Observed and modelled historical trends in the water‐use efficiency of plants and ecosystems

Plant water‐use efficiency (WUE, the carbon gained through photosynthesis per unit of water lost through transpiration) is a tracer of the plant physiological controls on the exchange of water and carbon dioxide between terrestrial ecosystems and the atmosphere. At the leaf level, rising CO₂ concentrations tend to increase carbon uptake (in the absence of other limitations) and to reduce stomatal conductance, both effects leading to an increase in leaf WUE. At the ecosystem level, indirect effects (e.g. increased leaf area index, soil water savings) may amplify or dampen the direct effect of CO₂. Thus, the extent to which changes in leaf WUE translate to changes at the ecosystem scale remains unclear. The differences in the magnitude of increase in leaf versus ecosystem WUE as reported by several studies are much larger than would be expected with current understanding of tree physiology and scaling, indicating unresolved issues. Moreover, current vegetation models produce inconsistent and often unrealistic magnitudes and patterns of variability in leaf and ecosystem WUE, calling for a better assessment of the underlying approaches. Here, we review the causes of variations in observed and modelled historical trends in WUE over the continuum of scales from leaf to ecosystem, including methodological issues, with the aim of elucidating the reasons for discrepancies observed within and across spatial scales. We emphasize that even though physiological responses to changing environmental drivers should be interpreted differently depending on the observational scale, there are large uncertainties in each data set which are often underestimated. Assumptions made by the vegetation models about the main processes influencing WUE strongly impact the modelled historical trends. We provide recommendations for improving long‐term observation‐based estimates of WUE that will better inform the representation of WUE in vegetation models.     

Water Relations,CO2 Exchange,Water-use Efficiency and Growth of Welwitschia mirabilis Hook. fil. in three Contrasting Habitats of the Namib Desert1

CO₂ exchange, transpiration and leaf water potential of Welwitschia mirabilis were measured in three contrasting habitats of the Namib desert. From these measurements stomatal conductance, internal CO₂concentration and WUE were calculated. In two of the three habitats photosynthetic CO₂ uptake decreased and transpiration increased with increasing leaf age while in the third habitat CO₂ uptake increased and transpiration decreased with leaf age. Except for the stomata of young leaf sections in this habitat, stomata closed with increasing δw leading to a pronounced midday depression of CO₂ uptake. The high stomatal limitation of photosynthetic CO₂ uptake of glasshouse-grown plants was verified in the natural habitat. Photosynthetic CO₂ uptake saturated between 800 and 1300 μmol photons m^?2 s^?1depending on leaf age and habitat. CO₂ uptake had a broad temperature optimum declining significantly beyond 32 °C. Predawn leaf water potential reflected water availability and atmospheric conditions in the three habitats and ranged from ? 2.5 to ? 6.2 MPa. There was a pronounced diurnal course of leaf water potential in all habitats. During the day a gradient in water potential developed along the leaf axis with the lowest potential at the leaf's tip. With respect to whole plant balances of CO₂ exchange and transpiration, there were marked differences between Welwitschias in the three habitats. Despite a negative CO₂ balance over a period of five months, leaves in the driest habitat grew constantly at the expense of carbon reserves in the plant. Only at the wettest site did carbon gain exceed carbon demand for growth. The WUE of whole plants was insignificant in all habitats. The results were as contrasting as the habitats and plants and did not allow generalisations about adaptational features of Welwitschia mirabilis.     

