Using robust, pairwise comparisons and a global dataset, we show that nitrogen concentration per unit leaf mass for nitrogen-fixing plants (N₂FP; mainly legumes plus some actinorhizal species) in nonagricultural ecosystems is universally greater (43–100%) than that for other plants (OP). This difference is maintained across Koppen climate zones and growth forms and strongest in the wet tropics and within deciduous angiosperms. N₂FP mostly show a similar advantage over OP in nitrogen per leaf area (N ), even in arid climates, despite diazotrophy being sensitive to drought. We also show that, for most N₂FP, carbon fixation by photosynthesis (A ) and stomatal conductance (g ) are not related to N —in distinct challenge to current theories that place the leaf nitrogen–A relationship at the center of explanations of plant fitness and competitive ability. Among N₂FP, only forbs displayed an N –g relationship similar to that for OP, whereas intrinsic water use efficiency (WUE ; A /g ) was positively related to N for woody N₂FP. Enhanced foliar nitrogen (relative to OP) contributes strongly to other evolutionarily advantageous attributes of legumes, such as seed nitrogen and herbivore defense. These alternate explanations of clear differences in leaf N between N₂FP and OP have significant implications (e.g., for global models of carbon fluxes based on relationships between leaf N and A ). Combined, greater WUE and leaf nitrogen—in a variety of forms—enhance fitness and survival of genomes of N₂FP, particularly in arid and semiarid climates.
A widespread perception is that, with increasing wind speed, transpiration from plant leaves increases. However, evidence suggests that increasing wind speed enhances carbon dioxide (CO 2 ) uptake while reducing transpiration because of more efficient convective cooling (under high solar radiation loads). We provide theoretical and experimental evidence that leaf water use efficiency (WUE, carbon uptake per water transpired) commonly increases with increasing wind speed, thus improving plants' ability to conserve water during photosynthesis. Our leaf‐scale analysis suggests that the observed global decrease in near‐surface wind speeds could have reduced WUE at a magnitude similar to the increase in WUE attributed to global rise in atmospheric CO 2 concentrations. However, there is indication that the effect of long‐term trends in wind speed on leaf gas exchange may be compensated for by the concurrent reduction in mean leaf sizes. These unintuitive feedbacks between wind, leaf size and water use efficiency call for re‐evaluation of the role of wind in plant water relations and potential re‐interpretation of temporal and geographic trends in leaf sizes. The study establishes theoretically and experimentally that, contrary to common expectations, wind increases leaf water use efficiency (WUE) under a wide range of environmental conditions. The increase reflects improved cooling efficiency by sensible heat and improved CO 2 uptake (both controlled by boundary layer conductance). Results suggest that the potential influence of global stilling on WUE is of similar magnitude but acts in opposite direction to the effect of rising atmospheric CO 2 concentrations. Additionally, reported decreases in leaf sizes over time are consistent with plant compensation for decreasing trends in wind speed that highlight the ecological significance of global stilling for leaf gas exchange. The results suggest that wind speed affects leaf WUE at the short time‐scale and leaf sizes at the long time‐scale.
Terrestrial plants remove CO2 from the atmosphere through photosynthesis, a process that is accompanied by the loss of water vapour from leaves(1). The ratio of water loss to carbon gain, or water-use efficiency, is a key characteristic of ecosystem function that is central to the global cycles of water, energy and carbon(2). Here we analyse direct, long-term measurements of whole-ecosystem carbon and water exchange(3). We find a substantial increase in water-use efficiency in temperate and boreal forests of the Northern Hemisphere over the past two decades. We systematically assess various competing hypotheses to explain this trend, and find that the observed increase is most consistent with a strong CO2 fertilization effect. The results suggest a partial closure of stomata(1)-small pores on the leaf surface that regulate gas exchange-to maintain a near-constant concentration of CO2 inside the leaf even under continually increasing atmospheric CO2 levels. The observed increase in forest water-use efficiency is larger than that predicted by existing theory and 13 terrestrial biosphere models. The increase is associated with trends of increasing ecosystem-level photosynthesis and net carbon uptake, and decreasing evapotranspiration. Our findings suggest a shift in the carbon-and water-based economics of terrestrial vegetation, which may require a reassessment of the role of stomatal control in regulating interactions between forests and climate change, and a re-evaluation of coupled vegetation-climate models.
