Leaf-level Water-use Efficiency Research Articles

Benchmarking Water-Use Efficiency for Wheat at Leaf and Ecosystem Scales

The processes coupled with carbon and water exchange are linked to crop assimilation, water consumption, controlling crop growth and development, and ultimately determining crop yield. Therefore, studying the characteristics of crop water constraints and their controlling factors at multiple scales is of great significance for regional and global food production stability and food security. Employing field observations and a comprehensive literature review, this study investigates the maximum water-use efficiency of wheat and its governing factors at both leaf and canopy (ecosystem) scales. The results demonstrate remarkable consistency and well-defined boundaries in maximum water-use efficiency across diverse climate regions and wheat varieties, both at the leaf and agricultural ecosystem scales. At the leaf scale, the maximum water-use efficiency of wheat was 4.5 μg C mg−1 H2O, while for wheat agricultural ecosystems, on a daily scale, the maximum water-use efficiency was 4.5 g C kg−1 H2O. Meanwhile, the maximum water-use efficiency of wheat agricultural ecosystems decreased continuously with increasing time scales, with values of 6.5, 4.5, 3.5, and 2 g C kg−1 H2O for instantaneous, daily, weekly, and monthly scales, respectively. Environmental factors, primarily vapor pressure deficit, light, and soil water content, exert significant control over leaf-level water-use efficiency. Similarly, the maximum water-use efficiency of agricultural ecosystems fluctuates in response to daily variations in meteorological elements. C3 crops like wheat exhibit remarkable resilience in their carbon–water exchange patterns across diverse environmental conditions. The findings in the current research can serve as a reference for improving crop water-use efficiency.

Climate gradient and leaf carbon investment influence the effects of climate change on water use efficiency of forests: A meta-analysis.

Forest ecosystems cover a large area of the global land surface and are important carbon sinks. The water-carbon cycles of forests are prone to climate change, but uncertainties remain regarding the magnitude of water use efficiency (WUE) response to climate change and the underpinning mechanism driving WUE variation. We conducted a meta-analysis of the effects of elevated CO2 concentration (eCO2 ), drought and elevated temperature (eT) on the leaf- to plant-level WUE, covering 80 field studies and 95 tree species. The results showed that eCO2 increased leaf intrinsic and instantaneous WUE (WUEi, WUEt), whereas drought enhanced both leaf- and plant-level WUEs. eT increased WUEi but decreased carbon isotope-based WUE, possibly due to the influence of mesophyll conductance. Stimulated leaf-level WUE by drought showed a progressing trend with increasing latitude, while eCO2 -induced WUE enhancement showed decreasing trends after >40° N. These latitudinal gradients might influence the spatial pattern of climate and further drove WUE variation. Moreover, high leaf-level WUE under eCO2 and drought was accompanied by low leaf carbon contents. Such a trade-off between growth efficiency and defence suggests a potentially compromised tolerance to diseases and pests. These findings add important ecophysiological parameters into climate models to predict carbon-water cycles of forests.

The effect of the vertical gradients of photosynthetic parameters on the CO2 assimilation and transpiration of a Panamanian tropical forest.

Terrestrial biosphere models (TBMs) include the representation of vertical gradients in leaf traits associated with modeling photosynthesis, respiration, and stomatal conductance. However, model assumptions associated with these gradients have not been tested in complex tropical forest canopies. We compared TBM representation of the vertical gradients of key leaf traits with measurements made in a tropical forest in Panama and then quantified the impact of the observed gradients on simulated canopy-scale CO2 and water fluxes. Comparison between observed and TBM trait gradients showed divergence that impacted canopy-scale simulations of water vapor and CO2 exchange. Notably, the ratio between the dark respiration rate and the maximum carboxylation rate was lower near the ground than at the top-of-canopy, leaf-level water-use efficiency was markedly higher at the top-of-canopy, and the decrease in maximum carboxylation rate from the top-of-canopy to the ground was less than TBM assumptions. The representation of the gradients of leaf traits in TBMs is typically derived from measurements made within-individual plants, or, for some traits, assumed constant due to a lack of experimental data. Our work shows that these assumptions are not representative of the trait gradients observed in species-rich, complex tropical forests.

