ABSTRACT Water‐use contributes significantly to plant growth and productivity, yet the extent to which variation in water‐use is a function of adaptive differentiation is unknown. Here, we studied natural variation of water use in Arabidopsis thaliana to understand how climatic history impacts water‐use strategies. We performed a survey of vegetative water use (VWU) and life‐history traits across A. thaliana ecotypes in controlled and outdoor settings. Performance of select ecotypes in controlled experiments was reflective of performance in outdoor conditions. Through trait‐climate and genome‐wide association (GWAS) analyses, we tested for signals of environmental adaptation in water use. Ecotypes from warmer environments were noted in displaying enhanced water use, independent of precipitation. GWAS identified the transcription factor MYB59 as a determiner of VWU. Functionally significant MYB59 SNPs showed associations with temperature, but not precipitation, and myb59 mutants demonstrated reduced water use under high temperatures. Our study suggests intraspecific variation in water‐use can be explained in part by climatic history, where temperature is the most significant driver. MYB59 appears to contribute to this association and represents a promising candidate for future investigation in crops.

Context : In a context of increasing droughts, identifying the causes of variations in individual growth performance is crucial. Aims : We investigated whether early-life intrinsic water-use efficiency (iWUE), and its plasticity to drought, estimated from tree-ring δ 13 C, can predict long-term growth performance in sessile oak trees. Methods: Two mature stands with contrasting ecological and management contexts were studied: (i) a fertile, high-density stand without thinning since a century, and (ii) a low-density stand, undergoing chronic drought-related dieback. All trees in both plots (n ~ 100 per plot) were cored, enabling reconstruction of the whole within-population variability of iWUE and radial growth trajectories. Results : Although the sampled trees represent only survivors, inter-individual variability in iWUE was remarkably conserved over a century, suggesting that such a diversity may persist naturally within stands, even under environmental filtering. Early-life iWUE and diameter were both moderate predictors of cumulative adult growth in the low-density stand. No relationship was observed between early-life iWUE and cumulative adult growth in the high-density stand. In both sites, iWUE plasticity to drought was uninformative for explaining inter-individual differences in growth performance. Conclusion At best, early-life iWUE appeared a modest predictor of adult growth. However, this effect did not extend to tree survival or health.

Stomata and photosynthesis respond at different rates to rapid changes in light availability, resulting in different dynamics of water and carbon fluxes. Here, we investigated how growth conditions, drought, and potassium availability affect the dynamics of stomatal conductance and photosynthesis in response to rapid changes of irradiance in beech saplings (Fagus sylvatica L.). Two greenhouse experiments were conducted, one comparing saplings grown under sunlit conditions to those grown in shaded conditions, and the other testing the effects of edaphic drought and potassium supplementation on saplings grown on potassium-poor forest soils. When irradiance was changed abruptly, stomatal conductance (gs) changed from an initial steady-state value to a final steady state; this change has two characteristic time constants: a lag time and a response time. Environmental conditions strongly shaped stomatal dynamics. Shade-grown leaves had reduced changes of steady-state g s, but more reactive stomata, characterized by shorter lag and response times. This allowed relatively greater CO2 uptake during transient light periods than if they had the response and lag times of sunlit-grown leaves. Droughted saplings with sufficient potassium also showed reduced g s amplitude and shorter lag and response times enhancing conservation of water compared to well-watered plants. However, potassium-deficient saplings showed no significant changes in stomatal dynamics regardless of soil water status. Stomata closed about 20% faster than they reopened across all treatments. The differences in stomatal dynamics were not associated with any changes in stomatal size or density, indicating that physiological rather than morphological traits drive the observed plasticity of stomatal responsiveness in European beech.

