear membrane model, and paired with magnetic resonance imaging to visualise sap-filled and embolised vessels, while geometrical data of bordered pits were obtained using electron microscopy. E effective was transformed to the geometrically independent apparent elastic modulus E apparent and to aspiration pressure P b. The material stiffness (E apparent) of fresh pit membranes was with 57 MPa considerably lower than previously suggested. The estimated pressure for pit membrane aspiration was 2.20+28 MPa. Pit membranes from older growth rings were shrunken, had a higher material stiffness and a lower aspiration pressure than current year ones, suggesting an irreversible, mechanical ageing process. This study provides an experimental-stiffness analysis of hydrated intervessel pit membranes in their native state. The estimated aspiration pressure suggests that membranes are not deflected under normal field conditions. Although absolute values should be interpreted carefully, our data suggest that pit membrane shrinkage implies increasing material stiffness, and highlight the dynamic changes of pit membrane mechanics and their complex, functional behaviour for fluid transport.

Functional xylem characteristics associated with drought-induced embolism in angiosperms

Hydraulic failure resulting from drought-induced embolism in the xylem of plants is a key determinant of reduced productivity and mortality. Methods to assess this vulnerability are difficult to achieve at scale, leading to alternative metrics and correlations with more easily measured traits. These efforts have led to the longstanding and pervasive assumed mechanistic link between vessel diameter and vulnerability in angiosperms. However, there are at least two problems with this assumption that requires critical re-evaluation: (1) our current understanding of drought-induced embolism does not provide a mechanistic explanation why increased vessel width should lead to greater vulnerability, and (2) the most recent advancements in nanoscale embolism processes suggest that vessel diameter is not a direct driver. Here, we review data from physiological and comparative wood anatomy studies, highlighting the potential anatomical and physicochemical drivers of embolism formation and spread. We then put forward key knowledge gaps, emphasising what is known, unknown and speculation. A meaningful evaluation of the diameter-vulnerability link will require a better mechanistic understanding of the biophysical processes at the nanoscale level that determine embolism formation and spread, which will in turn lead to more accurate predictions of how water transport in plants is affected by drought.

Stem and leaf xylem of angiosperm trees experiences minimal embolism in temperate forests during two consecutive summers with moderate drought

Drought events may increase the likelihood that the plant water transport system becomes interrupted by embolism. Yet our knowledge about the temporal frequency of xylem embolism in the field is frequently lacking, as it requires detailed, long-term measurements. We measured xylem embolism resistance and midday xylem water potentials during the consecutive summers of 2019 and 2020 to estimate maximum levels of embolism in leaf and stem xylem of ten temperate angiosperm tree species. We also studied vessel and pit membrane characteristics based on light and electron microscopy to corroborate potential differences in embolism resistance between leaves and stems. Apart from A. pseudoplatanus and Q. petraea, eight species experienced minimum xylem water potentials that were close to or below those required to initiate embolism. Water potentials corresponding to ca. 12% loss of hydraulic conductivity (PLC) could occur in six species, while considerable levels of embolism around 50% PLC were limited to B. pendula and C. avellana. There was a general agreement in embolism resistance between stems and leaves, with leaves being equally or more resistant than stems. Also, xylem embolism resistance was significantly correlated to intervessel pit membrane thickness (T_PM ) for stems, but not to vessel diameter and total intervessel pit membrane surface area of a vessel. Our data indicate that low amounts of embolism occur in most species during moderate summer drought, and that considerable levels of embolism are uncommon. Moreover, our experimental and T_PM data show that leaf xylem is generally no more vulnerable than stem xylem.

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.

Nanoparticles are linked to polar lipids in xylem sap of temperate angiosperm species

