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The oxygen isotope signature of water is a powerful tracer of water movement from plants to the global scale. However, little is known about the short‐term variability of oxygen isotopes leaving the ecosystem via transpiration, as high‐frequency measurements are lacking. A laser spectrometer was coupled to a gas‐exchange chamber directly estimating branch‐level fluxes in order to evaluate the short‐term variability of the isotopic composition of transpiration (δE) and to investigate the role of isotopic non‐steady‐state transpiration under natural conditions in cork‐oak trees (Quercus suber) during distinct Mediterranean seasons. The measured δ¹⁸O of transpiration (δE) deviated from isotopic steady state throughout most of the day even when leaf water at the evaporating sites was near isotopic steady state. High agreement was found between estimated and modeled δE values assuming non‐steady‐state enrichment of leaf water. Isoforcing, that is, the influence of the transpirational δ¹⁸O flux on atmospheric values, deviated from steady‐state calculations but daily means were similar between steady state and non‐steady state. However, strong daytime isoforcing on the atmosphere implies that short‐term variations in δE are likely to have consequences for large‐scale applications, for example, partitioning of ecosystem fluxes or satellite‐based applications.

Increasing water scarcity challenges crop sustainability inmany regions. As a consequence, the enhancement of transpiration efficiency (TE)—that is, the biomass produced per unit of water transpired—has become crucial in breeding programs. This could be achieved by reducing plant transpiration through a better closure of the stomatal pores at the leaf surface. However, this strategy generally also lowers growth, as stomatal opening is necessary for the capture of atmospheric CO₂ that feeds daytime photosynthesis. Here, we considered the reduction in transpiration rate at night (En) as a possible strategy to limit water use without altering growth. For this purpose, we carried out a genetic analysis for En and TE in grapevine, a major crop in drought-prone areas. Using recently developed phenotyping facilities, potted plants of a cross between Syrah and Grenache cultivars were screened for 2 y under well-watered and moderate soil water deficit scenarios. High genetic variability was found for En under both scenarios and was primarily associated with residual diffusion through the stomata. Five quantitative trait loci (QTLs) were detected that underlay genetic variability in En. Interestingly, four of them colocalized with QTLs for TE. Moreover, genotypes with favorable alleles on these common QTLs exhibited reduced En without altered growth. These results demonstrate the interest of breeding grapevine for lower water loss at night and pave the way to breeding other crops with this underexploited trait for higher TE.

It was previously shown that pearl millet genotypes carrying a terminal drought tolerance quantitative trait locus (QTL) had a lower transpiration rate (Tr; g cm−2 d−1) under well-watered conditions than sensitive lines. Here experiments were carried out to test whether this relates to leaf abscisic acid (ABA) and Tr concentration at high vapour pressure deficit (VPD), and whether that leads to transpiration efficiency (TE) differences. These traits were measured in tolerant/sensitive pearl millet genotypes, including near-isogenic lines introgressed with a terminal drought tolerance QTL (NIL-QTLs). Most genotypic differences were found under well-watered conditions. ABA levels under well-watered conditions were higher in tolerant genotypes, including NIL-QTLs, than in sensitive genotypes, and ABA did not increase under water stress. Well-watered Tr was lower in tolerant than in sensitive genotypes at all VPD levels. Except for one line, Tr slowed down in tolerant lines above a breakpoint at 1.40–1.90 kPa, with the slope decreasing >50%, whereas sensitive lines showed no change in that Tr response across the whole VPD range. It is concluded that two water-saving (avoidance) mechanisms may operate under well-watered conditions in tolerant pearl millet: (i) a low Tr even at low VPD conditions, which may relate to leaf ABA; and (ii) a sensitivity to higher VPD that further restricts Tr, which suggests the involvement of hydraulic signals. Both traits, which did not lead to TE differences, could contribute to absolute water saving seen in part due to dry weight increase differences. This water saved would become critical for grain filling and deserves consideration in the breeding of terminal drought-tolerant lines.

