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Introduction: Many atmospheric aerosols are hygroscopic and play an important role in cloud formation. Similarly, aerosols become sites of micro-condensation when they deposit to the upper and lower surfaces of leaves. Deposited salts, in particular can trigger condensation at humidities considerably below atmospheric saturation, according to their hygroscopicity and the relative humidity within the leaf boundary layer. Salt induced water potential gradients and the resulting dynamics of concentrated salt solutions can be expected to affect plant water relations. Methods: Hydroponic sunflowers were grown in filtered (FA) and unfiltered, ambient air (AA). Sap flow was measured for 18 days and several indicators of incipient drought stress were studied. Results: At 2% difference in mean vapor pressure deficit (D), AA sunflowers had 49% higher mean transpiration rates, lower osmotic potential, higher proline concentrations, and different tracer transport patterns in the leaf compared to FA sunflowers. Aerosols increased plant conductance particularly at low D. Discussion: The proposed mechanism is that thin aqueous films of salt solutions from deliquescent deposited aerosols enter into stomata and cause an extension of the hydraulic system. This hydraulic connection leads -- parallel to stomatal water vapor transpiration -- to wick-like stomatal loss of liquid water and to a higher impact of D on plant water loss. Due to ample water supply by hydroponic cultivation, AA plants thrived as well as FA plants, but under more challenging conditions, aerosol deposits may make plants more susceptible to drought stress. [ABSTRACT FROM AUTHOR]

Grapevine adaptation to drought involves morphological, anatomical, and physiological modifications that could be viewed as a measure of drought avoidance. The main vine responses to drought consist of the regulation of carbon assimilation as a consequence of limited stomatal conductance, which is reflected in changes in plant water status. In this factorial study (2020–2021 growing seasons), two red cultivars, the local 'Aglianico', widely grown in Central-South Italy, and the international 'Cabernet Sauvignon', were used to evaluate how their interaction in three different environments can modify physiological adaptations and how yields and their qualitative traits can be modified. The lowest leaf water potential (−0.68 Mpa) for the two cultivars was registered in Molise, while the most stressed vine was found in Sicily for Aglianico (−1.86 MPa). At least in two of three locations, Molise and Campania, the detected stomatal conductance and the leaf water potential have shown that Cabernet Sauvignon can be classified as a near-isohydric cultivar, whereas Aglianico can be categorized as a near-anisohydric cultivar. The interactions between genotype x environment highlight different levels of adaptability between the two cultivars in different sites during each season. The data presented here contribute to a better understanding of the effects of genotype and environment interactions in progressive dry cultivation and how these interactions can modify the qualitative traits of grapes. [ABSTRACT FROM AUTHOR]

Key message: Picea abies and Pinus sylvestris seedlings conserve soil water and are more sensitive to drought showing photoinhibition even under moderate stress, while Quercus robur and Fagus sylvatica seedlings have higher soil water uptake, they show photoinhibition only under severe drought. Drought is an important factor in ecological change and species distribution shifts. We conducted a greenhouse experiment with seedlings of four Central European tree species: Pinus sylvestris (PS), Picea abies (PA), Fagus sylvatica (FS), and Quercus robur (QR) to investigate their response to drought. We monitored maximum quantum yield of photosystem II (Fv/Fm) during a 60-day drought treatment and measured above- and below-ground characteristics as morphophysiological responses to drought stress. Due to the fast, juvenile growth of the deciduous species (FS and QR), they had higher soil water uptake and suffered more quickly from severe drought than conifers (PS and PA). The deciduous species maintained a higher Fv/Fm, until volumetric water content (VWC) was very low (< 5%), oscillating within a narrow safety margin. Both conifers PA and PS conserved soil water; photoinhibition in these species occurred at VWC of 14.5% and 5.5%, respectively. There were no differences in height between drought-stressed and irrigated seedlings, while drought reduced all root characteristics of the deciduous seedlings. Our study revealed trade-offs between different water management strategies, growth rate, and photoinhibition during the juvenile growth stage of our focal species. For climate change adaptation, anisohydric deciduous tree species seem to be more suitable. However, PS, with its water-conserving management and low photoinhibition threshold, holds promise for successful regeneration on drought-prone sites. Since species selection is critical for forest sustainability, our study contributes to the broader discussion of tree species' drought resistance during the vulnerable juvenile phase in the face of climate change. [ABSTRACT FROM AUTHOR]

Quercus rubra L. is a moderately shade-tolerant tree species native to eastern North America, readily regenerating since its introduction in the Central and Southern European forests to displace the native pedunculate oak (Quercus robur). Under a scenario of increasing drought, understanding the seedling responses of these two species to water limitation is critical for forest conservation and management. To this aim, morphological, physiological and non-structural carbohydrates analyzes were performed on very-fine and fine roots of Q. robur and Q. rubra seedlings grown under controlled conditions and exposed to two levels of drought before allowing them to recover. Results show significant differences between species for all the investigated traits. The alien Q. rubra showed lower shoot and root biomasses than the native Q. robur, particularly for the thicker fine root 1–2 mm diameter class. However, both species equally invested more biomass in the shoot than the root system (R:S ratio

IntroductionMany atmospheric aerosols are hygroscopic and play an important role in cloud formation. Similarly, aerosols become sites of micro-condensation when they deposit to the upper and lower surfaces of leaves. Deposited salts, in particular can trigger condensation at humidities considerably below atmospheric saturation, according to their hygroscopicity and the relative humidity within the leaf boundary layer. Salt induced water potential gradients and the resulting dynamics of concentrated salt solutions can be expected to affect plant water relations.MethodsHydroponic sunflowers were grown in filtered (FA) and unfiltered, ambient air (AA). Sap flow was measured for 18 days and several indicators of incipient drought stress were studied.ResultsAt 2% difference in mean vapor pressure deficit (D), AA sunflowers had 49% higher mean transpiration rates, lower osmotic potential, higher proline concentrations, and different tracer transport patterns in the leaf compared to FA sunflowers. Aerosols increased plant conductance particularly at low D.DiscussionThe proposed mechanism is that thin aqueous films of salt solutions from deliquescent deposited aerosols enter into stomata and cause an extension of the hydraulic system. This hydraulic connection leads – parallel to stomatal water vapor transpiration – to wick-like stomatal loss of liquid water and to a higher impact of D on plant water loss. Due to ample water supply by hydroponic cultivation, AA plants thrived as well as FA plants, but under more challenging conditions, aerosol deposits may make plants more susceptible to drought stress.

Several important environmental influences of tree growth and carbon sequestration have changed over the past several decades in eastern North America, specifically, more frequent pluvial conditions, increased carbon dioxide (CO2) concentrations, and decreased acidic deposition. These factors could lead to changes in the relationship between tree growth and water availability, and perhaps even decouple the two, having large implications on how future climate change will impact forest productivity and carbon sequestration. Here, we examine the concurrent influence of the climatic water balance (precipitation minus potential evapotranspiration), CO2 concentrations, and sulfate and nitrogen deposition on radial tree growth, carbon isotopes, and intrinsic water-use efficiency (iWUE) for several hardwood tree species in the Midwestern United States. We found that when considering the simultaneous influence of these factors, the climatic water balance is the dominant influence on annual growth. Therefore, the recent pluvial period is the primary cause of the weakening relationship between radial growth and water availability. Even during pluvial periods, water availability is the primary control on growth, with increasing CO2 concentrations and decreased SO4 deposition being secondary factors. Importantly, the weakening in the climate-growth relationship is species specific, with Acer species having stable relationships with the climatic water balance, Liriodendron tulipifera showing a strengthening relationship, and Quercus species and Populus grandidentata exhibiting weakening. Thus, interannual variations in soil moisture unevenly impact tree growth and carbon sequestration. Our findings suggest that, despite recent pluvial conditions, increasing CO2 concentrations and decreasing acidic deposition have not buffered the impact of water availability on tree growth and carbon sequestration.

