Proxy For Water Use Efficiency Research Articles

Natural alleles of the abscisic acid catabolism gene ZmAbh4 modulate water use efficiency and carbon isotope discrimination in maize.

Altering plant water use efficiency (WUE) is a promising approach for achieving sustainable crop production in changing climate scenarios. Here, we show that WUE can be tuned by alleles of a single gene discovered in elite maize (Zea mays) breeding material. Genetic dissection of a genomic region affecting WUE led to the identification of the gene ZmAbh4 as causative for the effect. CRISPR/Cas9-mediated ZmAbh4 inactivation increased WUE without growth reductions in well-watered conditions. ZmAbh4 encodes an enzyme that hydroxylates the phytohormone abscisic acid (ABA) and initiates its catabolism. Stomatal conductance is regulated by ABA and emerged as a major link between variation in WUE and discrimination against the heavy carbon isotope (Δ13C) during photosynthesis in the C4 crop maize. Changes in Δ13C persisted in kernel material, which offers an easy-to-screen proxy for WUE. Our results establish a direct physiological and genetic link between WUE and Δ13C through a single gene with potential applications in maize breeding.

Responses of Carbon Isotope Composition of Common C3 and C4 Plants to Climatic Factors in Temperate Grasslands

Investigating relationships between climatic factors and plant δ13C of both C3 and C4 plants simultaneously is critical for accurately predicting the effects of climate change on plant ecophysiology and ecosystem functioning and reconstructing past vegetation and climate conditions. We selected common C3 and C4 plants in temperate grasslands in Inner Mongolia, China, i.e., Stipa spp., Carex spp., Leymus chinensis and Cleistogenes spp., and investigated the relationships between climatic factors and plant δ13C of each genus/species. The results showed that precipitation, especially growing season precipitation (GSP), was the dominant factor affecting plant δ13C in this region. For C3 plants, there were significantly negative relationships between precipitation and plant δ13C. For C4 plants, plant δ13C of Cleistogenes spp. firstly increased, then decreased with precipitation at a breakpoint GSP 204.84 mm. Our findings emphasize that C4 plant δ13C is sensitive to precipitation, but responses are species-specific and environment-specific, and suggest that C4 plant δ13C can be used as a proxy for water use efficiency (WUE), but care should be taken in evaluating WUE. Moreover, our findings provide basic information for accurately predicting the effects of climate change on ecosystem structure and function and reconstructing past vegetation and climate conditions from bulk materials in arid and semiarid regions.

Sex and tree rings: Females neither grow less nor are less water-use efficient than males in four dioecious tree species

Females of woody dioecious species usually expend more resources on reproduction than males. Therefore, it is expected that females incur greater costs of reproduction than males, and, as a result, trade-offs between reproduction and growth should emerge. The aim of this study is to test those hypotheses by analyzing the differences between genders regarding radial growth (basal area increment) and wood carbon isotope composition (δ13C), a proxy of water-use efficiency. We compared these two variables in males and females of four dioecious tree species inhabiting drought-prone Mediterranean sites in Spain (Pistacia terebinthus, Ilex aquifolium, Juniperus thurifera and Ailanthus altissima). We analyzed the influence of sex on the radial growth patterns throughout the tree life considering the growth stage of individuals, the differences in the response of genders to climate variables (air temperature, precipitation and drought severity), and the δ13C during a severe drought period. One site was studied for each species and 21–33 trees per species were sampled in each site. No differences in growth were found between genders for any of the four species throughout their life span. No significant interactions between gender and precipitation were found, although A. altissima males were more responsive to summer (June-July) temperature. No differences in δ13C were found between genders excepting for P. terebinthus, indicating that the males of this species show a less efficient water use during drought events than the females. These results do not support the broad assumption that females of woody dioecious plants show lower growth and are less water-use efficient than males or that they respond differently to precipitation variability, except for P. terebinthus during drought events. Further analyses could be performed in other dioecious species inhabiting seasonally dry regions to confirm or reject our conclusions.