Recent decades have been characterized by increasing temperatures worldwide, resulting in an exponential climb in vapor pressure deficit (VPD). VPD has been identified as an increasingly important driver of plant functioning in terrestrial biomes and has been established as a major contributor in recent drought-induced plant mortality independent of other drivers associated with climate change. Despite this, few studies have isolated the physiological response of plant functioning to high VPD, thus limiting our understanding and ability to predict future impacts on terrestrial ecosystems. An abundance of evidence suggests that stomatal conductance declines under high VPD and transpiration increases in most species up until a given VPD threshold, leading to a cascade of subsequent impacts including reduced photosynthesis and growth, and higher risks of carbon starvation and hydraulic failure. Incorporation of photosynthetic and hydraulic traits in ‘next-generation’ land-surface models has the greatest potential for improved prediction of VPD responses at the plant- and global-scale, and will yield more mechanistic simulations of plant responses to a changing climate. By providing a fully integrated framework and evaluation of the impacts of high VPD on plant function, improvements in forecasting and long-term projections of climate impacts can be made.

The non-protein amino acid γ-aminobutyric acid (GABA) has been proposed to be an ancient messenger for cellular communication conserved across biological kingdoms. GABA has well-defined signalling roles in animals; however, whilst GABA accumulates in plants under stress it has not been determined if, how, where and when GABA acts as an endogenous plant signalling molecule. Here, we establish endogenous GABA as a bona fide plant signal, acting via a mechanism not found in animals. Using Arabidopsis thaliana , we show guard cell GABA production is necessary and sufficient to reduce stomatal opening and transpirational water loss, which improves water use efficiency and drought tolerance, via negative regulation of a stomatal guard cell tonoplast-localised anion transporter. We find GABA modulation of stomata occurs in multiple plants, including dicot and monocot crops. This study highlights a role for GABA metabolism in fine tuning physiology and opens alternative avenues for improving plant stress resilience. GABA accumulates during stress in plants but how, where and when GABA acts is not clear. Here the authors show that GABA production in Arabidopsis guard cells reduces stomatal opening and transpirational water loss, thereby improving water use efficiency.

Trees play a key role in the global hydrological cycle and measurements performed with the thermal dissipation method (TDM) have been crucial in providing whole-tree water-use estimates. Yet, different data processing to calculate whole-tree water use encapsulates uncertainties that have not been systematically assessed. We quantified uncertainties in conifer sap flux density (F d) and stand water use caused by commonly applied methods for deriving zero-flow conditions, dampening and sensor calibration. Their contribution has been assessed using a stem segment calibration experiment and 4 yr of TDM measurements in Picea abies and Larix decidua growing in contrasting environments. Uncertainties were then projected on TDM data from different conifers across the northern hemisphere. Commonly applied methods mostly underestimated absolute F d. Lacking a site- and species-specific calibrations reduced our stand water-use measurements by 37% and induced uncertainty in northern hemisphere F d. Additionally, although the interdaily variability was maintained, disregarding dampening and/or applying zero-flow conditions that ignored night-time water use reduced the correlation between environment and F d. The presented ensemble of calibration curves and proposed dampening correction, together with the systematic quantification of data-processing uncertainties, provide crucial steps in improving whole-tree water-use estimates across spatial and temporal scales.

Water saving under drought stress is assured by stomatal closure driven by active (ABA-mediated) and/or passive (hydraulic-mediated) mechanisms. There is currently no comprehensive model nor any general consensus about the actual contribution and relative importance of each of the above factors in modulating stomatal closure in planta . In the present study, we assessed the contribution of passive (hydraulic) vs active (ABA mediated) mechanisms of stomatal closure in V. vinifera plants facing drought stress. Leaf gas exchange decreased progressively to zero during drought and embolism-induced loss of hydraulic conductance in petioles peaked to ~50% in correspondence with strong daily limitation of stomatal conductance. Foliar ABA significantly increased only after complete stomatal closure had already occurred. Rewatering plants after complete stomatal closure and after foliar ABA reached maximum values did not induced stomatal re-opening, despite embolism recovery and water potential rise. Our data suggest that in grapevine stomatal conductance is primarily regulated by passive hydraulic mechanisms. Foliar ABA apparently limits leaf gas exchange over long-term, also preventing recovery of stomatal aperture upon rewatering, suggesting the occurrence of a mechanism of long-term down-regulation of transpiration to favor embolism repair and preserve water under conditions of fluctuating water availability and repeated drought events.

