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Recent European heatwaves have significantly impacted forest ecosystems, leading to increased plant water stress. Advances in land surface models aim to improve the representation of vegetation drought responses by incorporating plant hydraulics into the plant functional type (PFT) classification system. However, reliance on PFTs may inadequately capture the diverse plant hydraulic traits (PHTs), potentially biasing transpiration and vegetation water stress representations. The detection of vegetation drought stress is further complicated by the mixing of different tree species and forest patches. This study uses the Community Land Model version 5.0 to simulate an experimental mixed‐forest catchment with configurations representing standalone, patched mixed, and fully‐mixed forests. Biome‐generic, PFT‐specific, or species‐specific PHTs are employed. Results emphasize the crucial role of accurately representing mixed forests in reproducing observed vegetation water stress and transpiration fluxes for both broadleaf and needleleaf tree species. The dominant vegetation fraction is a key determinant, influencing aggregated vegetation response patterns. Segregation level in PHT parameterizations shapes differences between observed and simulated transpiration fluxes. Simulated root water potential emerges as a potential metric for detecting vegetation stress periods. However, the model's plant hydraulic system has limitations in reproducing the long‐term effects of extreme weather events on needleleaf tree species. These findings highlight the complexity of modeling mixed forests and underscore the need for improved representation of plant diversity in land surface models to enhance the understanding of vegetation water stress under changing climate conditions. Plain Language Summary: Numerical simulation models for large‐scale ecosystems often oversimplify mixed forests, neglecting the diversity of species and structural complexity. This oversight impacts the accuracy of simulated plant water use, especially during droughts and heatwaves. This study focused on the specific traits of key tree species at a Luxembourg site, aiming to enhance the model's ability to represent the vegetation response to extreme conditions. By incorporating detailed plant traits, the model improved in replicating observed tree water use and identifying periods of water deficit. The findings highlight the importance of considering the functional diversity of mixed forest ecosystems for accurate simulations. Moreover, the study introduces a simple metric using the model's structure to pinpoint periods when different species experience severe water deficit. The proposed metric provides a practical tool for identifying critical periods of water stress for various species within mixed forests. This approach enhances our understanding of mixed forest dynamics under extreme conditions, emphasizing the need for nuanced representations in large‐scale models. Key Points: We show that the model's dominant fraction of a mixed ecosystem masks the water status of smaller fractions within a grid cellWe demonstrate that refining the plant hydraulic traits based on species presence improves the representation of mixed forests in CLM5We evidence the limitations of CLM5 in reproducing the needleleaf water stress using tree water deficit measurements [ABSTRACT FROM AUTHOR]

Historically, wheat yields in drought-prone Australian environments have been consistently increasing for over a century. There is currently an agreement that approximately half of that increase is attributable to breeding programmes, but their physiological basis remains poorly documented. In this investigation, we hypothesized that limited whole-plant transpiration rate ( TR) under high atmospheric vapour pressure deficit ( VPD) could result in advantageous water conservation and crop yield increase under south Australian conditions. Therefore, TR response to VPD was measured in the 0.9-3.2 kPa range for a group of 23 wheat cultivars that were released from 1890 to 2008. Consistent with a water-conservation hypothesis, all genotypes displayed a VPD break point ( BP) in TR with increasing VPD such that TR was limited at VPD above a BP of about 2 kPa. The BP and slope of TR with increasing VPD above the break point were correlated with the year of release, although the changes were in different directions. Such changes in these transpiration parameters were independent of plant leaf area and only marginally correlated with Zadok's stages. These results indicated that selection over 120 years by breeders for yield increase unconsciously resulted in genotype selection for the expression of the limited- TR trait. [ABSTRACT FROM AUTHOR]

Non-negligible nighttime transpiration rates (TR N ) have been identified in grasses such as wheat and barley. Evidence from the last 30 years indicate that in drought-prone environments with high evaporative demand, TR N could amount to 8-55% of daytime TR, leading several investigators to hypothesize that reducing TR N might represent a viable water-saving strategy that minimizes seemingly 'wasteful' water loss that is not traded for CO 2 fixation. More recently however, evidence suggests that actual increases in TR N during pre-dawn hours, which are presumably controlled by the circadian clock, mediate drought tolerance - not through water conservation - but by enabling maximized gas exchange early in the morning before midday depression sets in. Finally, new findings point to a previously undocumented role for leaf sheaths as substantial contributors (up to 45%) of canopy TR N , although the extent of their involvement in these two strategies remains unknown. In this paper, we synthesize and reconcile key results from experimental and simulation-based modeling efforts conducted at scales ranging from the leaf tissue to the field plot on wheat and barley to show that both strategies could in fact concomitantly enable yield gains under limited water supply. We propose a simple framework highlighting the role played by TR N dynamics in drought tolerance and provide a synthesis of potential research directions, with an emphasis on the need for further examining the role played by the circadian clock and leaf sheath gas exchange.