Species-specific responses of plant intrinsic water-use efficiency (iWUE) to multiple environmental drivers associated with climate change, including soil moisture (θ), vapor pressure deficit (D), and atmospheric CO2 concentration (ca ), are poorly understood. We assessed how the iWUE and growth of several species of deciduous trees that span a gradient of isohydric to anisohydric water-use strategies respond to key environmental drivers (θ, D and ca ). iWUE was calculated for individual tree species using leaf-level gas exchange and tree-ring δ13 C in wood measurements, and for the whole forest using the eddy covariance method. The iWUE of the isohydric species was generally more sensitive to environmental change than the anisohydric species was, and increased significantly with rising D during the periods of water stress. At longer timescales, the influence of ca was pronounced for isohydric tulip poplar but not for others. Trees' physiological responses to changing environmental drivers can be interpreted differently depending on the observational scale. Care should be also taken in interpreting observed or modeled trends in iWUE that do not explicitly account for the influence of D.

Partitioning of evapotranspiration (ET) into transpiration (T) and residual evaporation (E) is a challenging but important task in order to assess the dynamics of increasingly scarce water resources in forest ecosystems. The T/ET ratio has been linked to the ecosystem water use efficiency of temperate forests, and thus is an important index for understanding utilization of water resources under global climate change. We used concurrent sap flow and eddy-covariance measurements to quantify the ET partitioning in pure European beech forest during the 2019–2020 period. The sap flow data were upscaled to stand level T and combined with stand level ET to calculate the T/ET ratio. We analysed intra-annual dynamics, the effect of seasonality and the impact of meteorological conditions on T, ET and T/ET. Annual T/ET of a pure European beech ecosystem was 0.48, falling at the lower end of reported global T/ET values for forest ecosystems. T/ET showed significant seasonal differences throughout spring (T/ET = 0.28), summer (T/ET = 0.62) and autumn (T/ET = 0.35). Air temperature (R2 = 0.45–0.63), VPD (R2 = 0.47–0.6) and PAR (R2 = 0.32–0.63) affected the daily dynamics of T, ET and T/ET; however, soil water content (SWC) had no significant effect. Mature European beech trees showed more anisohydric behaviour and relatively stable T/ET, even under decreasing SWC. The results improve the understanding of ecosystem scale T, ET and T/ET intra-annual dynamics and environmental constraints in anisohydric mature European beech. [ABSTRACT FROM AUTHOR]

Understanding of plant hydraulic strategies (i.e. the degree of iso‐/anisohydricity) is crucial to predict the response of plants to changing environmental conditions such as climate‐change induced extreme drought. Several abiotic factors, including evaporative demand, have been shown to seasonally modify the isohydricity of plants. However, the impact of biotic factors such as plant–plant interactions on hydraulic strategies has seldom been explored.Here, we investigated adaptations and changes in hydraulic strategies of two woody species in response to seasonal abiotic conditions, experimental drought and plant invasion in a Mediterranean cork oak (Quercus suber) ecosystem with a combined shrub invasion (Cistus ladanifer) and rain exclusion experiment.From the dry to wet season, Q. suber shifted from a partial isohydric to an anisohydric behaviour while C. ladanifer shifted from strict anisohydric to partial isohydric. During drought, water competition by plant invasion significantly modified the hydraulic strategy of invaded Q. suber, which was accompanied by lower pre‐dawn leaf water potentials, sap flow density, leaf area index and trunk increment rates.This altered isohydricity of invaded Q. suber trees was most likely caused by interspecific competition for water resources by water spending C. ladanifer shrubs. Both species do have the highest proportion of fine roots in the topsoil and thus, an additional water consumer, such as C. ladanifer can lead to more stressful conditions for Q. suber during times of water scarcity. Further underlying mechanisms of the altered isohydricity of Q. suber, such as potential allelopathic effects of C. ladanifer exudates on root growth of Q. suber, have to be investigated in the future.In conclusion, we demonstrate that the degree of isohydricity of two woody Mediterranean plant species is dynamically determined by the interplay of species‐specific hydraulic traits and their abiotic and biotic environment. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Predicting the impact of drought on forest ecosystem processes requires an understanding of trees' species-specific responses to drought, especially in the Eastern USA, where species composition is highly dynamic due to historical changes in land use and fire regime. Here, we adapted a framework that classifies trees' water-use strategy along the spectrum of isohydric to anisohydric behavior to determine the responses of three canopy-dominant species to drought. We used a collection of leaf-level gas exchange, tree-level sap flux and stand-level eddy covariance data collected in south-central Indiana from 2011 to 2013, which included an unusually severe drought in the summer of 2012. Our goal was to assess how patterns in the radial profile of sap flux and reliance on hydraulic capacitance differed among species of contrasting water-use strategies. In isohydric species, which included sugar maple (Acer saccharum Marsh.) and tulip poplar (Liriodendron tulipifera L.), we found that the sap flux in the outer xylem experienced dramatic declines during drought, but sap flux at inner xylem was buffered from reductions in water availability. In contrast, for anisohydric oak species (Quercus alba L. and Quercus rubra L.), we observed relatively smaller variations in sap flux during drought in both inner and outer xylem, and higher nighttime refilling when compared with isohydric species. This reliance on nocturnal refilling, which occurred coincident with a decoupling between leaf- and tree-level water-use dynamics, suggests that anisohydric species may benefit from a reliance on hydraulic capacitance to mitigate the risk of hydraulic failure associated with maintaining high transpiration rates during drought. In the case of both isohydric and anisohydric species, our work demonstrates that failure to account for shifts in the radial profile of sap flux during drought could introduce substantial bias in estimates of tree water use during both drought and non-drought periods.

SUMMARY: The leaf of a deciduous species completes its life cycle in a few months. During leaf maturation, osmolyte accumulation leads to a significant reduction of the turgor loss point (ΨTLP), a known marker for stomatal closure. Here we exposed two grapevine cultivars to drought at three different times during the growing season to explore if the seasonal decrease in leaf ΨTLP influences the stomatal response to drought. The results showed a significant seasonal shift in the response of stomatal conductance to stem water potential (gs~Ψstem), demonstrating that grapevines become increasingly tolerant to low Ψstem as the season progresses in coordination with the decrease in ΨTLP. We also used the SurEau hydraulic model to demonstrate a direct link between osmotic adjustment and the plasticity of gs~Ψstem. To understand the possible advantages of gs~Ψstem plasticity, we incorporated a seasonally dynamic leaf osmotic potential into the model that simulated stomatal conductance under several water availabilities and climatic scenarios. The model demonstrated that a seasonally dynamic stomatal closure threshold results in trade‐offs: it reduces the time to turgor loss under sustained long‐term drought, but increases overall gas exchange particularly under seasonal shifts in temperature and stochastic water availability. A projected hotter future is expected to lower the increase in gas exchange that plants gain from the seasonal shift in gs~Ψstem. These findings show that accounting for dynamic stomatal regulation is critical for understanding drought tolerance. [ABSTRACT FROM AUTHOR]

The increase in the frequency and severity of extreme weather events affects populations of tree species. As a result, many forests around the world have suffered massive mortality due to extreme droughts. Therefore, it is important to understand the potential adjustments of dominant woody species of temperate forests to climate change. The objective was to show in seedlings inhabiting the most contrasting habitats, forest types, and precipitation regimes the relevance of species-specific and intra-specific adaptive trait variation. Ecophysiological differences between seedlings collected from distinct humid and dry provenances of Nothofagus pumilio and N. dombeyi in northern Patagonia were assessed using common garden and water stress manipulative experiments. Levels of genetic diversity and divergence were evaluated with microsatellite analysis. Seedlings from humid provenances of both species showed increased growth, and leaf traits differed significantly between N. dombeyi provenances. Studied species behaved differently under water stress, isohydric for N. dombeyi and anisohydric for N. pumilio, while no intraspecific differences were measured in the regulation of water status. Humid provenance of N. dombeyi had higher genetic diversity than the dry one, whereas those of N. pumilio were similarly diverse. Yet, low but significantly different from zero genetic divergence was measured between provenances of the latter. Our results show species-specific responses to water stress and that adaptive variation at distinct provenances has a genetic basis that is maintained despite potential gene flow. The strategy of using seedlings from "regional admixture provenancing" is suggested for long-term management of vulnerable taxa to desiccation as the mesic N. dombeyi. [ABSTRACT FROM AUTHOR]