Altitudinal variation of leaf carbon isotope for Dendrosenecio keniensis and Lobelia gregoriana in Mount Kenya alpine zone

AbstractAbiotic factors vary along altitudinal gradients, and this may influence plant morphology, physiology and function. This study aimed to test the hypothesis that leaf δ13C—a common proxy for water use efficiency—was indirectly influenced by morphological adjustments with changing climatic factors along an altitudinal gradient on Mount Kenya. We sampled leaves of Dendrosenecio keniensis and Lobelia gregoriana using seventy‐two 10 × 10 m plots situated every 100 m starting from 3600 to 4300 m. We determined leaf δ13C using stable isotope mass spectrometry. We also quantified the following morphological factors; leaf area, leaf mass per area, specific leaf area and leaf thickness. Climate data included mean annual temperature and precipitation, diurnal temperature range and water vapor pressure. Our results revealed that there was a leaf δ13C enrichment of 1.76 ‰ km−1 and 1.62 ‰ km−1 with altitude for D. keniensis and L. gregoriana, respectively. Leaf δ13C was enrichment by 0.01 ‰ mm−1 with mean annual precipitation along the altitude gradient for D. keniensis and 0.02 ‰ mm−1 for L. gregoriana. D. keniensis and L. gregoriana have high‐water use efficiency, an adaptation for surviving near freezing alpine temperatures and high‐diurnal range. Leaf δ13C exhibited a depletion of −0.37 ‰ per °C increase of mean annual temperature along the altitude gradient for D. keniensis and −0.34 ‰ per °C increase for L. gregoriana. Our results also showed a negative relationship between pCO2 and leaf δ13C and positive relationship between pCO2 and ∆13C for both species. Low temperatures led to the increase in leaf thickness and specific leaf area for these two species, factors that influenced leaf δ13C and ∆13C.Abstract in Chinese is available with online material.

Improved photosynthesis by leaf nutrient content enhances water use efficiency in Juniperus tibetica along elevation gradients above 4000 m a.s.l.

ABSTRACT Background Leaf functional traits can influence the ability of plants to grow when facing stresses. Climate changes can impose fundamental impacts on plant growth, especially at high mountains. Yet little is known about relationships between leaf functional traits and elevations in tree species above 4000 m a.s.l. Aims Our objective was to investigate and compare the trend in water use efficiency (WUE) in the Tibetan juniper (Juniperus tibetica) along elevation gradients under different climate conditions, and by presenting evidence from leaf functional traits, to simultaneously clarify the underlying mechanisms. Methods We investigated various leaf functional traits, i.e. leaf carbon (δ13C) and oxygen (δ18O) isotopic composition, leaf N and P concentration and specific leaf area (SLA) in J. tibetica, a dominant tree species of the treeline, along elevation gradients at three study sites between 4150 and 4950 m a.s.l. in the Qinghai-Tibet Plateau. Results As elevation increased, leaf δ13C, used as a proxy for WUE, increased, whereas leaf δ18Odecreased in J. tibetica. Leaf N area also increased with elevation. Conclusions We deduced that increasing WUE with elevation was due to enhanced photosynthetic capacity at all three study sites, and increased N area may enhance photosynthesis and hence WUE.

Effects of nitrogen addition and mowing on nitrogen- and water-use efficiency of Artemisia frigida in a grassland restored from an abandoned cropland

Abstract Aims Competition among plants in a community usually depends on their nitrogen (N)-use efficiency (NUE) and water-use efficiency (WUE) in arid and semi-arid regions. Artemisia frigida is an indicator species in heavily degraded grassland, however, how its NUE and WUE respond to N addition in different successional stages is still unclear, especially with mowing, a common management practice in semi-arid grasslands. Methods Based on a long-term controlled experiment with N addition and mowing in an abandoned cropland from 2006 to 2013, we investigated the NUE and WUE of A. frigida in two patches (i.e. grass and herb patches) in 2013 which represented two potential successional stages from herb to grass communities. The coverage of A. frigida was higher (about 50%) in the herb patch than in the grass patch (about 10%). Stable isotopic C (δ 13C) and N (δ 15N) as well as C and N pools were measured in plants and soils. NUE was calculated as leaf C/N, and leaf δ 13C values were used as a proxy for WUE. Important Findings N addition did not affect WUE of A. frigida, but significantly decreased NUE by 42.9% and 26.6% in grass and herb patches, respectively. The response of NUE to N addition was related to altering utilization of different N sources (NH4+vs. NO3−) by A. frigida according to the changed relationship between leaf δ 15N/soil δ 15N and NUE. Mowing had no effect on NUE regardless of N addition, but significantly increased WUE by 2.3% for A. frigida without N addition in the grass patch. The addition of N reduced the positive effect of mowing on its WUE in grass patch. Our results suggested that decreased NUE and/or WUE of A. frigida under mowing and N addition could reduce its competition, and further accelerate restoration succession from the abandoned cropland to natural grassland in the semi-arid region.