Photosynthesis is one of the most sensitive processes to temperature and reduced water availability, strongly influencing forest productivity under present and future conditions. Accurate prediction of plant responses to these drivers requires accurate parameterisation of the maximum rate of RubisCo carboxylation (Vcmax) and maximum rate of electron transport (Jmax). In the present study, we evaluated the response of photosynthesis to moderate water deficit and elevated temperature in four Quercus species: Q. robur, Q. cerris, Q. pubescens, and Q. pyrenaica. Leaf gas exchange and chlorophyll fluorescence measurements were used to derive photosynthetic parameters. Three estimations of Vcmax and Jmax were compared, considering: (E1) finite mesophyll conductance (gₘ), (E2) infinite gₘ using Cc-based (chloroplast CO2 concentration) compensation point (Γ*), and (E3) infinite gₘ with Ci-based (substomatal CO2 concentration) compensation point (Ci*). Net CO2 assimilation (AN) and gₘ showed moderate, species-specific responses to water deficit and elevated temperature. Infinite gₘ produced lower Vcmax and higher Jmax values compared to finite gm, although differences disappeared under moderate drought or elevated temperature. The strong coupling between Vcmax and Jmax was maintained across all treatments, suggesting a conserved coordination between carboxylation and electron transport processes, while the statistical strength of this relationship depended on the gₘ estimation approach. These findings highlight the importance of methodological considerations related to mesophyll conductance in ensuring realistic photosynthetic characterisation and improving the robustness of vegetation models.

The rate of stomatal opening and closure in response to changes in light affects leaf photosynthesis and water use. However, it is unclear how strongly stomatal size (SS) and density (SD) influence stomatal conductance (gs) kinetics, and whether variation arises from methodological differences, guard cell type or degree of amphistomaty. We divided published records combining stomatal kinetics and anatomical traits from 89 species into kidney and dumbbell-shaped guard cells, and evaluated four dynamic gs models on them. We derived the time constant for an exponential response of gs (τ) and the maximum rate of change (Slmax) as well as the ratio of adaxial/abaxial SD (rSD). We found significant differences in parameter estimation between models. Stomatal anatomical traits and kinetic parameters showed large variation across species. While individual anatomical features (SS, SD, rSD and guard cell types) were weakly correlated with stomatal response speed (τ and Slmax), interactions between these features showed significant effects, demonstrating that kinetic performance arises from synergistic rather than additive anatomical relationships. Our results call for the use of our unified modeling approach, challenge the generality of the observation that smaller stomata move faster across species and suggest rSD as an understudied driver of stomatal kinetics.

Abstract Key Message Leaves of seedlings from five oak species (Quercus robur L.; Q. pubescens L.; Q. suber L.; Q. afares Pomel; Q. ilex L.) displayed large, mainly inter-specific, differences in leaf mass-to-area ratio (LMA) and lignin content, as well as in the 13C composition of bulk leaf biomass. The variation in leaf lignin content and LMA did not impact the offset between the 13C composition measured in bulk leaf material versus soluble sugars. This observation, as well as the similar correlations between intrinsic leaf water use efficiency and the 13C compositions of bulk material or soluble sugars extracted from leaves, confirms their reliable use as a proxy for the former even when there is a large variation in LMA or lignin among samples. Context Carbon isotope composition (δ 13 C) of bulk leaf biomass is frequently used as a proxy for intrinsic water use efficiency (iWUE) in large-scale intra- and inter-specific comparisons. However, post-photosynthetic 13C discrimination during the synthesis of lignin combined with differences in leaf mass-to-area ratio (LMA) may bias the relationship between δ 13 C of bulk leaf matter and iWUE and thus its use as a proxy of iWUE. Aims To quantify the impact of differences in lignin content and LMA on the relationship between δ 13 C of bulk leaf biomass and iWUE over a large gradient of lignin contents across five oak species (deciduous: Quercus robur, Q. pubescens, Q. afares and evergreen: Q. ilex and Q. suber). Methods We measured lignin content, LMA, and δ 13 C of bulk leaf biomass and of soluble sugars extracted from the leaves, as well as intrinsic water use efficiency (derived from leaf gas exchange) in seedlings of the five oak species grown under common conditions in a greenhouse and measured in a climate chamber. Results There was a large range (mainly across species) in lignin content (4 to 33%) and LMA (60–180 g m−2). δ 13 C of bulk leaf biomass and soluble sugars were tightly correlated, showing a significant mean offset of − 0.4‰. This offset was stable across species and not correlated to the lignin content of the leaves. A very loose correlation was found between the offset and LMA, mainly due to one species. Conclusion Our results are a demonstration that potential variations in leaf lignin content or LMA have no or only a little effect on the δ 13 C of bulk leaf biomass. They are unlikely to cause a bias when using bulk leaf δ 13 C as a proxy for variations in intrinsic water use efficiency among Mediterranean and temperate broad-leaf forest tree species.