Ephedra alata subsp. alenda is the most important pioneer plant of the moving and semi-stable sand dunes in the deserts and steppes of south Tunisia and occurs naturally in the Grand Erg Oriental, one of the most extreme habitats for plant growth on the planet. A new analysis of physiological performance of this medically important and internationally threatened xerophytic shrub was conducted to assess possible mechanisms of drought tolerance and how these relate to its ecological success. Five-month old plants, grown under controlled climatic conditions, were subjected to a well-watered control treatment or progressive drought by withholding water for 14d with subsequent recovery for 7d. Soil water depletion significantly reduced stem relative water content (RWC) water potential (Ψw) and osmotic potential (Ψπ). Ephedra displayed more negative Ψw and Ψπ values of ca. -3.5 and -4.1MPa, respectively, at the end of the drought treatment, and were associated with turgor loss. Low stem Ψw reduced stomatal conductance (gs), photosynthetic CO2 assimilation rates (ACO2), transpiration (E) and internal CO2 concentration (Ci). However, instantaneous (WUE; ACO2 E-1) and intrinsic (WUEi; ACO2 gs-1) water use efficiency (WUE) increased gradually as water deficit was intensified. Stomatal closure therefore only exerted limited control against dehydration and could not compensate for decreases in soil water status, typical of anisohydric behavior. Drought-stressed stems accumulated high levels of proline up to 480% of control values, highlighting a pivotal role in osmotic adjustment during intense water deficit. In contrast, the osmotic adaptation to soluble sugars was limited. Drought-stressed plants increased ACO2, E, gs and Ci and decreased WUE and WUEi during the first 48h after re-watering, such that they reached similar values to those of control plants by the end of the experiment. Stem proline levels of drought-stressed plants returned to near control values with re-watering. Overall, rapid recovery of photosynthesis following drought-breaking moisture appears to be a critical mechanism allowing E. alata to withstand and survive dry environments.

Ephedra alata subsp. alenda is the most important pioneer plant of the moving and semi-stable sand dunes in the deserts and steppes of south Tunisia and occurs naturally in the Grand Erg Oriental, one of the most extreme habitats for plant growth on the planet. A new analysis of physiological performance of this medically important and internationally threatened xerophytic shrub was conducted to assess possible mechanisms of drought tolerance and how these relate to its ecological success. Five-month old plants, grown under controlled climatic conditions, were subjected to a well-watered control treatment or progressive drought by withholding water for 14d with subsequent recovery for 7d. Soil water depletion significantly reduced stem relative water content (RWC) water potential (Ψw) and osmotic potential (Ψπ). Ephedra displayed more negative Ψw and Ψπ values of ca. -3.5 and -4.1MPa, respectively, at the end of the drought treatment, and were associated with turgor loss. Low stem Ψw reduced stomatal conductance (gs), photosynthetic CO2 assimilation rates (ACO2), transpiration (E) and internal CO2 concentration (Ci). However, instantaneous (WUE; ACO2 E-1) and intrinsic (WUEi; ACO2 gs-1) water use efficiency (WUE) increased gradually as water deficit was intensified. Stomatal closure therefore only exerted limited control against dehydration and could not compensate for decreases in soil water status, typical of anisohydric behavior. Drought-stressed stems accumulated high levels of proline up to 480% of control values, highlighting a pivotal role in osmotic adjustment during intense water deficit. In contrast, the osmotic adaptation to soluble sugars was limited. Drought-stressed plants increased ACO2, E, gs and Ci and decreased WUE and WUEi during the first 48h after re-watering, such that they reached similar values to those of control plants by the end of the experiment. Stem proline levels of drought-stressed plants returned to near control values with re-watering. Overall, rapid recovery of photosynthesis following drought-breaking moisture appears to be a critical mechanism allowing E. alata to withstand and survive dry environments.

Abstract Multiple factors, such as the vineyard environment and winemaking practices, are known to affect the development of vines as well as the final composition of grapes. Water stress promotes the synthesis of phenols and is associated with grape quality as long as it does not inhibit production. To identify the key parameters for managing water stress and grape quality, multivariate statistical analysis is essential. Classification and regression trees are methods for constructing prediction models from data, especially when data are complex and when constructing a single global model is difficult and models are challenging to interpret. The models were obtained by recursively partitioning the data space and fitting a simple prediction model within each partition. The partitioning can be represented graphically as a decision tree. This approach permitted the most decisive variables for predicting the most vulnerable vineyards and wine quality parameters associated with water stress. In Priorat AOC, Carignan grapevines had the highest water potential and abscisic acid concentration in the early growth plant stages and permitted vineyards to be classified by mesoclimate. This information is useful for identifying which measurements could most easily differentiate between early and late‐ripening vineyards. LWP and Ts during an early physiological stage (pea size) permitted warm and cold areas to be differentiated.

Multiple factors, such as the vineyard environment and winemaking practices, are known to affect the development of vines as well as the final composition of grapes. Water stress promotes the synthesis of phenols and is associated with grape quality as long as it does not inhibit production. To identify the key parameters for managing water stress and grape quality, multivariate statistical analysis is essential. Classification and regression trees are methods for constructing prediction models from data, especially when data are complex and when constructing a single global model is difficult and models are challenging to interpret. The models were obtained by recursively partitioning the data space and fitting a simple prediction model within each partition. The partitioning can be represented graphically as a decision tree. This approach permitted the most decisive variables for predicting the most vulnerable vineyards and wine quality parameters associated with water stress. In Priorat AOC, Carignan grapevines had the highest water potential and abscisic acid concentration in the early growth plant stages and permitted vineyards to be classified by mesoclimate. This information is useful for identifying which measurements could most easily differentiate between early and late-ripening vineyards. LWP and Ts during an early physiological stage (pea size) permitted warm and cold areas to be differentiated. [ABSTRACT FROM AUTHOR]

Under conditions of high transpiration and low soil water availability, the demand for water can exceed supply causing a reduction in water potential and a loss of cell turgor (wilting). Regulation of stomatal aperture mediates the loss of water vapour (g s), which in turn is dependent in part on the anatomical characteristics of stomatal density (SD) and stomatal size (SS). Anisohydric sugar beet (Beta vulgaris) is atypical, exhibiting wilting under high soil water availability. Spinach (Spinacia oleracea) belongs to the same family Chenopodiaceae s.s. but demonstrates a more typical wilting response. To investigate the role of stomatal dynamics in such behaviours, sugar beet and spinach leaves were exposed to step-changes in photosynthetic photon flux density (PPFD) from 250 to 2500 µmol m−2 s−1. Using a four log-logistic function, the maximum rate of stomatal opening was estimated. Concurrent measurements of SD and SS were taken for both species. While sugar beet coupled faster opening with smaller, more numerous stomata, spinach showed the converse. After exposure to drought, maximum g s was reduced in sugar beet but still achieved a similar speed of opening. It is concluded that sugar beet stomata respond rapidly to changes in PPFD with a high rate and magnitude of opening under both non-droughted and droughted conditions. Such a response may contribute to wilting, even under high soil water availability, but enables photosynthesis to be better coupled with increasing PPFD. [ABSTRACT FROM AUTHOR]

PREMISE: The xylem tissue of plants performs three principal functions: transport of water, support of the plant body, and nutrient storage. Tradeoffs may arise because different structural requirements are associated with different functions or because suites of traits are under selection that relate to resource acquisition, use, and turnover. The structural and functional basis of xylem storage is not well established. We hypothesized that greater starch storage would be associated with greater sapwood parenchyma and reduced fibers, which would compromise resistance to xylem tensions during dehydration. METHODS: We measured cavitation resistance, minimum water potential, starch content, and sapwood parenchyma and fiber area in 30 species of southern California chaparral shrubs (evergreen and deciduous). RESULTS: We found that species storing greater starch within their xylem tended to avoid dehydration and were less cavitation resistant, and this was supported by phylogenetic independent contrasts. Greater sapwood starch was associated with greater parenchyma area and reduced fiber area. For species without living fibers, the associations with parenchyma were stronger, suggesting that living fibers may expand starch storage capacity while also contributing to the support function of the vascular tissue. Drought‐deciduous species were associated with greater dehydration avoidance than evergreens. CONCLUSIONS: Evolutionary forces have led to an association between starch storage and dehydration resistance as part of an adaptive suite of traits. We found evidence for a tradeoff between tissue mechanical traits and starch storage; moreover, the evolution of novel strategies, such as starch‐storing living fibers, may mitigate the strength of this tradeoff. [ABSTRACT FROM AUTHOR]