Both Mature Patches and Expanding Areas of Juniperus thurifera Forests Are Vulnerable to Climate Change But for Different Reasons

Research Highlights: Water use efficiency (WUE) varied along a gradient of Juniperus thurifera (L.) forest expansion, being higher in recently colonised areas. Background and Objectives: WUE is a classic physiological process of plants that reflects the compromise between carbon assimilation and water loss and has a profound influence on their performance in water-limited environments. Forest expansion in Mediterranean regions associated with land abandonment can influence the WUE of plants due to the existence of two opposing gradients: one of favourable–unfavourable environmental conditions and another one of increased–decreased intraspecific competition, the former increasing and the latter decreasing towards the expanding front. The main objective of this study was to elucidate how the WUE of Juniperus thurifera varied along the stages of forest expansion and to provide insight on how this variation is influenced by intraspecific competition and abiotic factors. Materials and Methods: Seventeen plots at different distances from the mature forest core were selected at three sites located in the centre of the Iberian Peninsula. For 30 individuals within each plot, we measured biometric characteristics, age, tree vigour, and C/N ratio in leaves, and the leaf carbon isotope signature (δ13C (‰)) as a proxy for WUE. Around each individual, we scored the percentage cover of bare soil, stoniness, conspecifics, and other woody species. Results: WUE of J. thurifera individuals varied along the forest expansion gradient, being greater for the individuals at the expanding front than for those at the mature forest. WUE was influenced by the cover of conspecifics, tree age, and C/N ratio in leaves. This pattern reveals that less favourable environmental conditions (i.e., rocky soils and higher radiation due to lower vegetation cover) and younger trees at the expanding front are associated with increased WUE. The increased cover of conspecifics decreases irradiance at the mature forest, involving milder stress conditions than at the expanding front. Conclusions: Lower WUE in mature forests due to more favourable conditions and higher WUE due to abiotic stress at expanding fronts revealed high constraints on water economy of this tree species in these two contrasting situations. Climate change scenarios bringing increased aridity are a serious threat to Juniperus thurifera forests, affecting both mature and juvenile populations although in different ways, which deserve further research to fully unveil.

Adaptation to drought is coupled with slow growth, but independent from phenology in marginal silver fir (Abies alba Mill.) populations.

Drought is one of the most important selection pressures for forest trees in the context of climate change. Yet, the different evolutionary mechanisms, and their environmental drivers, by which certain populations become more drought tolerant than others is still little understood. We studied adaptation to drought in 16 silver fir (Abies alba Mill.) populations from the French Mediterranean Alps by combining observations on seedlings from a greenhouse experiment (N = 8,199) and on adult tress in situ (N = 315). In the greenhouse, we followed half‐sib families for four growing seasons for growth and phenology traits, and tested their water stress response in a "drought until death" experiment. Adult trees in the field were assessed for δ 13C, a proxy for water use efficiency, and genotyped at 357 SNP loci. SNP data was used to generate a null expectation for seedling trait divergence between populations in order to detect the signature of selection, and 31 environmental variables were used to identify the selective environment. We found that seedlings originating from populations with low soil water capacity grew more slowly, attained a smaller stature, and resisted water stress for a longer period of time in the greenhouse. Additionally, adult trees of these populations exhibited a higher water use efficiency as evidenced by their δ 13C. These results suggest a correlated evolution of the growth‐drought tolerance trait complex. Population divergence in bud break phenology was adaptive only in the second growing season, and evolved independently from the growth‐drought tolerance trait complex. Adaptive divergence in bud break phenology was principally driven by the inter‐ and intra‐annual variation in temperature at the geographic origin of the population. Our results illustrate the different evolutionary strategies used by populations to cope with drought stress at the range limits across a highly heterogeneous landscape, and can be used to inform assisted migration programs.