Summary In a context of climate change, it is necessary to decipher the strategies established by plants to cope with limited water supply. Transcriptome, methylome and small RNA data were generated for two oak species with contrasting levels of drought tolerance ( Quercus robur and Quercus petraea ), under control and drought stress conditions All data are in line with a species-specific response to drought stress consistent with their ecological preferences. The biological processes associated with genomic regions identified in all datasets were mainly associated with parietal processes in Q. petraea, which may explain in part its better tolerance to water deprivation. A significant proportion of DNA methylation differences observed in control conditions between the two oak species were maintained during DS which may constitute a pool of epigenetic markers discriminating these two oak species. These markers were enriched in highly differentiated SNPs suggesting that some of them may be associated both with the ecological differences or intrinsic barriers to reproduction between the two species. An integrative approach of the three datasets revealed genomic co-locations of potential importance for forest three adaptation to drought stress.

Water use efficiency (WUE) is an important adaptive trait for soil water deficit. The molecular and physiological bases of WUE regulation in crops have been studied in detail in the context of plant breeding. Knowledge for most forest tree species lags behind, despite the need to identify populations or genotypes able to cope with the longer, more intense drought periods likely to result from climate warming. We aimed to bridge this gap in knowledge for sessile oak (Quercus petraea (Matt.) Liebl.), one of the most ecologically and economically important tree species in Europe, using a factorial design including trees with contrasted phenotypic values (low and high WUE) and two watering regimes (control and drought). By monitoring the ecophysiological response, we first qualified genotypes for their WUE (by using instantaneous and long-term measures). We then performed RNA-seq to quantify gene expression for the three most extreme genotypes exposed to the two watering regimes. By analyzing the interaction term, we were able to capture the molecular strategy of each group of plants for coping with drought. We identified putative candidate genes potentially involved in the regulation of transpiration rate in high-WUE phenotypes. Regardless of water availability, trees from the high-WUE phenotypic class overexpressed genes associated with drought responses, and in the control of stomatal density and distribution, and displayed a downregulation of genes associated with early stomatal closure and high transpiration rate. Fine physiological screening of sessile oaks with contrasting WUE, and their molecular characterization (i) highlighted subtle differences in transcription between low- and high-WUE genotypes, identifying key molecular players in the genetic control of this trait and (ii) revealed the genes underlying the molecular strategy that evolved in each group to potentially cope with water deficit, providing new insight into the within-species diversity in drought adaptation strategies.

Abies alba (Mill.) has a high potential for mitigating climate change in European mountain forests; yet, its natural regeneration is severely limited by ungulate browsing. Here, we simulated browsing in a common garden experiment to study growth and physiological traits, measured from bulk needles, using a randomized block design with two levels of browsing severity and seedlings originating from 19 populations across Switzerland. Genetic factors explained most variation in growth (on average, 51.5%) and physiological traits (10.2%) under control conditions, while heavy browsing considerably reduced the genetic effects on growth (to 30%), but doubled those on physiological traits related to carbon storage. While browsing reduced seedling height, it also lowered seedling water-use efficiency (decreased $\delta ^{13}$C) and increased their $\delta ^{15}$N. Different populations reacted differently to browsing stress, and for seedling height, starch concentration and $\delta ^{15}$N, population differences appeared to be the result of natural selection. First, we found that populations originating from the warmest regions recovered the fastest from browsing stress, and they did so by mobilizing starch from their needles, which suggests a genetic underpinning for a growth-storage trade-off across populations. Second, we found that seedlings originating from mountain populations growing on steep slopes had a higher $\delta ^{15}$N in the common garden than those originating from flat areas, indicating that they have been selected to grow on N-poor, potentially drained, soils. This finding was corroborated by the fact that nitrogen concentration in adult needles was lower on steep slopes than on flat ground, strongly indicating that steep slopes are the most N-poor environments. These results suggest that adaptation to climate and soil nitrogen availability, as well as ungulate browsing pressure, co-determine the regeneration and range limit of silver fir.