In wheat, night-time transpiration rate (TRN) could amount to 14-55% of daytime transpiration rate (TR), depending on the cultivar and environment. Recent evidence suggests that TRN is much less responsive to soil drying than daytime TR, and that such 'wasteful' water losses would increase the impact of drought on yields. In contrast, other evidence indicates that pre-dawn, circadian increases in TRN may enable enhanced radiation use efficiency, resulting in increased productivity under water deficit. Until now, there have been no attempts to evaluate these seemingly conflicting hypotheses in terms of their impact on yields in any crop. Here, using the Mediterranean environment of Tunisia as a case study, we undertook a simulation modelling approach using SSM-Wheat to evaluate yield outcomes resulting from these TRN trait modifications. TRN represented 15% of daytime TR-generated yield penalties of up to 20%, and these worsened when TRN was not sensitive to soil drying TR. For the same TRN level (15%), simulating a predawn increase in TRN alleviated yield penalties, leading to yield gains of up to 25%. Overall, this work suggests that decreasing TRN but increasing pre-dawn circadian control would be a viable breeding target to increase drought tolerance in a Mediterranean environment.

Background and Aims: The ability of wheat genotypes to save water by reducing their transpiration rate (TR) at times of the day with high vapour pressure deficit (VPD) has been linked to increasing yields in terminal drought environments. Further, recent evidence shows that reducing nocturnal transpiration (TRN) could amplify water saving. Previous research indicates that such traits involve a root-based hydraulic limitation, but the contribution of hormones, particularly auxin and abscisic acid (ABA), has not been explored to explain the shoot-root link. In this investigation, based on physiological, genetic and molecular evidence gathered on a mapping population, we hypothesized that root auxin accumulation regulates whole-plant water use during both times of the day., Methods: Eight double-haploid lines were selected from a mapping population descending from two parents with contrasting water-saving strategies and root hydraulic properties. These spanned the entire range of slopes of TR responses to VPD and TRN encountered in the population. We examined daytime/night-time auxin and ABA contents in the roots and the leaves in relation to hydraulic traits that included whole-plant TR, plant hydraulic conductance (KPlant), slopes of TR responses to VPD and leaf-level anatomical traits., Key Results: Root auxin levels were consistently genotype-dependent in this group irrespective of experiments and times of the day. Daytime root auxin concentrations were found to be strongly and negatively correlated with daytime TR, KPlant and the slope of TR response to VPD. Night-time root auxin levels significantly and negatively correlated with TRN. In addition, daytime and night-time leaf auxin and ABA concentrations did not correlate with any of the examined traits., Conclusions: The above results indicate that accumulation of auxin in the root system reduces daytime and night-time water use and modulates plant hydraulic properties to enable the expression of water-saving traits that have been associated with enhanced yields under drought., (© The Author(s) 2019. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com.)

Main Conclusion: Nocturnal transpiration, through its circadian control, plays a role in modulating daytime transpiration response to increasing evaporative demand, to potentially enable drought tolerance in wheat. Limiting plant transpiration rate (TR) in response to increasing vapor pressure deficit (VPD) has been suggested to enable drought tolerance through water conservation. However, there is very little information on the extent of diversity of TR response curves to "true" VPD (i.e., independent from temperature). Furthermore, new evidence indicate that water-saving could operate by modulating nocturnal TR (TR N ), and that this response might be coupled to daytime gas exchange. Based on 3 years of experimental data on a diverse group of 77 genotypes from 25 countries and 5 continents, a first goal of this study was to characterize the functional diversity in daytime TR responses to VPD and TR N in wheat. A second objective was to test the hypothesis that these traits could be coupled through the circadian clock. Using a new gravimetric phenotyping platform that allowed for independent temperature and VPD control, we identified three and fourfold variation in daytime and nighttime responses, respectively. In addition, TR N was found to be positively correlated with slopes of daytime TR responses to VPD, and we identified pre-dawn variation in TR N that likely mediated this relationship. Furthermore, pre-dawn increase in TR N positively correlated with the year of release among drought-tolerant Australian cultivars and with the VPD threshold at which they initiated water-saving. Overall, the study indicates a substantial diversity in TR responses to VPD that could be leveraged to enhance fitness under water-limited environments, and that TR N and its circadian control may play an important role in the expression of water-saving.