Seasonal responses of terrestrial ecosystem water‐use efficiency to climate change

Ecosystem water‐use efficiency (EWUE) is an indicator of carbon–water interactions and is defined as the ratio of carbon assimilation (GPP) to evapotranspiration (ET). Previous research suggests an increasing long‐term trend in annual EWUE over many regions and is largely attributed to the physiological effects of rising CO₂. The seasonal trends in EWUE, however, have not yet been analyzed. In this study, we investigate seasonal EWUE trends and responses to various drivers during 1982–2008. The seasonal cycle for two variants of EWUE, water‐use efficiency (WUE, GPP/ET), and transpiration‐based WUE (WUE_t, the ratio of GPP and transpiration), is analyzed from 0.5° gridded fields from four process‐based models and satellite‐based products, as well as a network of 63 local flux tower observations. WUE derived from flux tower observations shows moderate seasonal variation for most latitude bands, which is in agreement with satellite‐based products. In contrast, the seasonal EWUE trends are not well captured by the same satellite‐based products. Trend analysis, based on process‐model factorial simulations separating effects of climate, CO₂, and nitrogen deposition (NDEP), further suggests that the seasonal EWUE trends are mainly associated with seasonal trends of climate, whereas CO₂ and NDEP do not show obvious seasonal difference in EWUE trends. About 66% grid cells show positive annual WUE trends, mainly over mid‐ and high northern latitudes. In these regions, spring climate change has amplified the effect of CO₂ in increasing WUE by more than 0.005 gC m⁻² mm⁻¹ yr⁻¹ for 41% pixels. Multiple regression analysis further shows that the increase in springtime WUE in the northern hemisphere is the result of GPP increasing faster than ET because of the higher temperature sensitivity of GPP relative to ET. The partitioning of annual EWUE to seasonal components provides new insight into the relative sensitivities of GPP and ET to climate, CO_2, and NDEP.     

Regulation of Transpiration to Improve Crop Water Use

Decreasing fresh water supplies and increasing agricultural drought threaten sustainable worldwide crop production. Consequently, there is a global priority to develop crops with higher water use efficiency (WUE): biomass production or yield per unit of water used. Water use efficiency varies substantially among species and genotypes within a species, and a major effort is now underway to identify the genetic determinants of WUE. Today, it is known that genotypes in primary gene pools exhibit allelic variation for WUE through mechanisms that regulate transpiration, which is the conductance of water through stomata, the cuticle, and the boundary layer. Because of the differential diffusion properties of water and carbon dioxide (CO₂) through these pathways, it is feasible that WUE could be improved by decreasing transpiration without a concomitant reduction in CO₂ uptake. Since CO₂ uptake and transpirational water loss occur predominantly through stomatal pores, it is not surprising that genes involved in stomatal development and stomatal opening/closing impact WUE. Furthermore, loss- and gain-of-function genetic screens have identified genes that regulate transpiration and WUE by yet undetermined mechanisms. This review will discuss the genetic determinants that regulate transpiration and WUE in the context of the modern agricultural goal of improving WUE while sustaining biomass and yield.     

Mesophyll conductance to CO2 and Rubisco as targets for improving intrinsic water use efficiency in C3 plants

Water limitation is a major global constraint for plant productivity that is likely to be exacerbated by climate change. Hence, improving plant water use efficiency (WUE) has become a major goal for the near future. At the leaf level, WUE is the ratio between photosynthesis and transpiration. Maintaining high photosynthesis under water stress, while improving WUE requires either increasing mesophyll conductance (g_m) and/or improving the biochemical capacity for CO₂ assimilation—in which Rubisco properties play a key role, especially in C₃ plants at current atmospheric CO₂. The goals of the present analysis are: (1) to summarize the evidence that improving g_m and/or Rubisco can result in increased WUE; (2) to review the degree of success of early attempts to genetically manipulate g_m or Rubisco; (3) to analyse how g_m, g_sw and the Rubisco's maximum velocity (V_cmax) co‐vary across different plant species in well‐watered and drought‐stressed conditions; (4) to examine how these variations cause differences in WUE and what is the overall extent of variation in individual determinants of WUE; and finally, (5) to use simulation analysis to provide a theoretical framework for the possible control of WUE by g_m and Rubisco catalytic constants vis‐à‐vis g_sw under water limitations.     