Context As a widespread species, sessile oak ( Quercus petraea ) populations occupy a wide range of ecological conditions, with large gradients of soil water availability. Drought acclimation involves a plastic increase in water-use efficiency (WUE), a trait that is easily measured using the carbon isotope composition (δ 13 C). However, the question remains whether WUE is an adaptive trait that impacts the fitness of trees in natural environments. Objectives and Methods To investigate whether WUE was a drought-adaptive trait, we studied a sample of 600 trees originating from 16 provenances, grown for 21 years in a common garden. Intrinsic WUE (WUE i ), estimated from tree ring δ 13 C, was compared among and within populations for three climatically contrasted years. The adaptive character of WUE i was evaluated by relating population mean WUE i , as well as its plasticity to drought, to the pedoclimatic conditions of their provenance sites. The contribution of WUE i to tree and population fitness was finally assessed from the relationship between WUE i and tree radial growth (GI). Results Significant differences in WUE i were found among populations but a much larger variability was observed within than among populations. The population WUE i of the juvenile oak trees growing in the relatively mesic conditions of the common garden showed no relationship with a modeled water deficit index for the provenance sites. However, a higher population WUE i plasticity to severe drought was related to a higher proportion of silt and carbon and a lower proportion of sand in the soil of the provenance sites. In response to severe drought, populations with a higher increase in WUE i showed a lower decrease in GI. Populations with lower GI reduction were from sites with higher vapor pressure deficit in May–July (VPD). For the wet year only, populations with a higher WUE i also had a higher GI. Conclusion The correlations observed at the common garden site between (i) population means of WUE i plasticity to drought and soil texture of the provenance sites, and (ii) GI plasticity to drought and VPD, suggested a local adaptation of sessile oak.

Plants have developed different strategies to cope with a reduced water availability in the soil (Levitt 1980).Drought resistance of forest trees is mainly a question of survival and competition within an ecosystem.However, in forestry, it is also a question of maintaining growth.Functional traits can be termed "adaptive" to drought, providing that their genetic variation can be linked to an increased fitness under soil water deficit conditions.The observation of trees grown from seeds of different populations in one or several common environments (e.g.provenance trials or common garden experiments) is a means to control for environmental variation, and therefore to detect genetic differences among these different populations and potentially genotype x environment interactions, if trials are replicated at different locations.Populations from different environments may differ in their mean values of any functional trait in the sense that these mean values relate to the environmental differences of the original population sites.This can be taken as a first indication that these traits are involved in the adaptation of populations to the local conditions.Studying population differences involves measuring the phenotypes of large numbers of individual plants.In this context, the carbon isotope composition ( 13 C) of plant organic material has been widely used. 13 C variations of plant organic matter reflect variations in intrinsic water use efficiency (W i , the ratio between A, the net CO 2 assimilation rate and g s , the stomatal

Key message The history of the relationship between plant growth and water consumption is retraced by following the progression of scientific thought through the centuries: from a purely philosophical question, to conceptual and methodological developments, towards a research interest in plant functioning and the interaction with the environment. Context The relationship between plant growth and water consumption has for a long time occupied the minds of philosophers and natural scientists. The ratio between biomass accumulation and water consumption is known as water use efficiency and is widely relevant today in fields as diverse as plant improvement, forest ecology and climate change. Defined at scales varying from single leaf physiology to whole plants, it shows how botanical investigations changed through time, generally in tandem with developing disciplines and improving methods. The history started as a purely philosophical question by Greek philosophers of how plants grow, progressed through thought and actual experiments, towards an interest in the functioning of plants and the relationship to the environment. Aims This article retraces this history by following the progression of scientific questions posed through the centuries, and presents not only the main methodological and conceptual developments on biomass growth and transpiration but also the development of the carbon isotopic method of estimation. The history of research on photosynthesis is only touched briefly, but the development of research on transpiration and stomatal conductance is presented with more detail. Conclusion Research on water use efficiency, following a path from the whole plant to leaf-level functioning, was strongly involved in the historical development of the discipline of plant ecophysiology and is still a very active research field across nearly all levels of botanical research.