In previous research, xylem sap of angiosperms has been found to include low concentrations of nanoparticles and polar lipids. A major goal of this study was to test predictions arising from the hypothesis that the nanoparticles consist largely of polar lipids from the original cell content of vessel elements. These predictions included that polar lipid and nanoparticle concentrations would be correlated, that they both do not pass through pit membranes and that they do not vary seasonally because they originate from living vessel element cells. We collected xylem sap of six temperate angiosperm species over the whole year to consider seasonal variation. Concentrations of nanoparticles and lipids in xylem sap and contamination control samples were measured with a NanoSight device and mass spectrometry. We found that the concentration of nanoparticles and polar lipids was (i) diluted when an increasing amount of sap was extracted, (ii) significantly correlated to each other for three species, (iii) affected by vessel anatomy, (iv) very low and largely different in chemical composition from contamination controls and (v) hardly variable among seasons. Moreover, there was a minor freezing-thawing effect with respect to nanoparticle amount and size. Xylem sap lipids included polar galactolipids and phospholipids in all species and neutral triacylglycerols in two species. These findings support the predictions and, by implication, the underlying hypothesis that nanoparticles in xylem sap consist of polar lipids from the original cell content of living vessel element cells. Further research is needed to examine the formation and stability of nanoparticles concerning lipid composition and multiphase interactions among gas, liquid and solid phases in xylem conduits of living plants.

A Field Collection of Indigenous Grapevines as a Valuable Repository for Applied Research

The grapevine is an economically important plant, with a historical connection to the development of human culture. Currently, over 6000 accessions are known as individual grapevine varieties, some of which are important to national heritage, valuable for current viticultural practices, and as genetic resources to maintain plasticity under changing climatic conditions, environmental sustainability, and market demands. Recently, the diversity of cultivated grapevines has declined significantly, due to the increased focus of global wine industries on a few major cultivars. Moreover, due to biotic and abiotic stresses, the wild V. vinifera germplasm's genetic diversity has declined, with some varieties on the verge of extinction. Vitis germplasm conservation can be achieved via either in situ (e.g., protected areas) or Ex situ (e.g., field collections, seed banks, and tissue culture collections) methods. This study aims to highlight the importance of Vitis field bank collections. We demonstrate the research done in the Israeli indigenous Vitis vinifera collection. The multi-layer analysis of the varieties enabled the identification of drought stress-resistant varieties, and suggested a mechanism for this resistance through noting the dramatic phenological differences in foliage development between resistant and sensitive varieties. In addition, we show a general characterization of the varieties via major grape characteristics, including bunch and berry shape, as well as their possible utilization based on their aromatic and phenolic profiles.

Seasonal adjustment of leaf embolism resistance and its importance for hydraulic safety in deciduous trees

Embolism resistance is often viewed as seasonally stable. Here we examined the seasonality in the leaf xylem vulnerability curve (VC) and turgor loss point (Ψ_TLP ) of nine deciduous species that originated from Mediterranean, temperate, tropical, or sub-tropical habitats and were growing on the Volcani campus, Israel. All four Mediterranean/temperate species exhibited a shift of their VC to lower xylem pressures (Ψ_x ) along the dry season, in addition to two of the five tropical/sub-tropical species. In three of the species that exhibited VC seasonality, it was critical for avoiding embolism in the leaf. In total, seven out of the nine species avoided embolism. The seasonal VC adjustment was over two times higher as compared with the seasonal adjustment of Ψ_TLP , resulting in improved hydraulic safety as the season progressed. The results suggest that seasonality in the leaf xylem vulnerability is common in species that originate from Mediterranean or temperate habitats that have large seasonal environmental changes. This seasonality is advantageous because it enables a gradual seasonal reduction in the Ψ_x without increasing the danger of embolism. The results also highlight that measuring the minimal Ψ_x and the VC at different times can lead to erroneous estimations of the hydraulic safety margins. Changing the current hydraulic dogma into a seasonal dynamic in the vulnerability of the xylem itself should enable physiologists to understand plants' responses to their environment better.

Safety-efficiency tradeoffs? Correlations of photosynthesis, leaf hydraulics, and dehydration tolerance across species

The tradeoffs between carbon assimilation and hydraulic efficiencies and drought-tolerance traits on different scales are considered a central tenet in plant ecophysiology; however, no clear tradeoff between these traits has emerged in previous studies using woody angiosperms or grasses by investigating several hydraulic tolerance and gas exchange efficiency and/or water transport efficiency traits. In this study, we measured numerous efficiency, resistance, and leaf anatomical traits, including light-saturated gas exchange, leaf hydraulic vulnerability curves, pressure-volume curves, and leaf anatomical traits, in seven species with diverse drought tolerance. A substantial variation in photosynthetic rate, stomatal conductance, mesophyll conductance, maximum leaf hydraulic conductance (K_max), mesophyll anatomical traits, and leaf vein density across species was observed. Both mesophyll conductance and K_max were related to leaf anatomical traits, but other gas exchange traits were decoupled from K_max. Although the efficiency and tolerance traits varied widely across estimated species, no clear trade-off between safety traits and efficiency traits was observed. These findings suggested that postulated leaf-level drought tolerance-carbon assimilation and hydraulic efficiency tradeoff does not exist among distant species and that the fact that different leaf anatomical traits determine efficiency and tolerance capacity might contribute to the lack of such tradeoffs.

