Species-specific control of DBH and landscape characteristics on tree-to-tree variability of sap velocity

The substantial tree-to-tree variability of transpiration poses a major challenge to a reliable stand-scale quantification of transpiration. The diameter at breast height (DBH) and landscape characteristics have been identified as drivers of tree-to-tree variability, but it remains unclear if their control on sap velocity varies between species-specific water-use and environmental conditions. We hypothesized that their controls are specie-specific, such as the temporal dynamic of their relative importance. To test our hypotheses, we used a multi-species stand that include 37 trees equipped with sap-flow sensors from four species representing the dominant species in central Europe. We analysed the daily relative importance of DBH, landscape slope, aspect, flow accumulation, and topographical position. We found that tree-to-tree variability of sap velocity was mainly dependent on DBH for oak (twice higher relative importance than other species) and on landscape characteristics for beech (36% higher relative importance than other species) and conifers. The temporal dynamics of the relative importance of most tested drivers was found to be species-specific and linked to root-related aspects in response to hydro-meteorological conditions. During dry summer months, the daily relative importance of oak's DBH increased to almost 60% to be three times higher than the value for beech. In contrast, the relative importance of flow accumulation was always two to three times higher for beech trees than oak and conifer trees. This indicated that larger oak trees accessed deeper water sources than smaller oaks. However, the shallower root architecture of beech trees involved a higher dependence on shallow soil water because a larger DBH is seemingly not enhancing the tree's capacity to explore deeper soils. These new insights emphasize the critical importance of accounting for DBH and landscape characteristics through a species-specific and temporally dynamic correction in further approaches for upscaling of sap-flow data from individual tree to stand-scale

High resolution mapping of traits related to whole-plant transpiration under increasing evaporative demand in wheat

First published online: March 20, 2016Atmospheric 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.Rémy Schoppach, Julian D Taylor, Elisabeth Majerus, Elodie Claverie, Ute Baumann, Radoslaw Suchecki, Delphine Fleury and Walid Sado

Sleep tight and wake-up early: nocturnal transpiration traits to increase wheat drought tolerance in a Mediterranean environment

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

Transpiration sensitivity to evaporative demand across 120 years of breeding of Australian wheat cultivars

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 atmo- spheric vapour pressure deficit (VPD) could result in advantageous water conser- vation 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-conserva- tion 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.R. Schoppach, D. Fleury, T. R. Sinclair, W. Sado

Yield comparison of simulated rainfed wheat and barley across Middle-East

Rain-fed wheat and barley are key crops in the Middle-East. A slight improvement in the effective use of water and in grain yield could greatly improve lives of subsistence farmers. This study aimed to evaluate the relative merits of wheat and barley in this region by simulating yields across 404 uniformly spread locations across 30 growing seasons. The results emphasized the primary importance of sowing date in each location. In comparison to wheat, barley generally was capable of rapid progress through its development stages allowing it to avoid deleterious late-season droughts and to have greater yields in low rainfall regions. A large part of Middle-East appeared unsuited for rain-fed production of these two grain species if seasonal yield variability is a concern

Wheat drought-tolerance to enhance food security in Tunisia, birthplace of the Arab Spring

The beginning of the ‘Arab Spring’ in 2011, a regional revolution which started in the Tunisian city of Sidi Bouzid in late 2010, occurred in part as a result of drought-triggered high wheat prices, which in the past led to ‘bread riots’ across several Middle East and North Africa (MENA) nations. Here we present, for the first time, an analysis of possible amelioration of wheat yield loss and greater stability in bread supply resulting from the incorporation of putative drought-tolerant traits into wheat cultivars grown in Tunisia. To this end, we used a simulation crop modeling approach using SSM-Wheat to evaluate yield loss or gain resulting from three types of water-saving traits that have been recently identified in wheat. These consisted in partial stomatal closure at high soil water content, overall decrease in transpiration rate (TR), and partial stomatal closure under elevated vapor pressure deficit (VPD). To capture large gradients in seasonal precipitation across wheat growing areas over a small country such as Tunisia, a grid pattern of 29 × 29 km was established as a basis for the geospatial simulation. Surprisingly, the simulation reflected opposite strategies in terms of water use (water-saving vs aggressive water use). The highest yield gain (30%) resulting from water-saving modification was found to occur in the food-insecure region of Sidi Bouzid. Traits enabling aggressive water use were found to be generally favorable across Tunisia, with one trait leading to up to 80% and 40% increases in yield and its stability in the food-challenged south of the country. However, major yield penalties were found to occur if water-saving traits were to be deployed in the ‘wrong’ region. Those findings could be used as a blueprint to navigate complex trait × environment interactions and to better inform local breeding and management programs to improve wheat yield and it stability in Tunisia and the MENA region in general

Importance of tree diameter and species for explaining the temporal and spatial variations of xylem water δ18O and δ2H in a multi-species forest

Identifying the vegetation and topographic variables influencing the isotopic variability of xylem water of forest vegetation remains crucial to interpret and predict ecohydrological processes in landscapes. In this study, we used temporally and spatially distributed xylem stable water isotopes measurements from two growing seasons to examine the temporal and spatial variations of xylem stable water isotopes and their relationships with vegetation and topographic variables in a Luxembourgish temperate mixed forest. Species-specific temporal variations of xylem stable water isotopes were observed during both growing seasons with a higher variability for beeches than oaks. Principal Component Regressions revealed that tree diameter at breast height explains up to 55% of the spatial variability of xylem stable water isotopes while tree species explains up to 24% of the variability. Topographic variables had a marginal role in explaining the spatial variability of xylem stable water isotopes (up to 6% for elevation). During the drier growing season (2020), we detected a higher influence of vegetation variables on xylem stable water isotopes and a lower temporal variability of the xylem water isotopic signatures than during the wetter growing season (2019). Our results reveal the dominant influence of vegetation on xylem stable water isotopes across a forested area and suggest that their spatial patterns arise mainly from size- and species-specific as well as water availability-dependent water use strategies rather than from topographic heterogeneity. The identification of the key role of vegetation on xylem stable water isotopes has critical implications for the representativity of isotopes-based ecohydrological and catchments studies