Main Conclusion: Theoretical derivation predicted growth retardation due to pot water limitations, i.e., pot binding. Experimental observations were consistent with these limitations. Combined, these results indicate a need for caution in high-throughput screening and phenotyping. Pot experiments are a mainstay in many plant studies, including the current emphasis on developing high-throughput, phenotyping systems. Pot studies can be vulnerable to decreased physiological activity of the plants particularly when pot volume is small, i.e., "pot binding". It is necessary to understand the conditions under which pot binding may exist to avoid the confounding influence of pot binding in interpreting experimental results. In this paper, a derivation is offered that gives well-defined conditions for the occurrence of pot binding based on restricted water availability. These results showed that not only are pot volume and plant size important variables, but the potting media is critical. Artificial potting mixtures used in many studies, including many high-throughput phenotyping systems, are particularly susceptible to the confounding influences of pot binding. Experimental studies for several crop species are presented that clearly show the existence of thresholds of plant leaf area at which various pot sizes and potting media result in the induction of pot binding even though there may be no immediate, visual plant symptoms. The derivation and experimental results showed that pot binding can readily occur in plant experiments if care is not given to have sufficiently large pots, suitable potting media, and maintenance of pot water status. Clear guidelines are provided for avoiding the confounding effects of water-limited pot binding in studying plant phenotype.

Increasing evidence suggests that nocturnal transpiration rate (TR N ) is a non-negligible contributor to global water cycles. Short-term variation in nocturnal vapor pressure deficit (VPD N ) has been suggested to be a key environmental variable influencing TR N . However, the long-term effects of VPD N on plant growth and development remain unknown, despite recent evidence documenting long-term effects of daytime VPD on plant anatomy, growth and productivity. Here we hypothesized that plant anatomical and functional traits influencing leaf and root hydraulics could be influenced by long-term exposure to VPD N . A total of 23 leaf and root traits were examined on four wheat (Triticum aestivum) genotypes, which were subjected to two long-term (30 day long) growth experiments where daytime VPD and daytime/nighttime temperature regimes were kept identical, with variation only stemming from VPD N , imposed at two levels (0.4 and 1.4 kPa). The VPD N treatment did not influence phenology, leaf areas, dry weights, number of tillers or their dry weights, consistently with a drought and temperature-independent treatment. In contrast, vein densities, adaxial stomata densities, TR N and cuticular TR, were strongly increased following exposure to high VPD N . Simultaneously, whole-root system xylem sap exudation and seminal root endodermis thickness were decreased, hypothetically indicating a change in root hydraulic properties. Overall these results suggest that plants 'sense' and adapt to variations in VPD N conditions over developmental scales by optimizing both leaf and root hydraulics., (© 2016 Scandinavian Plant Physiology Society.)

Atmospheric vapor pressure deficit (VPD) is a key component of drought and has a strong influence on yields. Whole-plant transpiration rate (TR) response to increasing VPD has been linked to drought tolerance in wheat, but because of its challenging phenotyping, its genetic basis remains unexplored. Further, the genetic control of other key traits linked to daytime TR such as leaf area, stomata densities and - more recently - nocturnal transpiration remains unknown. Considering the presence of wheat phenology genes that can interfere with drought tolerance, the aim of this investigation was to identify at an enhanced resolution the genetic basis of the above traits while investigating the effects of phenology genes Ppd-D1 and Ppd-B1 Virtually all traits were highly heritable (heritabilities from 0.61 to 0.91) and a total of mostly trait-specific 68 QTL were detected. Six QTL were identified for TR response to VPD, with one QTL (QSLP.ucl-5A) individually explaining 25.4% of the genetic variance. This QTL harbored several genes previously reported to be involved in ABA signaling, interaction with DREB2A and root hydraulics. Surprisingly, nocturnal TR and stomata densities on both leaf sides were characterized by highly specific and robust QTL. In addition, negative correlations were found between TR and leaf area suggesting trade-offs between these traits. Further, Ppd-D1 had strong but opposite effects on these traits, suggesting an involvement in this trade-off. Overall, these findings revealed novel genetic resources while suggesting a more direct role of phenology genes in enhancing wheat drought tolerance., (© The Author 2016. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