Selecting for water use efficiency, wood chemical traits and biomass with genomic selection in a Eucalyptus breeding program

The selection of ideotypes combining high biomass production, high water use efficiency (WUE) and adequate wood properties is a major challenge in improving forest trees for industrial plantations. This issue was addressed in a field experiment evaluating 1130 clones of Eucalyptus urophylla * Eucalyptus grandis. Genomic selection using 3303 SNPs and the GBLUP model was carried out at age 55 months to select genitors and clones for stem volume (V55), WUE (using stable carbon isotope composition in stemwood, δ13C as a proxy, transformed into intrinsic WUE, Wi), as well as lignin (KL) and holocellulose (HCEL) contents. The variance components were mainly additive for δ13C(Wi), KL and HCEL, while they were highly non-additive for V55 (the ratio of dominance to additive variance was 130%). The narrow sense heritability was higher for Wi (h2 = 0.704) than for volume and wood traits (<0.5), showing that this trait was under strong genetic control. Additive and total genetic correlations among traits were low (between −0.260 and 0.260) as were the correlations due to environmental effects (between −0.267 and 0.344), which suggests a rather independence among traits. The equal emphasis and the desired gain index selection methods were used to assess economic weights and to estimate the expected additive and total genetic gains. Different breeding objectives were defined with percentages weighting the economic coefficients or the desired gain for the equal emphasis or desired gain method, respectively. When the breeding objective gave emphasis to volume production using the percentages of 70%, 10%, 10%, and 10% for V55, HCEL, KL and Wi, respectively, positive gains were observed in both methods, but the correlation between index rankings were 0.686 and 0.635 for breeding and clonal selection, respectively. With a more balanced breeding objective using percentages such as 25%, 25%, 25%, 25% for V55, HCEL, KL and Wi, respectively, both index methods gave close positive gains and similar rankings, the correlation between index rankings ranging from 0.941 to 0.982 for parent selection and from 0.883 to 0.903 for clone selection. With more contrasted percentages between V55 and Wi, such as (80%, 10%, 10%, 0%) or (90%, 10%, 10%, −10%), the gain increased for V55 and was close to the maximum and decreased markedly for Wi. This study explores the magnitude of economic coefficients for index selection and shows that positive genetic gains can be achieved by associating biomass, a proxy of WUE and wood chemical traits. It provides encouraging results for selecting Eucalyptus varieties adapted to dry zones while maintaining good performances regarding other economic traits.

Adaptation to local climate in multi-trait space: evidence from silver fir (Abies alba Mill.) populations across a heterogeneous environment.

Heterogeneous environments, such as mountainous landscapes, create spatially varying selection pressure that potentially affects several traits simultaneously across different life stages, yet little is known about the general patterns and drivers of adaptation in such complex settings. We studied silver fir (Abies alba Mill.) populations across Switzerland and characterized its mountainous landscape using downscaled historical climate data. We sampled 387 trees from 19 populations and genotyped them at 374 single-nucleotide polymorphisms (SNPs) to estimate their demographic distances. Seedling morphology, growth and phenology traits were recorded in a common garden, and a proxy for water use efficiency was estimated for adult trees. We tested whether populations have more strongly diverged at quantitative traits than expected based on genetic drift alone in a multi-trait framework, and identified potential environmental drivers of selection. We found two main responses to selection: (i) populations from warmer and more thermally stable locations have evolved towards a taller stature, and (ii) the growth timing of populations evolved towards two extreme strategies, 'start early and grow slowly' or 'start late and grow fast', driven by precipitation seasonality. Populations following the 'start early and grow slowly' strategy had higher water use efficiency and came from inner Alpine valleys characterized by pronounced summer droughts. Our results suggest that contrasting adaptive life-history strategies exist in silver fir across different life stages (seedling to adult), and that some of the characterized populations may provide suitable seed sources for tree growth under future climatic conditions.