Predictions of climate change indicate an increase in water scarcity in Mediterranean areas. Therefore, improving water use efficiency ( ) becomes crucial for sustainable viticulture in the Mediterranean for both grapevine growth and fruit productivity. Variability of between cultivars presents an opportunity to select the most appropriate cultivars in viticultural areas with increasing aridity. In this review, an update on the variability of in different grapevine cultivars and environmental conditions is presented. Most studies on are focused at the leaf level and frequently used to estimate whole-plant . However, there are large discrepancies when scaling-up from leaf to whole-plant level. There are several structural and physiological processes, not included in leaf measurements, considered as possible factors to solve the gap between leaf and whole-plant . Canopy structure and plant respiration are described as the most important components involved in whole-plant regulation, and proposed as potential targets for its improvement.
We revisit the relationship between plant water use efficiency and carbon isotope signatures (δ¹³C) of plant material. Based on the definitions of intrinsic, instantaneous and integrated water use efficiency, we discuss the implications for interpreting δ¹³C data from leaf to landscape levels, and across diurnal to decadal timescales. Previous studies have often applied a simplified, linear relationship between δ¹³C, ratios of intercellular to ambient CO₂ mole fraction , and water use efficiency. In contrast, photosynthetic ¹³C discrimination (Δ) is sensitive to the ratio of the chloroplast to ambient CO₂ mole fraction, (rather than ) and, consequently, to mesophyll conductance. Because mesophyll conductance may differ between species and over time, it is not possible to determine from the same gas exchange measurements as . On the other hand, water use efficiency at the leaf level depends on evaporative demand, which does not directly affect Δ. Water use efficiency and Δ can thus vary independently, making it difficult to obtain trends in water use efficiency from δ¹³C data. As an alternative approach, we offer a model available at http://carbonisotopes. googlepages.com to explore how water use efficiency and ¹³C discrimination are related across leaf and canopy scales. The model provides a tool to investigate whether trends in Δ indicate changes in leaf functional traits and/or environmental conditions during leaf growth, and how they are associated with trends in plant water use efficiency. The model can be used, for example, to examine whether trends in δ¹³C signatures obtained from tree rings imply changes in tree water use efficiency in response to atmospheric CO₂ increase. This is crucial for predicting how plants may respond to future climate change.
Predicted responses of transpiration to elevated atmospheric CO 2 concentration ( eCO 2 ) are highly variable amongst process‐based models. To better understand and constrain this variability amongst models, we conducted an intercomparison of 11 ecosystem models applied to data from two forest free‐air CO 2 enrichment ( FACE ) experiments at Duke University and Oak Ridge National Laboratory. We analysed model structures to identify the key underlying assumptions causing differences in model predictions of transpiration and canopy water use efficiency. We then compared the models against data to identify model assumptions that are incorrect or are large sources of uncertainty. We found that model‐to‐model and model‐to‐observations differences resulted from four key sets of assumptions, namely (i) the nature of the stomatal response to elevated CO 2 (coupling between photosynthesis and stomata was supported by the data); (ii) the roles of the leaf and atmospheric boundary layer (models which assumed multiple conductance terms in series predicted more decoupled fluxes than observed at the broadleaf site); (iii) the treatment of canopy interception (large intermodel variability, 2–15%); and (iv) the impact of soil moisture stress (process uncertainty in how models limit carbon and water fluxes during moisture stress). Overall, model predictions of the CO 2 effect on WUE were reasonable (intermodel μ = approximately 28% ± 10%) compared to the observations (μ = approximately 30% ± 13%) at the well‐coupled coniferous site (Duke), but poor (intermodel μ = approximately 24% ± 6%; observations μ = approximately 38% ± 7%) at the broadleaf site (Oak Ridge). The study yields a framework for analysing and interpreting model predictions of transpiration responses to eCO 2 , and highlights key improvements to these types of models.
Water-use efficiency (WUE) is often considered an important determinant of yield under stress and even as a component of crop drought resistance. It has been used to imply that rainfed plant production can be increased per unit water used, resulting in “more crop per drop”. This opinionated review argues that selection for high WUE in breeding for water-limited conditions will most likely lead, under most conditions, to reduced yield and reduced drought resistance. As long as the biochemistry of photosynthesis cannot be improved genetically, greater genotypic transpiration efficiency (TE) and WUE are driven mainly by plant traits that reduce transpiration and crop water-use, processes which are crucially important for plant production. Since biomass production is tightly linked to transpiration, breeding for maximized soil moisture capture for transpiration is the most important target for yield improvement under drought stress. Effective use of water (EUW) implies maximal soil moisture capture for transpiration which also involves reduced non-stomatal transpiration and minimal water loss by soil evaporation. Even osmotic adjustment which is a major stress adaptive trait in crop plants is recognized as enhancing soil moisture capture and transpiration. High harvest index (HI) expresses successful plant reproduction and yield in terms of reproductive functions and assimilate partitioning towards reproduction. In most rainfed environments crop water deficit develops during the reproductive growth stage thus reducing HI. EUW by way of improving plant water status helps sustain assimilate partitions and reproductive success. It is concluded that EUW is a major target for yield improvement in water-limited environments. It is not a coincidence that EUW is an inverse acronym of WUE because very often high WUE is achieved at the expense of reduced EUW.