Plant Water Use Efficiency over Geological Time – Evolution of Leaf Stomata Configurations Affecting Plant Gas Exchange

Plant gas exchange is a key process shaping global hydrological and carbon cycles and is often characterized by plant water use efficiency (WUE - the ratio of CO₂ gain to water vapor loss). Plant fossil record suggests that plant adaptation to changing atmospheric CO₂ involved correlated evolution of stomata density (d) and size (s), and related maximal aperture, a_max. We interpreted the fossil record of s and d correlated evolution during the Phanerozoic to quantify impacts on gas conductance affecting plant transpiration, E, and CO₂ uptake, A, independently, and consequently, on plant WUE. A shift in stomata configuration from large s-low d to small s-high d in response to decreasing atmospheric CO₂ resulted in large changes in plant gas exchange characteristics. The relationships between gas conductance, g_ws, A and E and maximal relative transpiring leaf area, (a_max⋅d), exhibited hysteretic-like behavior. The new WUE trend derived from independent estimates of A and E differs from established WUE-CO₂ trends for atmospheric CO₂ concentrations exceeding 1,200 ppm. In contrast with a nearly-linear decrease in WUE with decreasing CO₂ obtained by standard methods, the newly estimated WUE trend exhibits remarkably stable values for an extended geologic period during which atmospheric CO₂ dropped from 3,500 to 1,200 ppm. Pending additional tests, the findings may affect projected impacts of increased atmospheric CO₂ on components of the global hydrological cycle.     

Responses of water use efficiency of 9 plant species to light and CO2 and their modeling

Photosynthesis coupled with transpiration determines water use efficiency (WUE) at leaf level, and the responses of WUE controlled by gas exchanges through stomata to environment are the basis of carbon and water cycles in the ecosystem. In this paper, by using Li-6400 Portable Photosynthesis System (LI-COR), WUE at leaf level was analyzed under controlled photosynthetic photons flux density (PPFD) and CO₂ concentration conditions across 9 plant species including maize (Zea mays), sorghum (Sorghum vulgare), millet (Setaria italica), soybean (Glycine max), peanut (Arachis phyogaea), sweet potato (Ipomoea batatas), rice (Oryza sativa), Masson pine (Pinus massoniana) and Schima superba. We had developed a new model to estimate the water use efficiency in response to the combined effects of light and CO₂ concentration. Our measured data validated that this model could simulate the changes of water use efficiency very well under combined effect of light and CO₂ concentration. It could be used to estimate contribution of photosynthesis increase and transpiration decline on water use efficiency with the rising of CO₂ concentration. Great differences in water use efficiency occurred in these different plant species under various CO₂ concentration levels. Based on water use efficiency at regional scale, we concluded that plants should be separated into C₃ plants and C₄ plants, and furthermore, C₃ plants should be separated into herbaceous plants and woody plants. Our separation criteria would do a great favor in modeling the evapotranspiration of terrestrial ecosystem with carbon and water balance.     

Water use efficiency as a function of leaf age and position within the cotton canopy

The gas exchange properties of whole plant canopies are an integral part of crop productivity and have attracted much attention in recent years. However, insufficient information exists on the coordination of transpiration and CO₂ uptake for individual leaves during the growing season. Single-leaf determinations of net photosynthesis (Pn), transpiration (E) and water use efficiency (WUE) for field-grown cotton (Gossypium hirsutum L.) leaves were recorded during a 2-year field study. Measurements were made at 3 to 4 day intervals on the main-stem and first three sympodial leaves at main-stem node 10 from their unfolding through senescence. Results indicated that all gas exchange parameters changed with individual main-stem and sympodial leaf age. Values of Pn, E and WUE followed a rise and fall pattern with maximum rates achieved at a leaf age of 18 to 20 days. While no significant position effects were observed for Pn, main-stem and sympodial leaves did differ in E and WUE particularly as leaves aged beyond 40 days. For a given leaf age, the main-stem leaf had a significantly lower WUE than the three sympodial leaves. WUE's for the main-stem and three sympodial leaves between the ages of 41 to 50 days were 0.85, 1.30, 1.36 and 1.95 μmol CO₂ mmol⁻¹ H₂O, respectively. The mechanisms which mediated leaf positional differences for WUE were not strictly related to changes in stomatal conductance (g_s·H₂O) since decreases in g_s·H₂O with leaf age were similar for the four leaves. However, significantly different radiant environments with distance along the fruiting branch did indicate the possible involvement of mutual leaf shading in determining WUE. The significance of these findings are presented in relation to light competition within the plant canopy during development.     