Genetic Variation in Water-Use Efficiency (WUE) and Growth in Mature Longleaf Pine

The genetic and physiological quality of seedlings is a critical component for longleaf pine (Pinus palustris Mill.) restoration, because planting genetic material that is adapted to environmental stress is required for long-term restoration success. Planting trees that exhibit high water-use efficiency (WUE) is a practice that could maximize this species’ survival and growth in a changing climate. Our study evaluates genetic variation in WUE and growth, as well as WUE-growth relationships, a key step to determine potential for breeding and planting trees with high WUE. We measured carbon isotope discrimination (∆)—a proxy for WUE—in 106 longleaf pine increment cores extracted from trees belonging to nine full-sib families. Tree diameter and total tree height were also measured at ages 7, 17, 30 and 40 years. Each increment core was divided into segments corresponding to ages 7–17, 18–30 and 31–40, representing early, intermediate and mature growth of the trees. We identified significant genetic variation in DBH and WUE among families that merit further exploration for identifying trees that can potentially withstand drought stress. Mean family growth rates were not associated with mean family values for carbon isotope discrimination. Family variation in both diameter growth and WUE but no relationship between family values for these traits, suggests it is possible to improve longleaf pines in both diameter growth and WUE through appropriate breeding.

Mapping QTLs for water-use efficiency reveals the potential candidate genes involved in regulating the trait in apple under drought stress

BackgroundImprovement of water-use efficiency (WUE) can effectively reduce production losses caused by drought stress. A better understanding of the genetic determination of WUE in crops under drought stress has great potential value for developing cultivars adapted to arid regions. To identify the genetic loci associated with WUE and reveal genes responsible for the trait in apple, we aim to map the quantitative trait loci (QTLs) for carbon isotope composition, the proxy for WUE, applying two contrasting irrigating regimes over the two-year experiment and search for the candidate genes encompassed in the mapped QTLs.ResultsWe constructed a high-density genetic linkage map with 10,172 markers of apple, using single nucleotide polymorphism (SNP) markers obtained through restriction site-associated DNA sequencing (RADseq) and a final segregating population of 350 seedlings from the cross of Honeycrisp and Qinguan. In total, 33 QTLs were identified for carbon isotope composition in apple under both well-watered and drought-stressed conditions. Three QTLs were stable over 2 years under drought stress on linkage groups LG8, LG15 and LG16, as validated by Kompetitive Allele-Specific PCR (KASP) assays. In those validated QTLs, 258 genes were screened according to their Gene Ontology functional annotations. Among them, 28 genes were identified, which exhibited significant responses to drought stress in ‘Honeycrisp’ and/or ‘Qinguan’. These genes are involved in signaling, photosynthesis, response to stresses, carbohydrate metabolism, protein metabolism and modification, hormone metabolism and transport, transport, respiration, transcriptional regulation, and development regulation. They, especially those for photoprotection and relevant signal transduction, are potential candidate genes connected with WUE regulation in drought-stressed apple.ConclusionsWe detected three stable QTLs for carbon isotope composition in apple under drought stress over 2 years, and validated them by KASP assay. Twenty-eight candidate genes encompassed in these QTLs were identified. These stable genetic loci and series of genes provided here serve as a foundation for further studies on marker-assisted selection of high WUE and regulatory mechanism of WUE in apple exposed to drought conditions, respectively.

Carbon Isotope Ratio Fractionation among Plant Tissues of Soybean

Carbon isotope ratio (δ13C) has been used as a proxy for water use efficiency (WUE) in several crops. We compared δ13C of soybean [Glycine max (L.) Merr.] tissues sampled at bloom with δ13C of mature seed (MS). Twenty Maturity Group IV accessions with contrasting δ13C values were evaluated in three environments. Genotype and environment effects on δ13C were significant for the uppermost mature trifoliolate leaf, center leaflets from main stems, whole shoot, and MS, but there were no significant genotype by environment interactions, indicating that genotype rankings were similar among environments. Repeatability of δ13C among tissues ranged from 0.79 to 0.94, and MS δ13C values were significantly correlated with δ13C from other tissues (r ≥ 0.69). For genotypes of similar maturity, MS δ13C may be used as a selection tool for WUE, simplifying the use of δ13C for high‐throughput phenotyping.