Addressing the Relationship between Leaf Nitrogen and Carbon Isotope Discrimination from the Three Levels of Community, Population and Individual.

The carbon, nitrogen and water cycles of terrestrial ecosystems are important biogeochemical cycles. Addressing the relationship of leaf nitrogen (N) and carbon isotope discrimination (Δ) will enhance the understanding of the links between these three cycles in plant leaves because Δ can reflect time-integrated leaf-level water-use efficiency (WUE) over the period when the leaf material is produced. Previous studies have paid considerable attention to the relationship. However, these studies have not effectively eliminated the interference of environmental factors, inter-species, and inter-individual differences in this relationship, so new research is necessary. To minimize these interferences, the present work explored the relationship at the three levels of community, population, and plant individual. Three patterns of positive, negative and no relationship were observed across communities, populations, and individuals, which is dependent on environmental conditions, species, and plant individuals. The results strongly suggested that there is no general pattern for the relationship between leaf N and Δ. Furthermore, the results indicated that there is often no coupling between leaf-level long-term WUE and leaf N in the metabolic process of carbon, N and water in leaves. The main reason for the lack of this relationship is that most plants do not invest large amounts of nitrogen into photosynthesis. In addition, the present study also observed that, for most plant species, leaf N was not related to photosynthetic rate, and that variations in photosynthetic rates are mainly driven by stomatal conductance.

Response of water use efficiency and plant-soil C:N:P stoichiometry to stand quality in Robinia pseudoacacia on the Loess Plateau of China

The evaluation of stand quality is of great significance for forest management and sustainable development. Water use efficiency (WUE) and the carbon (C), nitrogen (N), and phosphorus (P) stoichiometric characteristics of plants and soil are important indexes used to evaluate plant water and nutrients adaptation strategies. However, little is known about the effects of stand quality on WUE and plant‑soil C:N:P stoichiometry. In this study, the growth characteristics of Robinia pseudoacacia plantations and understory plants in an age series of 8-, 15-, 25-, and 35-year-old stands were investigated, and the leaf-level WUE and C:N:P stoichiometry in plants and soil were measured on the Loess Plateau. The results showed that the species richness, Shannon-Weiner diversity index, and evenness index reached their peaks in the 35-year-old stands with increasing afforestation age. WUE reached its highest value in the 35-year-old stands but was only significantly higher than that in the 8-year-old stands. Stand age had different effects on C, N, and P nutrients and stoichiometry in leaves, fine roots, and soil. The most obvious trend was that soil total P decreased significantly with increasing afforestation age, and the leaf N:P ratio was greater than 16 in the four stand ages, which indicated that P was the main limiting factor in R. pseudoacacia plantations. Leaf nutrients and stoichiometry are closely related to forest growth characteristics, while root nutrients and stoichiometry are more related to understory plant composition and diversity. Importantly, the WUE had no significant change with the increase in the stand quality index, and the response of soil stoichiometry to stand quality was stronger than that of plant stoichiometry. These results help us further understand coordinated plant-soil restoration after afforestation.

Water uptake depth is coordinated with leaf water potential, water-use efficiency and drought vulnerability in karst vegetation.

Root access to bedrock water storage or groundwater is an important trait allowing plant survival in seasonally dry environments. However, the degree of coordination between water uptake depth, leaf-level water-use efficiency (WUEi) and water potential in drought-prone plant communities is not well understood. We conducted a 135-d rainfall exclusion experiment in a subtropical karst ecosystem with thin skeletal soils to evaluate the responses of 11 co-occurring woody species of contrasting life forms and leaf habits to a severe drought during the wet growing season. Marked differences in xylem water isotopic composition during drought revealed distinct ecohydrological niche separation among species. The contrasting behaviour of leaf water potential in coexisting species during drought was largely explained by differences in root access to deeper, temporally stable water sources. Smaller-diameter species with shallower water uptake, more negative water potentials and lower WUEi showed extensive drought-induced canopy defoliation and/or mortality. By contrast, larger-diameter species with deeper water uptake, higher leaf-level WUEi and more isohydric behaviour survived drought with only moderate canopy defoliation. Severe water limitation imposes strong environmental filtering and/or selective pressures resulting in tight coordination between tree diameter, water uptake depth, iso/anisohydric behaviour, WUEi and drought vulnerability in karst plant communities.