不同植物叶片水分利用效率对光和CO2的响应与模拟

植物叶片水分利用效率的高低取决于气孔控制的光合作用和蒸腾作用两个相互耦合的过程,模拟水分利用效率对环境变化的响应特征和机制是理解生态系统碳循环和水循环及其耦合关系的基础.研究通过人工控制光强和CO2浓度,对叶片水分利用效率进行了研究.提出了植物水分利用效率在光强和CO2浓度共同作用下的估算模型.数据分析表明,该模型在包括C3和C4植物、草本和木本植物在内的9种植物上能很好地模拟水分利用效率对光强和CO2浓度共同作用的响应.该模型可以用于估算CO2浓度升高条件下光合速率的提高和蒸腾速率的降低对水分利用效率提高的贡献量.CO2浓度变化条件下,水分利用效率在不同植物之间有巨大差异,研究区域尺度植物的水分利用效率时至少需要将植物区分为C4植物和C3植物,其中C3植物区分为草本和木本植物3种生态功能型才能较为准确地估算植物的整体水分利用效率.应用本研究提出的水分利用效率估算模型和植物水分利用效率生态功能型分类标准,可以为建立以植物的水分利用效率为基本参数的陆地生态系统水循环模型和陆地生态系统生产力模型提供重要依据.     

The “isohydric trap”: A proposed feedback between water shortage,stomatal regulation,and nutrient acquisition drives differential growth and survival of European pines under climatic dryness

Climatic dryness imposes limitations on vascular plant growth by reducing stomatal conductance, thereby decreasing CO₂ uptake and transpiration. Given that transpiration‐driven water flow is required for nutrient uptake, climatic stress‐induced nutrient deficit could be a key mechanism for decreased plant performance under prolonged drought. We propose the existence of an “isohydric trap,” a dryness‐induced detrimental feedback leading to nutrient deficit and stoichiometry imbalance in strict isohydric species. We tested this framework in a common garden experiment with 840 individuals of four ecologically contrasting European pines (Pinus halepensis, P. nigra, P. sylvestris, and P. uncinata) at a site with high temperature and low soil water availability. We measured growth, survival, photochemical efficiency, stem water potentials, leaf isotopic composition (δ¹³C, δ¹⁸O), and nutrient concentrations (C, N, P, K, Zn, Cu). After 2 years, the Mediterranean species Pinus halepensis showed lower δ¹⁸O and higher δ¹³C values than the other species, indicating higher time‐integrated transpiration and water‐use efficiency (WUE), along with lower predawn and midday water potentials, higher photochemical efficiency, higher leaf P, and K concentrations, more balanced N:P and N:K ratios, and much greater dry‐biomass (up to 63‐fold) and survival (100%). Conversely, the more mesic mountain pine species showed higher leaf δ¹⁸O and lower δ¹³C, indicating lower transpiration and WUE, higher water potentials, severe P and K deficiencies and N:P and N:K imbalances, and poorer photochemical efficiency, growth, and survival. These results support our hypothesis that vascular plant species with tight stomatal regulation of transpiration can become trapped in a feedback cycle of nutrient deficit and imbalance that exacerbates the detrimental impacts of climatic dryness on performance. This overlooked feedback mechanism may hamper the ability of isohydric species to respond to ongoing global change, by aggravating the interactive impacts of stoichiometric imbalance and water stress caused by anthropogenic N deposition and hotter droughts, respectively.     