Combining carbon-13 and oxygen-18 to unravel triticale grain yield and physiological response to water stress

Water availability in semi-arid regions is increasingly becoming threatened by erratic rains and frequent droughts leading to over-reliance on irrigation to meet food demand. Improving crop water use efficiency (WUE) has become a priority but direct measurements remain a challenge. There is therefore a need to identify reliable proxies and screening traits for WUE. Carbon isotope discrimination (Δ13C) offers potential as a proxy for WUE, but its application is hindered by environmental factors and thus varies greatly among different studies. A two-year study was carried out with four moisture levels, ranging from well-watered (430–450mm) to severe stress (SS) (220–250mm), combined with four commercial triticale genotypes grown under field conditions in a hot, arid, steppe climate of Limpopo in South Africa. The study tested the use of Δ13C as a proxy of intrinsic WUE and grain yield of triticale. Second, δ13C and δ18O in combination with measured gas exchanges were used to test the functionality of the dual isotope model to interpret causes of variation in carbon isotope composition. Third, grain filling carbon assimilate sources were inferred from measured flag leaf and grain Δ13C.The results showed that moisture level significantly influenced grain yield, intrinsic WUE and Δ13C in triticale. Well-watered conditions increased grain yield, which ranged from 3.5 to 0.8tha−1 and 4.9–1.8tha−1 in 2013 and 2014 respectively. Δ13C was also high under well-watered conditions and decreased with decreasing moisture level while WUEintrinsic increased with decreasing moisture level. The relationship between Δ13C and grain yield was positive (P<0.01), but only significant under water stressed conditions, indicating dependence of the relationship on moisture level. The relationship between Δ13C and WUEintrinsic did not depend on the moisture level but showed a negative relationship when data for all moisture levels was combined. δ13C showed a negative relationship with photosynthetic rate (A), while the relationship between stomatal conductance (gs) and δ18O varied with season. Hence, the dual isotope model could only predict that variation observed in Δ13C and thus intrinsic water use efficiency was due to a concomitant decrease in both A and gs when transpiration was not limited by evaporative demand. Flag leaf Δ13C measured under SS at GS71 in the 2014 growing season, was significantly higher (2.2–3.6‰) than grain Δ13C, also measured under SS, suggesting minimal contribution of flag leaf photosynthesis to grain filling. No genotypic differences were observed in Δ13C, grain yield and WUEintrinsic, indicating a probable lack of diversity in the studied genotypes.The results of this study show that carbon isotope discrimination could be useful as a predictor of triticale grain yield in drought prone areas. Δ13C also offers potential as a proxy for WUEintrinsic and breeding for lower Δ13C values could result in varieties with higher WUEintrinsic in triticale. Flag leaf photosynthesis and pre-anthesis assimilates contribute much less carbon to grain filling under water stress than previously thought. Lastly, our results show that the dual isotope model is operational, but is not all encompassing but depends on evaporative demand.

Assessing the potential for maladaptation during active management of limber pine populations: a common garden study detects genetic differentiation in response to soil moisture in the Southern Rocky Mountains

Active management is needed to sustain healthy limber pine (Pinus flexilis E. James) forests in the Southern Rocky Mountains (henceforth, Southern Rockies), as they are threatened by the interaction of the mountain pine beetle (Dendroctonus ponderosae Hopkins) epidemic, climate change, and the spread of the non-native pathogen that causes white pine blister rust disease (Cronartium ribicola A. Dietr.). Appropriate source material for restoration and proactive introduction needs to be selected, taking into account potential genetic differentiation that would hamper management success. We conducted a common garden study in a greenhouse to determine the degree of genetic differentiation among limber pine populations in the Southern Rockies. We evaluated the differential responses of populations from northern and southern portions of the Southern Rockies to different moisture regimes during early seedling growth by measuring primary needle length, stem diameter, water potential, biomass allocation between root and shoot, and carbon isotope ratios (δ13C; a proxy for water-use efficiency). There were significant (p &lt; 0.05) effects of source region for root length, stem diameter, needle length, and total dry mass, with seedlings from southern sources bigger than seedlings from northern sources. Furthermore, there was a marginally significant interaction between soil moisture regime and source region for carbon isotope ratio (p = 0.0778), suggesting possible local adaptation. These data indicate that genetic differentiation exists among populations in the Southern Rockies, potentially increasing the risk of maladaptation when moving seed far from its source for active management.