C4 grasses adapted to low precipitation habitats show traits related to greater mesophyll conductance and lower leaf hydraulic conductance.

In habitats with low water availability, a fundamental challenge for plants will be to maximize photosynthetic C-gain while minimizing transpirational water-loss. This trade-off between C-gain and water-loss can in part be achieved through the coordination of leaf-level photosynthetic and hydraulic traits. To test the relationship of photosynthetic C-gain and transpirational water-loss, we grew, under common growth conditions, 18 C4 grasses adapted to habitats with different mean annual precipitation (MAP) and measured leaf-level structural and anatomical traits associated with mesophyll conductance (gm ) and leaf hydraulic conductance (Kleaf ). The C4 grasses adapted to lower MAP showed greater mesophyll surface area exposed to intercellular air spaces (Smes ) and adaxial stomatal density (SDada ) which supported greater gm . These grasses also showed greater leaf thickness and vein-to-epidermis distance, which may lead to lower Kleaf . Additionally, grasses with greater gm and lower Kleaf also showed greater photosynthetic rates (Anet ) and leaf-level water-use efficiency (WUE). In summary, we identify a suite of leaf-level traits that appear important for adaptation of C4 grasses to habitats with low MAP and may be useful to identify C4 species showing greater Anet and WUE in drier conditions.

Strong photosynthetic acclimation and enhanced water‐use efficiency in grassland functional groups persist over 21 years of CO2 enrichment, independent of nitrogen supply

Uncertainty about long-term leaf-level responses to atmospheric CO2 rise is a major knowledge gap that exists because of limited empirical data. Thus, it remains unclear how responses of leaf gas exchange to elevated CO2 (eCO2 ) vary among plant species and functional groups, or across different levels of nutrient supply, and whether they persist over time for long-lived perennials. Here, we report the effects of eCO2 on rates of net photosynthesis and stomatal conductance in 14 perennial grassland species from four functional groups over two decades in a Minnesota Free-Air CO2 Enrichment experiment, BioCON. Monocultures of species belonging to C3 grasses, C4 grasses, forbs, and legumes were exposed to two levels of CO2 and nitrogen supply in factorial combinations over 21years. eCO2 increased photosynthesis by 12.9% on average in C3 species, substantially less than model predictions of instantaneous responses based on physiological theory and results of other studies, even those spanning multiple years. Acclimation of photosynthesis to eCO2 was observed beginning in the first year and did not strengthen through time. Yet, contrary to expectations, the response of photosynthesis to eCO2 was not enhanced by increased nitrogen supply. Differences in responses among herbaceous plant functional groups were modest, with legumes responding the most and C4 grasses the least as expected, but did not further diverge over time. Leaf-level water-use efficiency increased by 50% under eCO2 primarily because of reduced stomatal conductance. Our results imply that enhanced nitrogen supply will not necessarily diminish photosynthetic acclimation to eCO2 in nitrogen-limited systems, and that significant and consistent declines in stomatal conductance and increases in water-use efficiency under eCO2 may allow plants to better withstand drought.