and carbon-related measurements can help elucidate drought-induced plant mortality. To study drought mortality mechanisms, seedlings of two woody species, including the anisohydric Robinia pseudoacacia and isohydric Quercus acutissima , were cultivated in a greenhouse and subjected to intense drought by withholding water and mild drought by adding half of the amount of daily water lost. Patterns of leaf and root gas exchange, leaf surface areas, growth, leaf and stem hydraulics, and carbohydrate dynamics were determined in drought-stressed and control seedlings. We detected a complete loss of hydraulic conductivity and partial depletion of total nonstructural carbohydrates contents (TNC) in the dead seedlings. We also found that intense drought triggered a more rapid decrease in plant water potential and a faster drop in net photosynthesis below zero, and a greater TNC loss in dead seedlings than mild drought. Additionally, anisohydric R. pseudoacacia suffered a rapider death than the isohydric Q. acutissima. Based on these findings, we propose that hydraulic conductivity loss and carbon limitation jointly contributed to drought-induced death, while the relative contributions could be altered by drought intensity. We thus believe that it is important to illustrate the mechanistic relationships between stress intensity and carbon-hydraulics coupling in the context of isohydric vs. anisohydric hydraulic strategies. • There were complete conductivity loss and partial C depletion in dead plants. • Intense drought also resulted in a smaller C loss and in a rapid conductivity loss when compared with mild drought. • Anisohydric R. pseudoacacia suffered higher risk of hydraulic failure, and a rapid mortality than isohydric Q. acutissima. [ABSTRACT FROM AUTHOR]

Both ozone (O3) and drought can limit carbon fixation by forest trees. To cope with drought stress, plants have isohydric or anisohydric water use strategies. Ozone enters plant tissues through stomata. Therefore, stomatal closure can be interpreted as avoidance to O3 stress. Here, we applied an optimization model of stomata involving water, CO2, and O3 flux to test whether isohydric and anisohydric strategies may affect avoidance of O3 stress by stomatal closure in four Mediterranean tree species during drought. The data suggest that stomatal closure represents a response to avoid damage to the photosynthetic mechanisms under elevated O3 depending on plant water use strategy. Under high‐O3 and well‐watered conditions, isohydric species limited O3 fluxes by stomatal closure, whereas anisohydric species activated a tolerance response and did not actively close stomata. Under both O3 and drought stress, however, anisohydric species enhanced the capacity of avoidance by closing stomata to cope with the severe oxidative stress. In the late growing season, regardless of the water use strategy, the efficiency of O3 stress avoidance decreased with leaf ageing. As a result, carbon assimilation rate was decreased by O3 while stomata did not close enough to limit transpirational water losses. In carbon‐starved red‐fleshed kiwifruit both carbohydrate and anthocyanin concentrations were greatly reduced. Our model suggests anthocyanin biosynthesis was actively repressed by MYB27 via trehalose 6‐phosphate signalling to conserve carbon. [ABSTRACT FROM AUTHOR]

Non-structural carbohydrate (NSC) pools fluctuate based on the interplay between photosynthesis, demand from various carbon (C) sinks and tree hydraulic status. Thus, it has been hypothesized that tree species with isohydric stomatal control (i.e., trees that close stomata rapidly in response to drought) rely heavily on NSC pools to sustain metabolism, which can lead to negative physiological consequences such as C depletion. Here, we seek to use a species' degree of isohydry or anisohydry as a conceptual framework for understanding the interrelations between photosynthetic C supply, hydraulic damage and fluctuations in NSC pools. We conducted a 6-week experimental drought, followed by a 6-week recovery period, in a greenhouse on seven tree species that span the spectrum from isohydric to anisohydric. Throughout the experiment, we measured photosynthesis, hydraulic damage and NSC pools. Non-structural carbohydrate pools were remarkably stable across species and tissues—even highly isohydric species that drastically reduced C assimilation were able to maintain stored C. Despite these static NSC pools, we still inferred an important role for stored C during drought, as most species converted starches into sugars during water stress (and back again post-drought). Finally, we did not observe any linkages between C supply, hydraulic damage and NSC pools, indicating that NSC was maintained independent of variation in photosynthesis and hydraulic function. Our results advance the idea that C depletion is a rare phenomenon due to either active maintenance of NSC pools or sink limitation, and thus question the hypothesis that reductions in C assimilation necessarily lead to C depletion. [ABSTRACT FROM AUTHOR]

Abstract As a major driving element of the structure and function of arid and semiarid ecosystems, rainfall is the essential factor limiting plant biological processes. To clarify the characteristics of transpiration and responses to summer rainfall, sap flow density (Fd) of Pinus tabulaeformis and Hippophae rhamnoides was monitored using thermal dissipation probes. In addition, midday leaf water potential (ψm) and leaf stomatal conductance (Gs) were also analyzed to determine water use strategies. The results indicated that the diurnal variation in the normalized Fd values exhibited a single‐peak curve for P. tabulaeformis, while H. rhamnoides showed multiple peaks. The normalized Fd for P. tabulaeformis remained relatively stable regardless of rainfall events. However, there was also a significant increase in the normalized Fd for H. rhamnoides in response to rainfall in June and August (p

Several important environmental influences of tree growth and carbon sequestration have changed over the past several decades in eastern North America, specifically, more frequent pluvial conditions, increased carbon dioxide (CO2) concentrations, and decreased acidic deposition. These factors could lead to changes in the relationship between tree growth and water availability, and perhaps even decouple the two, having large implications on how future climate change will impact forest productivity and carbon sequestration. Here, we examine the concurrent influence of the climatic water balance (precipitation minus potential evapotranspiration), CO2 concentrations, and sulfate and nitrogen deposition on radial tree growth, carbon isotopes, and intrinsic water‐use efficiency (iWUE) for several hardwood tree species in the Midwestern United States. We found that when considering the simultaneous influence of these factors, the climatic water balance is the dominant influence on annual growth. Therefore, the recent pluvial period is the primary cause of the weakening relationship between radial growth and water availability. Even during pluvial periods, water availability is the primary control on growth, with increasing CO2 concentrations and decreased SO4 deposition being secondary factors. Importantly, the weakening in the climate‐growth relationship is species specific, with Acer species having stable relationships with the climatic water balance, Liriodendron tulipifera showing a strengthening relationship, and Quercus species and Populus grandidentata exhibiting weakening. Thus, interannual variations in soil moisture unevenly impact tree growth and carbon sequestration. Our findings suggest that, despite recent pluvial conditions, increasing CO2 concentrations and decreasing acidic deposition have not buffered the impact of water availability on tree growth and carbon sequestration. Plain Language Summary: In recent decades, many factors that influence tree growth have changed across the Midwestern United States, including more precipitation, higher carbon dioxide concentrations in the atmosphere, and fewer pollutants. Changes in these environmental factors could result in tree growth being less sensitive to water availability. While a weakening relationship between water availability and tree growth is present, it is difficult to determine the cause. Here, we examine the simultaneous influence of a wetter climate, higher carbon dioxide concentrations, and a decrease in pollutants deposition on tree growth and how efficiently trees use water. We found that when considering all three variables, increased moisture was the leading influence on tree growth. Therefore, the recent wet period is behind trees being less sensitive to soil moisture, not increases in carbon dioxide or decreases in pollutants. It is important to note that some species such as oak and aspen did become less sensitive to soil moisture, other species such as maple and tuliptree did not. This is important because when drought conditions return to the region, trees will still be sensitive despite higher carbon dioxide concentrations or decreases in sulfate and nitrate deposition. Key Points: Recent weakening of the relationship between climatic water balance and radial growth and C isotopes was species specificWater availability is the dominant factor influencing long‐term radial growth even when considering the simultaneous influence of other factors such as increasing atmospheric CO2 and decreasing acidic depositionIncreasing atmospheric CO2 concentrations increased water‐use efficiency but not growth of most tree species [ABSTRACT FROM AUTHOR]