In crop plants, accumulating evidence indicates non-marginal night-time transpiration (TRNight) that is responsive to environmental conditions, especially in semiarid areas. However, the agronomical advantages resulting from such phenomenon remain obscure. Recently, drought-tolerance strategies directly stemming from daytime TR (TRDay) responses to daytime vapour pressure deficit VPD (VPDDay) were identified in wheat (Triticum spp.), but the existence of similar strategies resulting from TRNight response to night-time VPD (VPDNight) remains to be investigated, especially that preliminary evidence on this species indicates that TRNight might be responsive to VPDNight. Our study aims at investigating such strategies among a group of diverse lines including drought-tolerant genotypes. The study revealed that: (i) TRNight can be as high as 55% that of the maximal TRDay; (ii) VPDNight is the major driver of TRNight in a genotype-dependent fashion and has an impact on following daytime gas exchange; and (iii) a strong correlation exists between TR sensitivities to VPD under night-time and daytime conditions, revealing that tolerance strategies such as conservative water use do also exist under night-time environments. Overall, this report opens the way to further phenotyping and modelling work aiming at assessing the potential of using TRNight as a trait in breeding new drought-tolerant germplasm.

Efficient breeding of drought-tolerant wheat (Triticum spp.) genotypes requires identifying mechanisms underlying exceptional performances. Evidence indicates that the drought-tolerant breeding line RAC875 is water-use conservative, limiting its transpiration rate (TR) sensitivity to increasing vapour pressure deficit (VPD), thereby saving soil water moisture for later use. However, the physiological basis of the response remains unknown. The involvement of leaf and root developmental, anatomical and hydraulic features in regulating high-VPD, whole-plant TR was investigated on RAC875 and a drought-sensitive cultivar (Kukri) in 12 independent hydroponic and pot experiments. Leaf areas and stomatal densities were found to be identical between lines and de-rooted plants didn't exhibit differential TR responses to VPD or TR sensitivity to four aquaporin (AQP) inhibitors that included mercury chloride (HgCl2). However, intact plants exhibited a differential sensitivity to HgCl2 that was partially reversed by β-mercaptoethanol. Further, root hydraulic conductivity of RAC875 was found to be lower than Kukri's and root cross-sections of RAC875 had significantly smaller stele and central metaxylem diameters. These findings indicate that the water-conservation of RAC875 results from a root-based hydraulic restriction that requires potentially heritable functional and anatomical features. The study revealed links between anatomical and AQP-based processes in regulating TR under increasing evaporative demand.

Warmer climates are already contributing to significant decreases in wheat (Triticum spp.) yields worldwide, highlighting the need for more adapted germplasm. Although many studies have addressed the effects of warmer climates on grain physiology and photosynthesis, only a few have considered temperature effects on other key yield-related traits such as the sensitivity of transpiration rate (TR) to vapour pressure deficit (VPD)-a function of air temperature and relative humidity. In wheat, no reports are available to document such influences. More importantly, the relative contributions of heat-stress night and day conditions on such sensitivity and the plant's evaporative surface remain to be investigated. The objective of this study was to assess the response of these two physiological processes to long-term (i.e. 3 weeks) exposures to six warming scenarios, consisting of a combination of three target growth-period VPD (2, 2.7 and 4kPa), and two night temperature (20 and 30°C) regimes among 11 diverse bread and durum wheat lines having different origins. The study revealed (i) a large genetic variability in those responses; (ii) non-linear interactions between the effects of day and night conditions; and (iii) compensation mechanisms between leaf areas and transpiration sensitivities to VPD together with differential acclimation strategies of these sensitivities with respect to increasingly warmer scenarios. These findings open the way to implementing breeding strategies that can improve wheat yields under different warming scenarios.