Wand plant architecture in the Fynbos: Testing the rodent herbivory hypothesis

Throughout the Cape Floristic Region, in a range of local environments, can be found a distinctive growth form: “wand” plants. This curious plant architecture comprises perennial plants which have slender (wand-like) stems that extend high above the matrix vegetation. We explore whether the evolution of wand-plants may have been driven by plant–herbivore interactions with rodents, where such architecture reduces access to nutrient rich flowers and seeds. To test this idea, we determined if (i) wand-plants were more flexible than non-wand congeners, and (ii) a stabilised wand plant was favoured for climbing (by laboratory mice) over a free-standing wand plant in a laboratory setting. Under a phylogenetic independent contrast framework, wand-plants were not more flexible (across a range of diameters) than non-wand congeners. Also, laboratory mice showed no differences in preference for, or success on, free-standing and stabilised wand-plants. We discuss a range of alternative ideas, including that the architecture may be a coevolutionary relic, to increase pollination visitation, to increase leaf-level water-use efficiency or to maintain cooler leaf temperatures in warm and/or windless environments. However, we suggest that our study may be flawed as it did not include the “landscape of fear” — i.e. the predation risk associated with exposure along a bare stem — and that rodent herbivory may nonetheless be the dominant evolutionary driver. Although we do not find any support for the hypothesis of an arms race between wand plants and rodent herbivores, this fascinating architecture remains unexplained and we hope that our study will stimulate further exploration of the topic.

Additive effects of high growth rate and low transpiration rate drive differences in whole plant transpiration efficiency among black poplar genotypes

Poplar plantations, widely used for the production of woody biomass, might be at high risk from the climate change-induced increase in the frequency of drought periods. Therefore, selecting improved genotypes, which are highly productive but with a high water use efficiency (WUE), is becoming a major target. The use of automated weighing systems in controlled environments facilitates the estimation of cumulated water loss and whole plant transpiration efficiency (TE). Differences in TE and leaf level intrinsic WUE as well as the contribution of underlying ecophysiological traits were determined in three contrasting P. nigra genotypes. Strong differences in TE among the selected genotypes were congruent with differences in leaf level intrinsic WUE. Our data show that a high total leaf area was overcompensated by a low per leaf area transpiration rate, leading to higher TE in highly productive genotypes originating from cool locations. Nocturnal water loss was relatively low but contributed to variations in TE among genotypes. In response to drought, leaf level WUE increased but not TE, suggesting that carbon losses due to whole plant respiration could offset the drought-induced increase in intrinsic WUE.

Early life history responses and phenotypic shifts in a rare endemic plant responding to climate change.

Changes in species ranges are anticipated with climate change, where in alpine settings, fragmentation and contraction are likely. This is especially true in high altitude biodiversity hotspots, where warmer growing seasons and increased drought events may negatively impact populations by limiting regeneration. Here, we test for high-altitude species responses to the interactive effects of warming and drought in Heterotheca brandegeei, a perennial cushion plant endemic to alpine outcroppings in Sierra de San Pedro Mártir National Park, Baja California, México. We exposed H. brandegeei seedlings to experimental warming and drought conditions to document early life history responses and the species ability to tolerate climate change. Drought negatively influenced seedling growth, with overall reductions in above- and belowground biomass. Warming and drought each led to substantial reductions in leaf development. At the same time, individuals maintained high specific leaf area and carbon investment in leaves across treatments, suggesting that existing phenotypic variation within populations may be high enough to withstand climate change. However, warming and drought interacted to negatively influence leaf-level water-use efficiency (WUE). Seedling mortality rates were nearly three times higher in warming and drought treatments, suggesting bleak prospects for H. brandegeei populations in future climate conditions. Overall, our results suggest H. brandegeei populations may experience substantial declines under future warmer and drier conditions. Some individuals may be able to establish, albeit, as smaller, more stressed plants. These results further suggest that warming alone may not be as consequential to populations as drought will be in this already water-limited system.

Effects of extreme changes in precipitation on the physiology of C4 grasses.