Premise: Understanding plant hydraulic functioning and water balance during drought has become key in predicting species survival and recovery. However, there are few insightful studies that couple physiological and morphological attributes for many ecosystems, such as the vulnerable Tropical Montane Cloud Forests (TMCF). In this study, we evaluated drought resistance and recovery for saplings for five tree species spanning deciduous to evergreen habits from a Mexican TMCF. Methods: In drought simulations, water was withheld until plants reached species‐specific P50 or P88 values (pressures required to induce a 50 or 88% loss in hydraulic conductivity), then they were rewatered. Drought resistance was considered within the isohydric–anisohydric framework and compared to leaf gas exchange, water status, pressure–volume curves, specific leaf area, and stomatal density. Results: The TMCF species closed stomata well before significant losses in hydraulic conductivity (isohydric). Yet, despite the coordination of these traits, the traits were not useful for predicting the time needed for the species to reach critical hydraulic thresholds. Instead, maximum photosynthetic rates explained these times, reinforcing the linkage between hydraulic and carbon dynamics. Despite their varying hydraulic conductivities, stomatal responses, and times to hydraulic thresholds, 58 of the 60 study plants recovered after the rewatering. The recovery of photosynthesis and stomatal conductance can be explained by the P50 values and isohydry. Conclusions: This study raises new questions surrounding drought management strategies, recovery processes, and how lethal thresholds are defined. Further studies need to consider the role of water and carbon balance in allowing for both survival and recovery from drought. [ABSTRACT FROM AUTHOR]

• No information exists on the quince tree mechanisms to confront drought. • Quince trees were characterized by an extreme anisohydric behavior. • Active osmotic adjustment operated contributing to the maintenance of leaf turgor (stress tolerance mechanism). • Low apoplastic water contributed a steeper gradient in water potential between the leaf and the soil favouring water absorption. Quince tree (Cydonia oblonga Mill.) is known for bearing fruits that are rich in nutrients and health-promoting compounds while requiring low inputs of agrochemicals, and maintenance, but no information exists on the mechanisms developed at the level of leaf water relations to confront water stress and recovery. For this reason, the purpose of the present study was to identify the strategy (isohydric or anisohydric) by which quince plants cope with water stress and to further elucidate the resistance mechanisms developed in response to water stress and during recovery. In summer 2016, field-grown own rooted 17-years old quince trees (cv. BA-29) were subjected to two irrigation treatments. Control (T0) plants were drip irrigated (105% ETo) to ensure non-limiting soil water conditions, while T1 plants were irrigated at the same level as used in T0, except that irrigation was withheld for 42 days during the linear fruit growth phase, after which irrigation returned to the levels of T0 (recovery period). During the experimental period, T0 and T1 received a total of 374 and 143 mm water, respectively, including rain water. The quince trees exhibited extreme anisohydric behaviour under the experimental conditions. As water stress developed and during the recovery period, the plants exhibited high hydraulic conductivity, probably the result of resistance to cavitation. From the beginning of water stress to the time of maximum water stress, leaf turgor was maintained, possibly due to active osmotic adjustment (stress tolerance mechanism). This leaf turgor maintenance may have contributed to the high leaf conductance, and, therefore, good leaf productivity. The low quince leaf apoplastic water fraction under water stress could be considered as another drought tolerance characteristic because if the accumulation of water in the apoplasm is avoided a steeper gradient in water potential between the leaf and the soil can take place under water stress, thus favouring water absorption. [ABSTRACT FROM AUTHOR]

In the near future, stomatal behaviour will be crucial to counteract conditions arising from climate change. Grapevine varieties are classified as either isohydric or anisohydric, depending on the sensitivity of stomata to water deficit and on their water potential homeostasis. However, the great variability observed in different studies indicates that a continuum exists in the range of stomatal sensitivity to water stress. Thus, more knowledge about the hydric behaviour and the gas exchange of isohydric and anisohydric grapevine varieties under different water conditions could lead to the development of irrigation strategies oriented at improving water-use efficiency, yield and berry composition. In this study, research was conducted in order to characterise the stomatal regulation of four different Vitis vinifera L. varieties, namely Pinot noir, Sauvignon blanc, Chardonnay and Merlot, according to soil water status. Measurements of leaf gas exchange, together with measurements of stem water potential (Ψs) and leaf water potential (Ψl), were taken during two seasons. Under conditions of water stress, Chardonnay and Merlot reached a minimum Ψs of -1.67 and -1.68 MPa respectively, and higher levels of water-use efficiency (AN/gs), of 62.3 and 69.7 µmol CO2/mol H2O respectively. In Sauvignon blanc and Pinot noir, the minimum Ψs was -1.26 and -1.40 MPa respectively, with lower levels of AN/gs (53.1 and 50.5 µmol CO2/mol H2O, respectively). Under conditions of water stress (Ψl < -0.9 MPa and Ψs < -0.6 MPa), all varieties had a significantly increased AN/gs¹, despite a significant reduction in gas exchange. Therefore, the hydric behaviour and gas exchange observed in this study suggest that Chardonnay and Merlot could be characterised as anisohydric varieties, as they present less sensitive stomatal control, while Pinot noir can be classified as a near-anisohydric variety and Sauvignon blanc as an isohydric variety. New investigations should consider other characteristics of the varieties to classify them better. [ABSTRACT FROM AUTHOR]

Over the past decade, the concept of isohydry or anisohydry, which describes the link between soil water potential (ΨS), leaf water potential (ΨL), and stomatal conductance (gs), has soared in popularity. However, its utility has recently been questioned, and a surprising lack of coordination between the dynamics of ΨL and gs across biomes has been reported. Here, we offer a more expanded view of the isohydricity concept that considers effects of vapour pressure deficit (VPD) and leaf area index (AL) on the apparent sensitivities of ΨL and gs to drought. After validating the model with tree‐ and ecosystem‐scale data, we find that within a site, isohydricity is a strong predictor of limitations to stomatal function, though variation in VPD and leaf area, among other factors, can challenge its diagnosis. Across sites, the theory predicts that the degree of isohydricity is a good predictor of the sensitivity of gs to declining soil water in the absence of confounding effects from other drivers. However, if VPD effects are significant, they alone are sufficient to decouple the dynamics of ΨL and gs entirely. We conclude with a set of practical recommendations for future applications of the isohydricity framework within and across sites. We expand the mathematical basis of the popular but controversial isohydric–anisohydric framework for plant drought response to include variations in atmospheric aridity and drought deciduousness. We demonstrate that aridity effects are critical to consider when applying the framework across sites. [ABSTRACT FROM AUTHOR]

Understanding the physiological basis underlying the water stress responses in grapevine is becoming increasingly topical owing to the challenges that climate change will impose to grapevine agriculture. Here we used cv. Pinot gris (clone H1), grafted on a series of tolerant (1103Paulsen; P), sensitive (SO4) and recently selected (Georgikon28; G28, Georgikon121; G121, Zamor17; Z17) rootstocks. Plants were either subjected to reduced water availability (WS) or maintained at pot capacity (WW). Photosynthetic (light response curves), stomatal and in vivo gas exchange analysis were carried out as well as dynamics of daily water use (WU), leaf area accumulation with affordable RGB imaging pipelines and leaf water potential. Significant genotypic variation was recorded between rootstocks for most of the traits analyzed under optimal conditions with P and SO4 showing a more vigorous growth, higher CO2 assimilation rate, stomatal conductance and stomatal density per unit of leaf area than G28, G121, Z17 (p < 0.001). Under WS, rootstocks induced different water stress response in Pinot gris, with G28 and G121 showing a higher sensitivity of water use to reduced water availability (WS) (p = 0.021) and no variation for midday leaf water potential until severe WS. P, Z17 and to some extent SO4 induced a pronounced near-anisohydric response with a general WU maintenance followed by reduction in leaf water potential even at high levels of soil water content. In addition, G28 and G121 showed a less marked slope in the linear relationship between daily water use and VPD (p = 0.008) suggesting elevated sensitivity of transpiration to evaporative demand. This led to an insensitivity for total dry weight biomass of G28 and G121 under WS conditions (p < 0.001). This work provides: (i) an in-depth analysis for a series of preferable traits under WS in Pinot gris; (ii) a characterization of Pinot gris × rootstock interaction and a series of desirable traits under WS induced by several rootstocks; (iii) the potential benefit for the use a series of affordable methods (e.g., RGB imaging) to easily detect dynamic changes in biomass in grapevine and quickly phenotype genotypes with superior responses under WS. In conclusion, the near-isohydric and conservative behavior observed for G28 and G121 coupled with their low vigor suggest them as potential Pinot gris rootstock candidates for sustaining grapevine productivity in shallow soils likely to develop terminal stress conditions.