Key Message We provide phenotypic and genotypic data for a progeny trial of 5813 European beech seedlings, originating from 60 open-pollinated families collected at three altitudes (1020 m; 1140 m, 1340 m) on Mont Ventoux (44° 11′ N; 17° 5′ E).

Hybrid saplings were more reactive to soil water deficit than Japanese and European larch. European larch had hydraulically safer wood and anisohydric behavior, Japanese and hybrid larch showed isohydric strategy. Deciduous larch species could be an alternative to evergreen conifers in reforestation, but little is known about drought sensitivity of their saplings. The effect of an experimental drought on hydraulics and quantitative wood anatomy was tested on saplings of European larch (EL, Larix decidua), Japanese larch (JL, Larix kaempferi) and their hybrid (HL). Across species, biomass, transpiration rate and relative water content were higher in controls than in drought stressed trees, but transpiration efficiency was lower. JL had the highest transpiration efficiency under drought, and EL the lowest, coinciding with slower growth of EL. Wood of EL formed before drought was hydraulically safer as shown by higher wall/lumen ratio and lower pit cavity area. EL neither had a significant increase in transpiration efficiency nor a reduction in transpiration rate under drought, suggesting that the stomata remained open under soil water deficit. HL saplings were the most reactive to water shortage, indicated by intra-annual density fluctuations and a decrease in relative water content of the sapwood. Significant reduction in transpiration by HL suggested a higher stomatal sensitivity, while the same leaf surface area was maintained and radial growth was still similar to its best parent, the JL. The latter showed a significantly lower leaf surface area under drought than controls. EL, with its hydraulically safer wood, followed an anisohydric behavior, while JL and HL revealed an isohydric strategy. Altogether, our results suggest species dependent acclimations to drought stress, whereby HL followed the strategy of JL rather than that of EL.

Water use efficiency (WUE), oftentimes estimated as transpiration efficiency (TE): the amount of biomass produced with regard to the water used, has not yet been used as a breeding trait to select poplar genotypes with simultaneously high productivity and conservation of water. Before its application as a selection target, evidence must be presented showing that WUE or its estimators remain constant with age and across environmental conditions. We conducted a rainfall exclusion experiment in the field on two Populus euramericana (Moench.) and two Populus nigra (L.) genotypes, and assessed leaf-level (A/g(s)) and whole-plant WUE (DMT/WU as well as their components and related traits. Then, we aimed to compare these results with the same poplar genotypes grown in a glasshouse under contrasting water availability. Despite a reduction of soil water content and whole-plant transpiration, growth was stimulated in the rainfall exclusion plot, likely as a result of an increased nitrogen assimilation. However, TE values between the glasshouse and the field were similar, and genotype ranking remained fairly constant for transpiration, carbon isotopic discrimination (Delta, as a proxy for Wi) and TE. Moreover, even though the drivers of WUE in both experiments were different, increases of WUE measured as Delta or TE was not associated with lower biomass production. Relatively good agreement was found between Delta and TE in the field, absence of a similar correlation in the glasshouse is discussed. These results suggest that. may be a good proxy for TE, and could be used, both as a breeding target for genotype selection in glasshouses without impacting biomass production when planted in the field. However, reduced water availability modified the genotype ranking more significantly than between the field/glasshouse experiments, suggesting a diversity of poplar response to drought that should be considered in breeding strategies.

The relationship between plant growth and water consumption has for a long time occupied the minds of philosophers and natural scientists. The ratio between biomass accumulation and water consumption is known as water use efficiency and is widely relevant today in fields as diverse as crop improvement, forest ecology and climate change. Defined at scales varying from single leaf physiology to whole plants, it shows how botanical investigations changed through time, generally in tandem with developing disciplines and improving methods. The history started as a purely philosophical question by Greek philosophers of how plants grow, progressed through thought and actual experiments, towards an interest in plant functioning and their relationship to the environment. This article retraces this history by elucidating the progression of scientific questions posed through the centuries, presents the main methodological and conceptual developments.