Climatic patterns are expected to become more extreme, with changes in precipitation characterized by heavier rainfall and prolonged dry periods. Yet, most studies focus on persistent moderate changes in precipitation, limiting our understanding of how ecosystems will function in the future. We examined the effects of extreme changes in precipitation on leaf-level and ecosystem CO2 and H2O exchange of three native C4 bunchgrasses (Andropogon gerardii, Panicum virgatum, and Sorghastrum nutans) over 3years. Grasses were grown in three precipitation treatments: extreme dry, mean, and extreme wet based on historical rainfall records. After 3years, plants were 45% smaller in the extreme dry treatment relative to the mean and extreme high treatment, which did not differ. We also found that an extreme decrease in precipitation caused reductions of 55, 40, and 40% in leaf-level photosynthesis (Anet), stomatal conductance (gs), and water use efficiency (WUE), respectively. Extreme increases in precipitation inhibited leaf-level WUE, with a 44% reduction relative to the mean treatment. At the ecosystem level, both an extreme increase and decrease in precipitation reduced net CO2 and water fluxes relative to plants grown with mean levels of precipitation. Net water fluxes (ET) were reduced by an average of 74% in the extreme dry and extreme wet treatment relative to mean treatment; net carbon fluxes followed a similar trend, with average reductions of 68% (NEE) and 100% (Re). Unlike moderate climate change, extreme increases in precipitation may be just as detrimental as extreme decreases in precipitation in shifting grassland physiology.

Dissimilar stem and leaf hydraulic traits suggest varying drought tolerance among co-occurring Eucalyptus grandis × E. urophylla clones

In South Africa, there is currently a phenomenon where a Eucalyptus grandis × E. urophylla clone is dying on a large scale, whereas co-occurring clones in the same plantation area remain healthy. No primary biotic pathogen has been isolated from these dying trees. However, the plantation region has endured severe drought conditions. We hypothesised that the specific E. grandis × E. urophylla clone dieback is due in part to the affected trees having an elevated drought vulnerability. We tested this hypothesis retrospectively by examining how the dying (branch dieback started) as compared with two healthy and co-occurring E. grandis × E. urophylla clones vary in branch xylem anatomy (water transport) and leaf stable carbon isotopes (gas exchange) across two levels of mean annual precipitation in the planted landscape. There was significant intra-hybrid variation in the range of studied hydraulic traits. It appeared that the dying clone is maladapted to drier field conditions relative to the two healthy growing clones. Individuals of the drought-susceptible clone had increased hydraulic conductivity, having both wider vessel lumens and larger lumen fractions, and also did not regulate leaf-level transpiration in the drier landscape as efficiently as the two unaffected clones (lower δ13C ratios). In turn, one of the healthy growing clones had a completely different hydraulic strategy, having higher wood density, lower lumen fractions, and higher δ13C ratios that are widely considered as hydraulically resilient during water stress. The other clone appeared to also gain drought tolerance via higher leaf-level water-use efficiency, although drought tolerance was less clear regarding xylem anatomy. We highlight how variation in relative drought vulnerability can be highly significant even between clones within a single hybrid group. Knowledge of this variation in plant hydraulics between commercially planted and closely related Eucalyptus trees would complement clone deployment programmes in a target landscape.

The relationship between cotton canopy temperature and yield, fibre quality and water-use efficiency

Crop canopy temperature (Tc) is associated with transpiration and Tc has been used in crop water stress detection. This study investigates the effect of surface drip and furrow irrigation regimes on cotton Tc. It outlines the relationship between Tc and lint yield, fibre quality and both agronomic (WUEa, kgha−1mm−1 total applied water) and leaf level water-use efficiency (WUEl, μmol[CO2] mmol[H2O]−1) in a high input, high yielding (>1800kgha−1) cotton system. Canopy temperature between flowering and crop maturity was monitored. Yield reductions occurred when Tc exceeded 28°C. Reductions in fibre length outside the ideal range (>28.6mm) occurred when Tc exceeded 31°C, while desirable micronaire (3.8–4.5) was observed at Tc between 25 and 32°C. Desirable fibre quality and peak lint yield can be realised if an irrigation scheduling protocol maintains average canopy temperatures below 28°C. However, maximum WUEa was observed at a higher average Tc (30°C) than peak lint yield (28°C), which would correspond to a predicted 23% reduction in lint yield from the peak (3030kgha−1). Therefore, the trade-off between peak yield and WUEa needs to be considered in conjunction with irrigation water costs and availability when scheduling irrigations based on canopy temperature.