Genetic architecture of carbon isotope composition and growth in Eucalyptus across multiple environments.

In the context of climate change, the water-use efficiency (WUE) of highly productive tree varieties, such as eucalypts, has become a major issue for breeding programmes. This study set out to dissect the genetic architecture of carbon isotope composition (δ(13) C), a proxy of WUE, across several environments. A family of Eucalyptus urophylla × E. grandis was planted in three trials and phenotyped for δ(13) C and growth traits. High-resolution genetic maps enabled us to target genomic regions underlying δ(13) C quantitative trait loci (QTLs) on the E. grandis genome. Of the 15 QTLs identified for δ(13) C, nine were stable across the environments and three displayed significant QTL-by-environment interaction, suggesting medium to high genetic determinism for this trait. Only one colocalization was found between growth and δ(13) C. Gene ontology (GO) term enrichment analysis suggested candidate genes related to foliar δ(13) C, including two involved in the regulation of stomatal movements. This study provides the first report of the genetic architecture of δ(13) C and its relation to growth in Eucalyptus. The low correlations found between the two traits at phenotypic and genetic levels suggest the possibility of improving the WUE of Eucalyptus varieties without having an impact on breeding for growth.

Impact of interspecific interactions on the soil water uptake depth in a young temperate mixed species plantation

• We studied whether differences in water uptake exists among tree species in a mixed plantation. • We tested the influence of species interactions on the water extraction depth. • Complementarity for water extraction was found between functional groups. • Species interactions did not clearly influence soil water extraction depth. • Complementarity could facilitate the coexistence of tree species in mature forests. Interactions between tree species in forests can be beneficial to ecosystem functions and services related to the carbon and water cycles by improving for example transpiration and productivity. However, little is known on below- and above-ground processes leading to these positive effects. We tested whether stratification in soil water uptake depth occurred between four tree species in a 10-year-old temperate mixed species plantation during a dry summer. We selected dominant and co-dominant trees of European beech, Sessile oak, Douglas fir and Norway spruce in areas with varying species diversity, competition intensity, and where different plant functional types (broadleaf vs. conifer) were present. We applied a deuterium labelling approach that consisted of spraying labelled water to the soil surface to create a strong vertical gradient of the deuterium isotope composition in the soil water. The deuterium isotope composition of both the xylem sap and the soil water was measured before labelling, and then again three days after labelling, to estimate the soil water uptake depth using a simple modelling approach. We also sampled leaves and needles from selected trees to measure their carbon isotope composition (a proxy for water use efficiency) and total nitrogen content. At the end of the summer, we found differences in the soil water uptake depth between plant functional types but not within types: on average, coniferous species extracted water from deeper layers than did broadleaved species. Neither species diversity nor competition intensity had a detectable influence on soil water uptake depth, foliar water use efficiency or foliar nitrogen concentration in the species studied. However, when coexisting with an increasing proportion of conifers, beech extracted water from progressively deeper soil layers. We conclude that complementarity for water uptake could occur in this 10-year-old plantation because of inherent differences among functional groups (conifers and broadleaves). Furthermore, water uptake depth of beech was already influenced at this young development stage by interspecific interactions whereas no clear niche differentiation occurred for the other species. This finding does not preclude that plasticity-mediated responses to species interactions could increase as the plantation ages, leading to the coexistence of these species in adult forest stands.

Maize productivity and nutrient and water use efficiencies across soil fertility domains on smallholder farms in Zimbabwe