Pit characters determine drought-induced embolism resistance of leaf xylem across 18 Neotropical tree species

Embolism spreading in xylem is an important component of plant drought resistance. Since embolism resistance has been shown to be mechanistically linked to pit membrane characters in stem xylem, we speculate that similar mechanisms account for leaf xylem. We conducted transmission electron microscopy to investigate pit membrane characters in leaf xylem across 18 Neotropical tree species. We also conducted gold perfusion and polar lipid detection experiments on three species covering the full range of leaf embolism resistance. We then related these observations to previously published data on embolism resistance of leaf xylem. We also incorporated previously published data on stem embolism resistance and stem xylem pit membranes to investigate the link between vulnerability segmentation (i.e. difference in embolism resistance) and leaf-stem anatomical variation. Maximum pit membrane thickness (Tpm,max) and the pit membrane thickness-to-diameter ratio (Tpm,max/Dpm) were predictive of leaf embolism resistance, especially when vestured pits were taken into account. Variation in Tpm,max/Dpm was the only trait predictive of vulnerability segmentation between leaves and stems. Gold particles of 5- and 10-nm infiltrated pit membranes in three species, while the entry of 50-nm particles was blocked. Moreover, polar lipids were associated with inner conduit walls and pits. Our results suggest that mechanisms related to embolism spreading are determined by Tpm, pore constrictions (i.e. the narrowest bottlenecks along pore pathways), and lipid surfactants, which are largely similar between leaf and stem xylem and between temperate and tropical trees. However, our mechanistic understanding of embolism propagation and the functional relevance of Tpm,max/Dpm remains elusive.

Coordination of hydraulic thresholds across roots, stems, and leaves of two co-occurring mangrove species

Mangroves are frequently inundated with saline water and have evolved different anatomical and physiological mechanisms to filter and, in some species, excrete excess salt from the water they take up. Because salts impose osmotic stress, interspecific differences in salt tolerance and salt management strategy may influence physiological responses to drought throughout the entire plant hydraulic pathway, from roots to leaves. Here, we characterized embolism vulnerability simultaneously in leaves, stems, and roots of seedlings of two mangrove species (Avicennia marina and Bruguiera gymnorrhiza) along with turgor-loss points in roots and leaves and xylem anatomical traits. In both species, the water potentials causing 50% of total embolism were less negative in roots and leaves than they were in stems, but the water potentials causing incipient embolism (5%) were similar in roots, stems, and leaves. Stomatal closure in leaves and turgor loss in both leaves and roots occurred at water potentials only slightly less negative than the water potentials causing 5% of total embolism. Xylem anatomical traits were unrelated to vulnerability to embolism. Vulnerability segmentation may be important in limiting embolism spread into stems from more vulnerable roots and leaves. Interspecific differences in salt tolerance affected hydraulic traits from roots to leaves: the salt-secretor A. marina lost turgor at more negative water potentials and had more embolism-resistant xylem than the salt-excluder B. gymnorrhiza. Characterizing physiological thresholds of roots may help to explain recent mangrove mortality after drought and extended saltwater inundation.

In situ, direct observation of seasonal embolism dynamics in Aleppo pine trees growing on the dry edge of their distribution

Xylem embolism impairs hydraulic conductivity in trees and drives drought-induced mortality. While embolism has been monitored in vivo in potted plants, and research has revealed evidence of embolism in field-grown trees, continuous in situ monitoring of cavitation in forests is lacking. Seasonal patterns of embolism were monitored in branchlets of Aleppo pine (Pinus halepensis) trees growing in a dry Mediterranean forest. Optical visualization (OV) sensors were installed on terminal branches, in addition to monthly sampling for micro computed tomography scans. We detected 208 cavitation events among four trees, which represented an embolism increase from zero to c. 12% along the dry season. Virtually all the cavitation events occurred during daytime hours, with 77% occurring between 10:00 and 17:00 h. The probability for cavitation in a given hour increased as vapor pressure deficit (VPD) increased, up to a probability of 42% for cavitation when VPD > 5 kPa. The findings uniquely reveal the instantaneous environmental conditions that lead to cavitation. The increased likelihood of cavitation events under high VPD in water-stressed pines is the first empirical support for this long hypothesized relationship. Our observations suggest that low levels of embolism are common in Aleppo pine trees at the dry edge of their distribution.