Competition for Water and Light in Closed-Canopy Forests: A Tractable Model of Carbon Allocation with Implications for Carbon Sinks

Abstract The dependence of forest productivity and community composition on rainfall is the result of complex interactions at multiple scales, from the physiology of carbon gain and water loss to competition among individuals and species. In an effort to understand the role of these multiscale interactions in the dependence of forest structure on rainfall, we build a tractable model of individual plant competition for water and light. With game-theoretic analyses, we predict the dominant plant allocation strategy, forest productivity, and carbon storage. We find that the amount and timing of rainfall are critical to forest structure. Comparing two forests that differ only in the total time plants spend in water saturation, the model predicts that the wetter forest has fewer fine roots, more leaves, and more woody biomass than the drier forest. In contrast, if two forests differ only in the amount of water available during water limitation, the model predicts that the wetter forest has more fine roots than the drier forest and equivalent leaves and woody biomass. The difference in these responses to increases in water availability has significant implications for potential carbon sinks with rising atmospheric CO2. We predict that enhanced productivity from increased leaf-level water-use efficiency during water limitation will be allocated to fine roots if plants respond competitively, producing only a small and short-lived carbon sink.

Genetic Variation in Ecophysiological and Survival Responses to Drought in Two Native Grasses:Koeleria macranthaandElymus elymoides

ABSTRACT. Genetic variation in ecophysiological and survival responses to drought was studied in 2 northern Arizona native grass species, Koeleria macrantha (Ledeb.) Schult, (prairie Junegrass) and Elymus elymoides (Raf.) Swezey. ssp. elymoides (squirreltail). Low- and high-elevation populations of each species were compared in a greenhouse common garden experiment that included simulated drought. Leaf gas-exchange characteristics were significantly affected by simulated drought and often by population elevation, but gas-exchange responses to drought were similar for high- and low-elevation populations. Compared to high-elevation populations, low-elevation populations of both species had higher net photosynthetic rate and predawn water potential, and for E. elymoides had higher stomatal conductance. Leaf-level water-use efficiency did not differ between populations for either species. Populations also differed significantly in leaf morphological characteristics related to water use. Compared to high-eleva...

Enhanced transpiration rate in the high pigment 1 tomato mutant and its physiological significance

Tomato high pigment (hp) mutants represent an interesting horticultural resource due to their enhanced accumulation of carotenoids, flavonoids and vitamin C. Since hp mutants are known for their exaggerated light responses, the molecules accumulated are likely to be antioxidants, recruited to deal with light and others stresses. Further phenotypes displayed by hp mutations are reduced growth and an apparent disturbance in water loss. Here, we examined the impact of the hp1 mutation and its near isogenic line cv Micro-Tom (MT) on stomatal conductance (gs), transpiration (E), CO(2) assimilation (A) and water use efficiency (WUE). Detached hp1 leaves lost water more rapidly than control leaves, but this behaviour was reversed by exogenous abscisic acid (ABA), indicating the ability of hp1 to respond to this hormone. Although attached hp1 leaves had enhanced gs, E and A compared to control leaves, genotypic differences were lost when water was withheld. Both instantaneous leaf-level WUE and long-term whole plant WUE did not differ between hp1 and MT. Our results indicate a link between exaggerated light response and water loss in hp1, which has important implications for the use of this mutant in both basic and horticultural research.

Global patterns in leaf 13 C discrimination and implications for studies of past and future climate