Climatic stresses limit plant growth and productivity. In the past decade, tree improvement programs were mainly focused on yield but it is obvious that enhanced stress resistance is also required. In this review we highlight important drought avoidance and tolerance mechanisms in forest trees. Genomes of economically important trees species with divergent resistance mechanisms can now be exploited to uncover the mechanistic basis of long-term drought adaptation at the whole plant level. Molecular tree physiology indicates that osmotic adjustment, antioxidative defense and increased water use efficiency are important targets for enhanced drought tolerance at the cellular and tissue level. Recent biotechnological approaches focused on overexpression of genes involved in stress sensing and signaling, such as the abscisic acid core pathway, and down-stream transcription factors. By this strategy, a suite of defense systems was recruited, generally enhancing drought and salt stress tolerance under laboratory conditions. However, field studies are still scarce. Under field conditions trees are exposed to combinations of stresses that vary in duration and magnitude. Variable stresses may overrule the positive effect achieved by engineering an individual defense pathway. To assess the usability of distinct modifications, large-scale experimental field studies in different environments are necessary. To optimize the balance between growth and defense, the use of stress-inducible promoters may be useful. Future improvement programs for drought resistance will benefit from a better understanding of the intricate networks that ameliorate molecular and ecological traits of forest trees.

Climatic stresses limit plant growth and productivity. In the past decade, tree improvement programs were mainly focused on yield but it is obvious that enhanced stress resistance is also required. In this review we highlight important drought avoidance and tolerance mechanisms in forest trees. Genomes of economically important trees species with divergent resistance mechanisms can now be exploited to uncover the mechanistic basis of long-term drought adaptation at the whole plant level. Molecular tree physiology indicates that osmotic adjustment, antioxidative defense and increased water use efficiency are important targets for enhanced drought tolerance at the cellular and tissue level. Recent biotechnological approaches focused on overexpression of genes involved in stress sensing and signaling, such as the abscisic acid core pathway, and down-stream transcription factors. By this strategy, a suite of defense systems was recruited, generally enhancing drought and salt stress tolerance under laboratory conditions. However, field studies are still scarce. Under field conditions trees are exposed to combinations of stresses that vary in duration and magnitude. Variable stresses may overrule the positive effect achieved by engineering an individual defense pathway. To assess the usability of distinct modifications, large-scale experimental field studies in different environments are necessary. To optimize the balance between growth and defense, the use of stress-inducible promoters may be useful. Future improvement programs for drought resistance will benefit from a better understanding of the intricate networks that ameliorate molecular and ecological traits of forest trees. [ABSTRACT FROM AUTHOR]

Irrigation management in Greenhouse Nursery Production (GNP) is based on empiric methods based on farmer personal experiences with over-irrigation results. The effects of irrigation volume and daily application were studied in a pot experiment carried out on rooted cuttings in a greenhouse The irrigation volume treatment was performed on Full and reduced Treatment. The treatment of water application was carried out with split supply and unsplit supply. The effects of the treatments were evaluated in terms of biomass accumulation and partitioning, leaf area, photosynthesis and stomatal response, chlorophyll content, and water productivity. Callistemon showed a good adaptation to the different treatments tested during the experiment. A positive relation was found between biomass accumulation and irrigation volume, moreover split water application increased plant Dry Weight. Therefore, the highest biomass accumulation was registered in full irrigation volume in split application treatment, and this behavior was confirmed by the photosynthetic rate. No statistical differences were found, in terms of Relative Water Content (RWC), between the treatments. Stem water potential and stomatal conductance values suggest in Callistemon an anysohidric water stress response behavior. Our results evidenced that, in Callistemon potted plants, an irrigation volume reduction is possible when a split application occurs during the daytime. A full irrigation volume amounts to 10.8 L per plant during the trial period of 90 days while the reduced volume amounts to 8.2 L per plant. Therefore, an increased water productivity can be obtained if the daily water requirement is split on two applications during the daytime. Our results highlighted a possible reduction in environmental impact of Callistemon greenhouse pot production, through the 25% reduction of the volume irrigation. [ABSTRACT FROM AUTHOR]

Main conclusion: Under high water availability, the maximum gas exchange was observed at noon in the expanding and expanded leaves. The expanded leaves showed lower gas exchange capacity due to the regulation of stomatal-movement genes.Under well-watered condition, stomatal conductance (gs) and photosynthetic rate (A) of expanding and expanded leaves of Atriplex halimus peaked at noon despite the midday decline in the leaf relative water content, suggesting deviation from typical isohydric behaviour. However, the expanding leaves had higher gs and A than the expanded ones. When light intensity was temporarily increased, A and gs were enhanced in both types of leaves though to a higher level in the expanding leaves. In well-watered expanded leaves: (1) A was mainly dependent on gs rather than photosynthetic capacity; gs was controlled by internal factors, thereby limiting water loss via transpiration (E); (2) the accumulation of total soluble sugars (TSS) along with increased Rubisco protein could be a subsidiary factor limiting A; (3) TSS and ABA seem to act in co-ordination to up-regulate ABA-dependent genes controlling gs and (4) the significant induction of DREBs suggests a role in maintaining high relative water content in these leaves compared to the expanding ones. In expanding leaves of well-watered plants, high A along with Rubisco down-regulation and elevated TSS suggests that A was regulated by signals coordinating carbon and nitrogen balance and the elevated ABA could be involved in regulating the hydraulic activity to enhance cell expansion and facilitate leaf growth. Both expanded and expanding leaves behaved in typical isohydric manner under water stress, which did not involve the accumulation of ABA suggesting that stomatal closure was primarily stimulated by hydraulic rather than chemical signals. [ABSTRACT FROM AUTHOR]

Plants are frequently classified as isohydric or anisohydric in an attempt to portray their water relations strategy or ecological niche. However, despite the popularity of the iso/anisohydric classification, the underlying biology remains unclear. We use here a simple hydraulic model and the extensive literature on grapevine hydraulics to illustrate that the iso/anisohydric classification of a plant depends on the definition used and the environment in which it is grown, rather than describing an intrinsic property of the plant itself. We argue that abandoning the iso/anisohydric terminology and returning to a more fundamental hydraulic framework would provide a stronger foundation for species comparisons and ecological predictions. [ABSTRACT FROM AUTHOR]

We report physiological, anatomical and molecular differences in two economically important grapevine (Vitis vinifera L.) cultivars cv. Grenache (near-isohydric) and Chardonnay (anisohydric) in their response to water-stress induced cavitation. The aim of the study was to compare organ vulnerability (petiole and stem) to cavitation by measuring ultrasonic acoustic emissions (UAE) and percent loss of conductance of potted grapevines subject to the onset of water-stress. Leaf (ψL) and stem water potential (ψS), stomatal conductance (gs), transpiration (E), petiole hydraulics (KPet), and xylem diameter were also measured. Chardonnay displayed hydraulic segmentation based on UAE, with cavitation occurring at a less negative ψL in the petiole than in the stem. Vulnerability segmentation was not observed in Grenache, with both petioles and stems equally vulnerable to cavitation. Leaf water potential that induced 50% of maximum UAE was significantly different between petioles and stems in Chardonnay (ψ50Petiole = -1.14 and ψ50Stem = -2.24 MPa) but not in Grenache (ψ50Petiole = -0.73 and ψ50Stem = -0.78 MPa). Grenache stems appeared more susceptible to water-stress induced cavitation than Chardonnay stems. Grenache displayed (on average) a higher KPet likely due to the presence of larger xylem vessels. A close relationship between petiole hydraulic properties and vine water status was observed in Chardonnay but not in Grenache. Transcriptional analysis of aquaporins in the petioles and leaves (VvPIP1;1, VvPIP2;1, VvPIP2;2 VvPIP2;3, VvTIP1;1, and VvTIP2;1) showed differential regulation diurnally and in response to water-stress. VvPIP2;1 showed strong diurnal regulation in the petioles and leaves of both cultivars with expression highest predawn. Expression of VvPIP2;1 and VvPIP2;2 responded to ψL and ψS in both cultivars indicating the expression of these two genes are closely linked to vine water status. Expression of several aquaporin genes correlated with gas exchange measurements, however, these genes differed between cultivars. In summary, the data shows two contrasting responses in petiole hydraulics and aquaporin expression between the near-isohydric cultivar, Grenache and anisohydric cultivar, Chardonnay.