Coordination of leaf hydraulic and economic traits in Cinnamomum camphora under impervious pavement

BACKGROUND

Paved urban environments can pose great threats to the physiological functioning and ecological services of street trees. In this context, assessment of leaf phenotypic plasticity is crucial for understanding the ecological strategy of tree species under impervious pavements.

RESULTS

In this study, we measured a set of leaf economic traits, hydraulic traits of Cinnamomum camphora, and surrounding environmental factors in a street site (the soil was covered by the impervious pavement) and a park site (the soil was covered by grass) in Hefei, eastern China. Compared with the park site, trees in the street site had higher stomatal length (SL), leaf thickness (LT), maximum photochemical quantum yield of photosystem II (Y(II)), and lower stomatal density (SD), specific leaf area (SLA), the leaf water potential at 50% loss of hydraulic conductance (P₅₀), and leaf turgor loss point (TLP). Redundancy analysis showed that air relative humidity and volumetric soil water content caused these traits to be altered.

CONCLUSIONS

Our results showed that C. camphora adapted to the street pavement environment through the coordination of leaf economic and leaf hydraulic traits, and adopted the slow investment return type in the leaf economic spectrum and high drought resistance to meet its actual physiological needs. This finding provides a new perspective for understanding the physiological strategies of street trees to adapt to urban pavement environments.

Drought acclimation of Quercus ilex leaves improves tolerance to moderate drought but not resistance to severe water stress

Increasing temperature and drought can result in leaf dehydration and defoliation even in drought-adapted tree species such as the Mediterranean evergreen Quercus ilex L. The stomatal regulation of leaf water potential plays a central role in avoiding this phenomenon and is constrained by a suite of leaf traits including hydraulic conductance and vulnerability, hydraulic capacitance, minimum conductance to water vapour, osmotic potential and cell wall elasticity. We investigated whether the plasticity in these traits may improve leaf tolerance to drought in two long-term rainfall exclusion experiments in Mediterranean forests. Osmotic adjustment was observed to lower the water potential at turgor loss in the rainfall-exclusion treatments, thus suggesting a stomatal closure at more negative water potentials and a more anisohydric behaviour in drier conditions. Conversely, leaf hydraulic conductance and vulnerability did not exhibit any plasticity between treatments so the hydraulic safety margins were narrower in the rainfall-exclusion treatments. The sequence of leaf responses to seasonal drought and dehydration was conserved among treatments and sites but trees were more likely to suffer losses of turgor and hydraulic functioning in the rainfall-exclusion treatments. We conclude that leaf plasticity might help the trees to tolerate moderate drought but not to resist severe water stress.

Unlocking Drought-Induced Tree Mortality: Physiological Mechanisms to Modeling

Drought-related tree mortality has become a major concern worldwide due to its pronounced negative impacts on the functioning and sustainability of forest ecosystems. However, our ability to identify the species that are most vulnerable to drought, and to pinpoint the spatial and temporal patterns of mortality events, is still limited. Model is useful tools to capture the dynamics of vegetation at spatiotemporal scales, yet contemporary land surface models (LSMs) are often incapable of predicting the response of vegetation to environmental perturbations with sufficient accuracy, especially under stressful conditions such as drought. Significant progress has been made regarding the physiological mechanisms underpinning plant drought response in the past decade, and plant hydraulic dysfunction has emerged as a key determinant for tree death due to water shortage. The identification of pivotal physiological events and relevant plant traits may facilitate forecasting tree mortality through a mechanistic approach, with improved precision. In this review, we (1) summarize current understanding of physiological mechanisms leading to tree death, (2) describe the functionality of key hydraulic traits that are involved in the process of hydraulic dysfunction, and (3) outline their roles in improving the representation of hydraulic function in LSMs. We urge potential future research on detailed hydraulic processes under drought, pinpointing corresponding functional traits, as well as understanding traits variation across and within species, for a better representation of drought-induced tree mortality in models.