Fractionation of carbon isotopes by plants during CO(2) uptake and fixation (Delta(leaf)) varies with environmental conditions, but quantitative patterns of Delta(leaf) across environmental gradients at the global scale are lacking. This impedes interpretation of variability in ancient terrestrial organic matter, which encodes climatic and ecological signals. To address this problem, we converted 3,310 published leaf delta(13)C values into mean Delta(leaf) values for 334 woody plant species at 105 locations (yielding 570 species-site combinations) representing a wide range of environmental conditions. Our analyses reveal a strong positive correlation between Delta(leaf) and mean annual precipitation (MAP; R(2) = 0.55), mirroring global trends in gross primary production and indicating stomatal constraints on leaf gas-exchange, mediated by water supply, are the dominant control of Delta(leaf) at large spatial scales. Independent of MAP, we show a lesser, negative effect of altitude on Delta(leaf) and minor effects of temperature and latitude. After accounting for these factors, mean Delta(leaf) of evergreen gymnosperms is lower (by 1-2.7 per thousand) than for other woody plant functional types (PFT), likely due to greater leaf-level water-use efficiency. Together, environmental and PFT effects contribute to differences in mean Delta(leaf) of up to 6 per thousand between biomes. Coupling geologic indicators of ancient precipitation and PFT (or biome) with modern Delta(leaf) patterns has potential to yield more robust reconstructions of atmospheric delta(13)C values, leading to better constraints on past greenhouse-gas perturbations. Accordingly, we estimate a 4.6 per thousand decline in the delta(13)C of atmospheric CO(2) at the onset of the Paleocene-Eocene Thermal Maximum, an abrupt global warming event approximately 55.8 Ma.

Effects of tree height on branch hydraulics, leaf structure and gas exchange in California redwoods

We examined changes in branch hydraulic, leaf structure and gas exchange properties in coast redwood (Sequoia sempervirens) and giant sequoia (Sequoiadendron giganteum) trees of different sizes. Leaf-specific hydraulic conductivity (k(L)) increased with height in S. sempervirens but not in S. giganteum, while xylem cavitation resistance increased with height in both species. Despite hydraulic adjustments, leaf mass per unit area (LMA) and leaf carbon isotope ratios (delta(13)C) increased, and maximum mass-based stomatal conductance (g(mass)) and photosynthesis (A(mass)) decreased with height in both species. As a result, both A(mass) and g(mass) were negatively correlated with branch hydraulic properties in S. sempervirens and uncorrelated in S. giganteum. In addition, A(mass) and g(mass) were negatively correlated with LMA in both species, which we attributed to the effects of decreasing leaf internal CO(2) conductance (g(i)). Species-level differences in wood density, LMA and area-based gas exchange capacity constrained other structural and physiological properties, with S. sempervirens exhibiting increased branch water transport efficiency and S. giganteum exhibiting increased leaf-level water-use efficiency with increasing height. Our results reveal different adaptive strategies for the two redwoods that help them compensate for constraints associated with growing taller, and reflect contrasting environmental conditions each species faces in its native habitat.

Leaf-level water use efficiency determined by carbon isotope discrimination in rice seedlings: genetic variation associated with population structure and QTL mapping

Increasing the water use efficiency (WUE) of our major crop species is an important target of agricultural research. Rice is a major water consumer in agriculture and it is also an attractive genetic model. We evaluated leaf-level WUE in young rice seedlings using carbon isotope discrimination (Delta(13)C) as an indicator of the trait. A survey of Delta(13)C was undertaken in 116 diverse germplasm accessions representing O. sativa, O. glaberrima and four wild Oryza species. O. sativa cultivars were classified into sub-populations based on SSR markers, and significant differences in Delta(13)C were observed among the five genetically defined groups. While individual accessions explained a greater proportion of the variation than did sub-population, indica rice varieties had the lowest Delta(13)C values overall, indicating superior WUE, while temperate japonica had the highest Delta(13)C. O sativa accessions had a similar or greater range of Delta(13)C values than wild Oryza species, while domesticated O. glaberrima had a narrower range. Correlation analysis identified leaf morphological and physiological traits that were significantly associated with Delta(13)C, including longer leaves, more drooping leaves, higher tillering ability, and lower leaf nitrogen content. These trait associations were investigated by quantitative trait locus (QTL) mapping using backcross inbred lines derived from a cross between Nipponbare (temperate japonica) and Kasalath (aus). Seven QTL for Delta(13)C were identified using composite interval analysis, located in five chromosomal regions. The QTL with the largest additive effect came from Kasalath and co-localized with QTL for leaf length, tiller number and nitrogen content.