Many temperate European tree species have their southernmost distribution limits in the Mediterranean Basin. The projected climatic conditions, particularly an increase in dryness, might induce an altitudinal and latitudinal retreat at their southernmost distribution limit. Therefore, characterizing the morphological and physiological variability of temperate tree species under dry conditions is essential to understand species’ responses to expected climate change. In this study, we compared branch-level hydraulic traits of four Scots pine and four sessile oak natural stands located at the western and central Mediterranean Basin to assess their adjustment to water limiting conditions. Hydraulic traits such as xylem- and leaf-specific maximum hydraulic conductivity (KS-MAX and KL-MAX), leaf-to-xylem area ratio (AL:AX) and functional xylem fraction (FX) were measured in July 2015 during a long and exceptionally dry summer. Additionally, xylem-specific native hydraulic conductivity (KS-N) and native percentage of loss of hydraulic conductivity (PLC) were measured for Scots pine. Interspecific differences in these hydraulic traits as well as intraspecific variability between sites were assessed. The influence of annual, summer and growing season site climatic aridity (P/PET) on intraspecific variability was investigated. Sessile oak displayed higher values of KS-MAX, KL-MAX, AL:AX but a smaller percentage of FX than Scots pines. Scots pine did not vary in any of the measured hydraulic traits across the sites, and PLC values were low for all sites, even during one of the warmest summers in the region. In contrast, sessile oak showed significant differences in KS-MAX, KL-MAX, and FX across sites, which were significantly related to site aridity. The striking similarity in the hydraulic traits across Scots pine sites suggests that no adjustment in hydraulic architecture was needed, likely as a consequence of a drought-avoidance strategy. In contrast, sessile oak displayed adjustments in the hydraulic architecture along an aridity gradient, pointing to a drought-tolerance strategy.

Aim: To evaluate the usability of various plant water potentials in table grape irrigation management under a semiarid climate. Methods and results: Two water regimes were set up. The « control » water regime was the one usually used in the vineyard. The « 50 % Irrigation » water regime delivered only half the quantity of water to the vines. Predawn leaf (ψ L PD), predawn stem (ψ S PD), midday leaf (ψ L M), and midday stem (ψ S M) water potentials were measured during the growing season. The results show that the four water potentials can accurately measure the vine water status in table grape vineyard at a daily and seasonal time scale. But, ψ L M appeared to be the most reliable indicator to differentiate between the two water regimes with a frequency of 73 %. The « 50 % Irrigation » water regime induced in the ‘Italia’ cultivar an anisohydric behavior and a decrease of 29.4 % in vine vigor and 11.5 % in berry weight. Under the Tunisian climate, ‘Italia’ cultivar may exhibit night time transpiration that decreases ψ L PD by 19.5 %. Conclusion: Preliminary minimum ψ L M threshold to produce high quality table grape would be -0.8 and -1.1 MPa for pre- and post-veraison, respectively. Significance and impact of the study: The pressure chamber is an effective device for irrigation management in commercial table grape vineyards under semiarid conditions.

As a major driving element of the structure and function of arid and semiarid ecosystems, rainfall is the essential factor limiting plant biological processes. To clarify the characteristics of transpiration and responses to summer rainfall, sap flow density ( Fd) of Pinus tabulaeformis and Hippophae rhamnoides was monitored using thermal dissipation probes. In addition, midday leaf water potential (ψm) and leaf stomatal conductance ( Gs) were also analyzed to determine water use strategies. The results indicated that the diurnal variation in the normalized Fd values exhibited a single-peak curve for P. tabulaeformis, while H. rhamnoides showed multiple peaks. The normalized Fd for P. tabulaeformis remained relatively stable regardless of rainfall events. However, there was also a significant increase in the normalized Fd for H. rhamnoides in response to rainfall in June and August ( p < .05), although no significant differences were observed in July. The normalized Fd values for P. tabulaeformis and H. rhamnoides fitted well with the derived variable of transpiration, an integrated index calculated from the vapor pressure deficit and solar radiation ( Rs), using an exponential saturation function. The differences in fitting coefficients suggested that H. rhamnoides showed more sensitivity to summer rainfall ( p < .01) than P. tabulaeformis. Furthermore, during the study period, P. tabulaeformis reduced Gs as soil water decreased, maintaining a relatively constant ψm; while H. rhamnoides allowed large fluctuations in ψm to maintain Gs. Therefore, P. tabulaeformis and H. rhamnoides should be considered isohydric and anisohydric species, respectively. And more consideration should be taken for H. rhamnoides in the afforestation activities and the local plantation management under the context of the frequently seasonal drought in the loess hilly region. [ABSTRACT FROM AUTHOR]

Topography may mitigate drought effects on vegetation along a hillslope gradient through redistribution of soil moisture. We examined the interaction of topography, climate, soil moisture, and transpiration in a low-elevation, mixed-hardwood forest in the southern Appalachian Mountains. The effects of meteorological variation (wet and dry years) and topographic position (upslope and cove) were tested on daily soil moisture amplitude and recession and plot and species-specific transpiration. Trees in the cove plot were 17% taller and had 45% greater sapwood area than those in the upslope plot. Lower rates of soil moisture recession following rainfall events were observed at the cove plot compared to the upper plot. Greater daily soil moisture amplitude and plot transpiration, even in dry years, suggest that lower slope positions may have been buffered against moderate drought. We also observed similar transpiration in Quercus spp., Carya spp., and Liriodendron tulipifera in the cove plot between dry and wet years. Plot transpiration was reduced by 51% in the dry year in the upslope plot only, and transpiration by individual species in the plot reflected this pattern, suggesting water stress in dry years may be exacerbated by topography. With drought predicted to increase for these systems, the different drought responses of species, in addition to topographic effects, may lead to complex shifts in species composition. [ABSTRACT FROM AUTHOR]

We report physiological, anatomical and molecular differences in two economically important grapevine (Vitis vinifera L.) cultivars cv. Grenache (near-isohydric) and Chardonnay (anisohydric) in their response to water-stress induced cavitation. The aim of the study was to compare organ vulnerability (petiole and stem) to cavitation by measuring ultrasonic acoustic emissions (UAE) and percent loss of conductance of potted grapevines subject to the onset of water-stress. Leaf (ψL) and stem water potential (ψS), stomatal conductance (gs), transpiration (E), petiole hydraulics (KPet), and xylem diameter were also measured. Chardonnay displayed hydraulic segmentation based on UAE, with cavitation occurring at a less negative ψL in the petiole than in the stem. Vulnerability segmentation was not observed in Grenache, with both petioles and stems equally vulnerable to cavitation. Leaf water potential that induced 50% of maximum UAE was significantly different between petioles and stems in Chardonnay (ψ50Petiole D=-1.14 and ψ50Stem =-2.24 MPa) but not in Grenache (ψ50Petiole = -0.73 and ψ50Stem =-0.78 MPa). Grenache stems appeared more susceptible to water-stress induced cavitation than Chardonnay stems. Grenache displayed (on average) a higher KPet likely due to the presence of larger xylem vessels. A close relationship between petiole hydraulic properties and vine water status was observed in Chardonnay but not in Grenache. Transcriptional analysis of aquaporins in the petioles and leaves (VvPIP1;1, VvPIP2;1, VvPIP2;2 VvPIP2;3, VvTIP1;1, and VvTIP2;1) showed differential regulation diurnally and in response to water-stress. VvPIP2;1 showed strong diurnal regulation in the petioles and leaves of both cultivars with expression highest predawn. Expression of VvPIP2;1 and VvPIP2;2 responded to %L and %S in both cultivars indicating the expression of these two genes are closely linked to vine water status. Expression of several aquaporin genes correlated with gas exchange measurements, however, these genes differed between cultivars. In summary, the data shows two contrasting responses in petiole hydraulics and aquaporin expression between the near-isohydric cultivar, Grenache and anisohydric cultivar, Chardonnay. [ABSTRACT FROM AUTHOR]