Gas exchange and water‐use efficiency in plant canopies

In this review, I first address the basics of gas exchange, water‐use efficiency and carbon isotope discrimination in C₃ plant canopies. I then present a case study of water‐use efficiency in northern Australian tree species. In general, C₃ plants face a trade‐off whereby increasing stomatal conductance for a given set of conditions will result in a higher CO₂ assimilation rate, but a lower photosynthetic water‐use efficiency. A common garden experiment suggested that tree species which are able to establish and grow in drier parts of northern Australia have a capacity to use water rapidly when it is available through high stomatal conductance, but that they do so at the expense of low water‐use efficiency. This may explain why community‐level carbon isotope discrimination does not decrease as steeply with decreasing rainfall on the North Australian Tropical Transect as has been observed on some other precipitation gradients. Next, I discuss changes in water‐use efficiency that take place during leaf expansion in C₃ plant leaves. Leaf phenology has recently been recognised as a significant driver of canopy gas exchange in evergreen forest canopies, and leaf expansion involves changes in both photosynthetic capacity and water‐use efficiency. Following this, I discuss the role of woody tissue respiration in canopy gas exchange and how photosynthetic refixation of respired CO₂ can increase whole‐plant water‐use efficiency. Finally, I discuss the role of water‐use efficiency in driving terrestrial plant responses to global change, especially the rising concentration of atmospheric CO₂. In coming decades, increases in plant water‐use efficiency caused by rising CO₂ are likely to partially mitigate impacts on plants of drought stress caused by global warming.     

Canopy radiation‐ and water‐use efficiencies as affected by elevated [CO2]

This study used an environmentally controlled plant growth facility, EcoCELLs, to measure canopy gas exchanges directly and to examine the effects of elevated [CO₂] on canopy radiation‐ and water‐use efficiencies. Sunflowers (Helianthus annus var. Mammoth) were grown at ambient (399 μmol mol^?1) and elevated [CO₂] (746 μmol mol^?1) for 53 days in EcoCELLs. Whole canopy carbon‐ and water‐fluxes were measured continuously during the period of the experiment. The results indicated that elevated [CO₂] enhanced daily total canopy carbon‐ and water‐fluxes by 53% and 11%, respectively, on a ground‐area basis, resulting in a 54% increase in radiation‐use efficiency (RUE) based on intercepted photosynthetic active radiation and a 26% increase in water‐use efficiency (WUE) by the end of the experiment. Canopy carbon‐ and water‐fluxes at both CO₂ treatments varied with canopy development. They were small at 22 days after planting (DAP) and gradually increased to the maxima at 46 DAP. When canopy carbon‐ and water‐fluxes were expressed on a leaf‐area basis, no effect of CO₂ was found for canopy water‐flux while elevated [CO₂] still enhanced canopy carbon‐flux by 29%, on average. Night‐time canopy carbon‐flux was 32% higher at elevated than at ambient [CO₂]. In addition, RUE and WUE displayed strong diurnal variations, high at noon and low in the morning or afternoon for WUE but opposite for RUE. This study provided direct evidence that plant canopy may consume more, instead of less, water but utilize both water and radiation more efficiently at elevated than at ambient [CO₂], at least during the exponential growth period as illustrated in this experiment.     

Drought response of mesophyll conductance in forest understory species – impacts on water‐use efficiency and interactions with leaf water movement

Regulation of stomatal (g_s) and mesophyll conductance (g_m) is an efficient means for optimizing the relationship between water loss and carbon uptake in plants. We assessed water‐use efficiency (WUE)‐based drought adaptation strategies with respect to mesophyll conductance of different functional plant groups of the forest understory. Moreover we aimed at assessing the mechanisms of and interactions between water and CO₂ conductance in the mesophyll. The facts that an increase in WUE was observed only in the two species that increased g_m in response to moderate drought, and that over all five species examined, changes in mesophyll conductance were significantly correlated with the drought‐induced change in WUE, proves the importance of g_m in optimizing resource use under water restriction. There was no clear correlation of mesophyll CO₂ conductance and the tortuosity of water movement in the leaf across the five species in the control and drought treatments. This points either to different main pathways for CO₂ and water in the mesophyll either to different regulation of a common pathway.     