Strategic targeting of scarce nutrient resources by smallholder farmers on different field types has remained poor partly due to knowledge limitations, resulting in inefficient use of the resources. We sought to establish efficient strategies for use of nutrient resources so as to narrow the yield gap in maize production on heterogeneous light-textured soils under rain-fed conditions in Eastern Zimbabwe. A nutrient omission study was implemented during two cropping seasons, across six on-farm sites with soil organic carbon (SOC) ranging from 3.5 to 8.9gkg−1, and clay content between 4 and 19%. Treatments used were: (i) a zero fertilizer control, (ii) NK, (iii) NPS, (iv) PKS, and (v) NPKS. Rainfall water productivity, RWP, (kggrainmm−1) was used as a proxy for water use efficiency for the different nutrient combinations. During both seasons, only 70kgha−1 N could be applied across all sites as prolonged mid-season droughts forced withholding of the second N topdressing targeted at maize anthesis. Maize productivity was influenced by both nutrient management and initial soil fertility. During the first season, maize yields across sites ranged from 0.25 to 0.84tha−1 for the control and 2.05–3.75tha−1 for the NPKS treatment that represented attainable yields. The corresponding RWP were 0.38–1.13kggrainmm−1 for the control and 3.15–7.66kggrainmm−1 for the NPKS treatment. For the second season, maize yields for the control ranged from 0.2 to 1.2tha−1, while those for the NPKS treatments ranged from 2.4 to 3.60tha−1. Across sites, response to N ranged 1.2–2.35tha−1, response to P ranged 0.71–2.10tha−1 and response to K ranged 0.08–0.30tha−1, indicating little response to K. Overall, balanced nutrient management has an overriding effect on maize grain and water productivity, but only for soils with SOC>4gkg−1 soil. Nitrogen and P remain the most limiting nutrients, in contrast, addition of K did not enhance grain yield, nor did it influence response to N or P. Variable N application strategies must be an integral component of farmer management if losses related to fertilizer investment are to be minimised under the risky rain-fed crop production systems.

Plasticity in bundle sheath extensions of heterobaric leaves

Leaf venation is linked to physiological performance, playing a critical role in ecosystem function. Despite the importance of leaf venation, associated bundle sheath extensions (BSEs) remain largely unstudied. Here, we quantify plasticity in the spacing of BSEs over irradiance and precipitation gradients. Because physiological function(s) of BSEs remain uncertain, we additionally explored a link between BSEs and water use efficiency (WUE). We sampled leaves of heterobaric trees along intracrown irradiance gradients in natural environments and growth chambers and correlated BSE spacing to incident irradiance. Additionally, we sampled leaves along a precipitation gradient and correlated BSE spacing to precipitation and bulk δ(13)C, a proxy for intrinsic WUE. BSE spacing was quantified using a novel semiautomatic method on fresh leaf tissue. With increased irradiance or decreased precipitation, Liquidambar styraciflua decreased BSE spacing, while Acer saccharum showed little variation in BSE spacing. Two additional species, Quercus robur and Platanus occidentalis, decreased BSE spacing with increased irradiance in growth chambers. BSE spacing correlated with bulk δ(13)C, a proxy for WUE in L. styraciflua, Q. robur, and P. occidentalis leaves but not in leaves of A. saccharum. We demonstrated that BSE spacing is plastic with respect to irradiance or precipitation and independent from veins, indicating BSE involvement in leaf adaptation to a microenvironment. Plasticity in BSE spacing was correlated with WUE only in some species, not supporting a function in water relations. We discuss a possible link between BSE plasticity and life history, particularly canopy position.

Variation in functional leaf traits among beech provenances during a Spanish summer reflects the differences in their origin

We assessed the response of 11-year-old saplings from six beech provenances growing in a common-garden trial at the southwestern range limit. Provenances from distinct climatic regions across the European beech distribution were selected. The local Spanish provenance appeared well suited to the site conditions, maintaining high rates of assimilation even in midsummer, but so did the provenance of southern continental origin, from Gotze-Delchev, Bulgaria. Those provenances from cooler sites in central Europe, a continental mountain climate in the Czech Republic and a continental range-edge site in eastern Poland, along with a German provenance of mild maritime origin, had good physiological functionality in early summer but reduced carbon assimilation (Aarea) and apparent soil–leaf hydraulic conductivity (KL) in midsummer. The northern maritime provenance from Sweden demonstrated severely-reduced photosynthetic capacity. These groupings of provenances according to their photosynthetic performance, stable carbon isotope composition (δ13C; a proxy for water-use efficiency) and leaf water potential under marginal conditions, during late summer in the trial, suggest that they have divergent strategies for water use. The research highlights large intraspecific differences among beech provenances of distinct origin and strategies which are expected to modify their response to drought, requiring future genetic studies to explicitly determine the basis of this ecophysiological differentiation.