Predicting the impact of drought on forest ecosystem processes requires an understanding of trees' species-specific responses to drought, especially in the Eastern USA, where species composition is highly dynamic due to historical changes in land use and fire regime. Here, we adapted a framework that classifies trees' water-use strategy along the spectrum of isohydric to anisohydric behavior to determine the responses of three canopy-dominant species to drought. We used a collection of leaf-level gas exchange, tree-level sap flux and stand-level eddy covariance data collected in south-central Indiana from 2011 to 2013, which included an unusually severe drought in the summer of 2012. Our goal was to assess how patterns in the radial profile of sap flux and reliance on hydraulic capacitance differed among species of contrasting water-use strategies. In isohydric species, which included sugar maple (Acer saccharum Marsh.) and tulip poplar (Liriodendron tulipifera L.), we found that the sap flux in the outer xylem experienced dramatic declines during drought, but sap flux at inner xylem was buffered from reductions in water availability. In contrast, for anisohydric oak species (Quercus alba L. and Quercus rubra L.), we observed relatively smaller variations in sap flux during drought in both inner and outer xylem, and higher nighttime refilling when compared with isohydric species. This reliance on nocturnal refilling, which occurred coincident with a decoupling between leaf- and tree-level water-use dynamics, suggests that anisohydric species may benefit from a reliance on hydraulic capacitance to mitigate the risk of hydraulic failure associated with maintaining high transpiration rates during drought. In the case of both isohydric and anisohydric species, our work demonstrates that failure to account for shifts in the radial profile of sap flux during drought could introduce substantial bias in estimates of tree water use during both drought and non-drought periods. [ABSTRACT FROM AUTHOR]

Key message: European beech presents intraspecific variations in drought resistance strategies that are mediated by the amount of precipitation in the habitat. Abstract: Climate change predictions forecast extended drought periods, which are expected to pose an enhanced risk to forest trees. Here, we investigated the drought response and fitness traits in European beech ( Fagus sylvatica L.) in response to a severe progressive drought. The capability of three beech provenances from habitats differing in annual precipitation (544, 665, and 766 mm year) to cope with drought was compared in a common garden experiment using beech seedlings. Soil and plant water status, maximum quantum yield of PSII, growth and biomass partitioning, stomatal conductance, and transcript abundance pattern of the kinase, Open Stomata 1 (OST1), of control (well-watered) and drought-treated (water withheld) plants from each provenance were repeatedly measured during a 60-day drought experiment. The lowest precipitation provenance displayed a more isohydric phenotype with a prompt stomatal closure, increased OST1 levels, high water potential and leaf water content, and a decrement in the maximum quantum yield of PSII. The other two provenances showed a more anisohydric stomatal regulation with a slow and delayed stomatal closure and a decrease in the water status. These findings suggest that intraspecific variations in beech for diverging drought resistance strategies exist and might be mediated by differences in the abscisic acid signaling pathway. The higher precipitation provenance maintained high quantum yield of PSII, and water potentials above −2.0 MPa for a longer period of time than the other two provenances, and consequently, mortality was delayed in this provenance. We concluded that lower precipitation adapted plants employ a drought resistance strategy suitable for the moderate drought, whereas the higher precipitation habitat plants revealed mechanisms, which could be better suited to cope with more severe drought events. [ABSTRACT FROM AUTHOR]

Here, we summarize studies on the effects of elevated [CO2] (CO2e) on the structure and function of plant hydraulic architecture and explore the implications of those changes using a model. Changes in conduit diameter and hydraulic conductance due to CO2e vary among species. Ring-porous species tend towards an increase in conduit size and consequently conductivity. The effect in diffuse-porous species is much more limited. In conifers, the results are mixed, some species show minor changes in xylem structure, while other studies found increases in tracheid density and diameter. Non-woody plants generally exhibited the reverse pattern with narrower conduits and lower hydraulic conductivity under CO2e. Further, changes in drought-resistance traits suggest that non-woody plants were the most affected by CO2e, which may permit them to better resist drought-induced embolism under future conditions. Due to their complexity, acclimation in hydraulic traits in response to CO2e is difficult to interpret when relying solely on measurements. When we examined how the observed tissues-specific trends might alter plant function, our modelling results suggest that these hydraulic changes would lead to reduced conductance and more frequent drought stress in trees that develop under CO2e with a more pronounced effect in isohydric than in anisohydric species. [ABSTRACT FROM AUTHOR]

Many temperate European tree species have their southernmost distribution limits in the Mediterranean Basin. The projected climatic conditions, particularly an increase in dryness, might induce an altitudinal and latitudinal retreat at their southernmost distribution limit. Therefore, characterizing the morphological and physiological variability of temperate tree species under dry conditions is essential to understand species’ responses to expected climate change. In this study, we compared branch-level hydraulic traits of four Scots pine and four sessile oak natural stands located at the western and central Mediterranean Basin to assess their adjustment to water limiting conditions. Hydraulic traits such as xylem- and leaf-specific maximum hydraulic conductivity (KS-MAX and KL-MAX), leaf-to-xylem area ratio (AL:AX) and functional xylem fraction (FX) were measured in July 2015 during a long and exceptionally dry summer. Additionally, xylem-specific native hydraulic conductivity (KS-N) and native percentage of loss of hydraulic conductivity (PLC) were measured for Scots pine. Interspecific differences in these hydraulic traits as well as intraspecific variability between sites were assessed. The influence of annual, summer and growing season site climatic aridity (P/PET) on intraspecific variability was investigated. Sessile oak displayed higher values of KS-MAX, KL-MAX, AL:AX but a smaller percentage of FX than Scots pines. Scots pine did not vary in any of the measured hydraulic traits across the sites, and PLC values were low for all sites, even during one of the warmest summers in the region. In contrast, sessile oak showed significant differences in KS-MAX, KL-MAX, and FX across sites, which were significantly related to site aridity. The striking similarity in the hydraulic traits across Scots pine sites suggests that no adjustment in hydraulic architecture was needed, likely as a consequence of a drought-avoidance strategy. In contrast, sessile oak displayed adjustments in the hydraulic architecture along an aridity gradient, pointing to a drought-tolerance strategy. [ABSTRACT FROM AUTHOR]

As dominant cultivated plants in low subtropical China, Eucalypts is facing challenges due to climate changes. Experiments were conducted to determine the hydrologic balance of a Eucalyptus urophylla plantation in response to periodical drought. Trees displayed lower stomatal conductance (GS) and leaf water potentials (ΨL) in the dry season. The maximum reference GS at D=1 kPa (GSref-max) decreased by 22.4%, while specific hydraulic conductivity (ks) of trees of different sizes decreased by 45.3 - 65.6% from the wet to the dry season. Fairly stable leaf stomatal conductance (gs) with decreasing ΨL was also observed at ΨL

Aims: Genetic variability in grapevine cultivars may influence their strategy to cope with drought through stomatal regulation of transpiration rate. The aim of the present study was to evaluate the stomatal sensitivity of five cultivars (Ekigaïna, Grenache, Marselan, Mourvèdre, and Syrah) to soil water status and air water vapor pressure deficit (VPD). Methods and results: Leaf gas exchange and canopy light interception efficiency (εi) were evaluated through a wide range of predawn leaf water potential (ΨPD) measurements in a field experiment in Southern France. Additionally, greenhouse experiments were carried out to monitor stomatal response to increasing VPD levels. Ekigaïna showed a strong isohydric behavior with the highest decrease in leaf gas exchange in response to soil water stress and VPD. Mourvèdre and Grenache showed a similar but relatively less extreme behavior. These three cultivars showed a constant leaf water status during the day through stomatal regulation and a strong decrease in εi. In contrast, Syrah and Marselan displayed anisohydric behavior as they presented a less sensitive stomatal control. Both cultivars showed fluctuating midday leaf water potential and Marselan was the least affected in terms of εi. Conclusion: This study demonstrated that grape cultivars differed in their stomatal response to soil water deficit and VPD. For a given cultivar, a similar stomatal behavior was found in response to both ΨPD and VPD. Significance and impact of the results: Adaptation to drought and viticulture viability in hot and dry environments could be achieved by identifying and breeding cultivars with drought tolerance traits.