The Earth's carbon and hydrologic cycles are intimately coupled by gas exchange through plant stomata(1-3). However, uncertainties in the magnitude(4-6) and consequences(7,8) of the physiological responses(9,10) of plants to elevated CO2 in natural environments hinders modelling of terrestrial water cycling and carbon storage(11). Here we use annually resolved long-term delta C-13 tree-ring measurements across a European forest network to reconstruct the physiologically driven response of intercellular CO2 (Ci) caused by atmospheric CO2 (Ca) trends. When removing meteorological signals from the delta C-13 measurements, we find that trees across Europe regulated gas exchange so that for one ppmv atmospheric CO2 increase, C-i increased by similar to 0.76 ppmv, most consistent with moderate control towards a constant C-i/C-a ratio. This response corresponds to twentiethcentury intrinsic water-use efficiency (iWUE) increases of 14 +/- 10 and 22 +/- 6% at broadleaf and coniferous sites, respectively. An ensemble of process-based global vegetation models shows similar CO2 effects on iWUE trends. Yet, when operating these models with climate drivers reintroduced, despite decreased stomatal opening, 5% increases in European forest transpiration are calculated over the twentieth century. This counterintuitive result arises from lengthened growing seasons, enhanced evaporative demand in a warming climate, and increased leaf area, which together oppose effects of CO2-induced stomatal closure. Our study questions changes to the hydrological cycle, such as reductions in transpiration and air humidity, hypothesized to result from plant responses to anthropogenic emissions.
Improvement in crop water-use efficiency (WUE) is a critical priority for regions facing increased drought or diminished groundwater resources. Despite new tools for the manipulation of stomatal development, the engineering of plants with high WUE remains a challenge. We used epidermal patterning factor (EPF) mutants exhibiting altered stomatal density to test whether WUE could be improved directly by manipulation of the genes controlling stomatal density. Specifically, we tested whether constitutive overexpression of reduced stomatal density and maximum stomatal conductance ( ) sufficiently to increase WUE. We found that a reduction in via reduced stomatal density in -overexpressing plants ( OE) increased both instantaneous and long-term WUE without altering significantly the photosynthetic capacity. Conversely, plants lacking both and expression ( ) exhibited higher stomatal density, higher and lower instantaneous WUE, as well as lower (but not significantly so) long-term WUE. Targeted genetic modification of stomatal conductance, such as in OE, is a viable approach for the engineering of higher WUE in crops, particularly in future high-carbon-dioxide (CO ) atmospheres.
A key objective for sustainable agriculture and forestry is to breed plants with both high carbon gain and water-use efficiency (WUE). At the level of leaf physiology, this implies increasing net photosynthesis (A N) relative to stomatal conductance (g s). Here, we review evidence for CO2 diffusional constraints on photosynthesis and WUE. Analyzing past observations for an extensive pool of crop and wild plant species that vary widely in mesophyll conductance to CO2 (g m), g s, and foliage A N, it was shown that both g s and g m limit A N, although the relative importance of each of the two conductances depends on species and conditions. Based on Fick’s law of diffusion, intrinsic WUE (the ratio A N/g s) should correlate on the ratio g m/g s, and not g m itself. Such a correlation is indeed often observed in the data. However, since besides diffusion A N also depends on photosynthetic capacity (i.e., V c,max), this relationship is not always sustained. It was shown that only in a very few cases, genotype selection has resulted in simultaneous increases of both A N and WUE. In fact, such a response has never been observed in genetically modified plants specifically engineered for either reduced g s or enhanced g m. Although increasing g m alone would result in increasing photosynthesis, and potentially increasing WUE, in practice, higher WUE seems to be only achieved when there are no parallel changes in g s. We conclude that for simultaneous improvement of A N and WUE, genetic manipulation of g m should avoid parallel changes in g s, and we suggest that the appropriate trait for selection for enhanced WUE is increased g m/g s.