Because of intense transpiration and growth, the needs of plants for water can be immense. Yet water in the soil is most often heterogeneous if not scarce due to more and more frequent and intense drought episodes. The converse context, flooding, is often associated with marked oxygen deficiency and can also challenge the plant water status. Under our feet, roots achieve an incredible challenge to meet the water demand of the plant’s aerial parts under such dramatically different environmental conditions. For this, they continuously explore the soil, building a highly complex, branched architecture. On shorter time scales, roots keep adjusting their water transport capacity (their so-called hydraulics) locally or globally. While the mechanisms that directly underlie root growth and development as well as tissue hydraulics are being uncovered, the signalling mechanisms that govern their local and systemic adjustments as a function of water availability remain largely unknown. A comprehensive understanding of root architecture and hydraulics as a whole (in other terms, root hydraulic architecture) is needed to apprehend the strategies used by plants to optimize water uptake and possibly improve crops regarding this crucial trait. One consequence of climate change is an increased frequency of flood and drought episodes. This Perspective explores how water availability regulates root architecture and water transport capacity (hydraulics), from sensing mechanisms to novel responses.

• The stomatal control of transpiration is one of the major strategies by which plants cope with water stress. Here, we investigated the genetic architecture of the rootstock control of scion transpiration‐related traits over a period of 3 yr. • The rootstocks studied were full sibs from a controlled interspecific cross (Vitis vinifera cv. Cabernet Sauvignon × Vitis riparia cv. Gloire de Montpellier), onto which we grafted a single scion genotype. After 10 d without stress, the water supply was progressively limited over a period of 10 d, and a stable water deficit was then applied for 15 d. Transpiration rate was estimated daily and a mathematical curve was fitted to its response to water deficit intensity. We also determined δ13C values in leaves, transpiration efficiency and water extraction capacity. These traits were then analysed in a multienvironment (year and water status) quantitative trait locus (QTL) analysis. • Quantitative trait loci, independent of year and water status, were detected for each trait. One genomic region was specifically implicated in the acclimation of scion transpiration induced by the rootstock. The QTLs identified colocalized with genes involved in water deficit responses, such as those relating to ABA and hydraulic regulation. • Scion transpiration rate and its acclimation to water deficit are thus controlled genetically by the rootstock, through different genetic architectures.

We report that phytochrome B ( phyB ) mutants exhibit improved drought tolerance compared to wild type (WT) rice ( Oryza sativa L. cv. Nipponbare). To understand the underlying mechanism by which phyB regulates drought tolerance, we analyzed root growth and water loss from the leaves of phyB mutants. The root system showed no significant difference between the phyB mutants and WT, suggesting that improved drought tolerance has little relation to root growth. However, phyB mutants exhibited reduced total leaf area per plant, which was probably due to a reduction in the total number of cells per leaf caused by enhanced expression of Orysa;KRP1 and Orysa;KRP4 (encoding inhibitors of cyclin-dependent kinase complex activity) in the phyB mutants. In addition, the developed leaves of phyB mutants displayed larger epidermal cells than WT leaves, resulting in reduced stomatal density. phyB deficiency promoted the expression of both putative ERECTA family genes and EXPANSIN family genes involved in cell expansion in leaves, thus causing greater epidermal cell expansion in the phyB mutants. Reduced stomatal density resulted in reduced transpiration per unit leaf area in the phyB mutants. Considering all these findings, we propose that phyB deficiency causes both reduced total leaf area and reduced transpiration per unit leaf area, which explains the reduced water loss and improved drought tolerance of phyB mutants.