Change in terrestrial ecosystem water‐use efficiency over the last three decades

Defined as the ratio between gross primary productivity (GPP) and evapotranspiration (ET), ecosystem‐scale water‐use efficiency (EWUE) is an indicator of the adjustment of vegetation photosynthesis to water loss. The processes controlling EWUE are complex and reflect both a slow evolution of plants and plant communities as well as fast adjustments of ecosystem functioning to changes of limiting resources. In this study, we investigated EWUE trends from 1982 to 2008 using data‐driven models derived from satellite observations and process‐oriented carbon cycle models. Our findings suggest positive EWUE trends of 0.0056, 0.0007 and 0.0001 g C m^?2 mm^?1 yr^?1 under the single effect of rising CO₂ (‘CO₂’), climate change (‘CLIM’) and nitrogen deposition (‘NDEP’), respectively. Global patterns of EWUE trends under different scenarios suggest that (i) EWUE‐CO₂ shows global increases, (ii) EWUE‐CLIM increases in mainly high latitudes and decreases at middle and low latitudes, (iii) EWUE‐NDEP displays slight increasing trends except in west Siberia, eastern Europe, parts of North America and central Amazonia. The data‐driven MTE model, however, shows a slight decline of EWUE during the same period (?0.0005 g C m^?2 mm^?1 yr^?1), which differs from process‐model (0.0064 g C m^?2 mm^?1 yr^?1) simulations with all drivers taken into account. We attribute this discrepancy to the fact that the nonmodeled physiological effects of elevated CO₂ reducing stomatal conductance and transpiration (TR) in the MTE model. Partial correlation analysis between EWUE and climate drivers shows similar responses to climatic variables with the data‐driven model and the process‐oriented models across different ecosystems. Change in water‐use efficiency defined from transpiration‐based WUE_t (GPP/TR) and inherent water‐use efficiency (IWUE_t, GPP×VPD/TR) in response to rising CO₂, climate change, and nitrogen deposition are also discussed. Our analyses will facilitate mechanistic understanding of the carbon–water interactions over terrestrial ecosystems under global change.     

Water use efficiency by alfalfa: Mechanisms involving anti-oxidation and osmotic adjustment under drought

Water use and mechanisms relating to osmotic adjustment and anti-oxidation were investigated in alfalfa (Medicago sativa L.) plants under reduced water availability. Water use efficiency (WUE), MDA and proline contents, and antioxidant enzyme activities were measured in three alfalfa cultivars under three levels of soil water availability in a greenhouse pot experiment. WUE was determined indirectly using discriminating carbon isotope composition. WUE increased with the severity of water deficit. Under all water regimes examined, cv. Longdong showed the greatest WUE values and the least reduction in biomass production under a 50% soil field water capacity. Stomatal density increased with increasing water deficit, but stomatal conductance decreased. This suggests that water stress can increase WUE by modifying stomatal regulation of the balance between the rates of CO₂ assimilation and water loss. The maintenance of leaf physiological function and leaf water status suggests that alfalfa has some mechanisms to maintain cell function when the plant is subjected to water deficit. The increase in the MDA content under drought conditions indicates that some degree of damage to cell membranes is unavoidable, whereas other results showing increases in the contents of proline and soluble sugars and activities of superoxide dismutase, peroxide dismutase, and catalase indicate how cell function may be to some extent maintained to result in the higher WUE. Alfalfa is shown to exhibit cultivar-specific differences in WUE with the maintenance of cell function under water deficit being related to anti-oxidation and osmotic adjustment.     

干旱区植物叶片大小对叶表面蒸腾及叶温的影响

利用热及物质交换原理, 并结合前人研究成果, 在单叶尺度上建立了简单的叶温和水气蒸腾模型。模型通过预设值驱动, 预设值参照干旱区环境及植物叶片特征设置。模拟结果显示: 随气孔阻力的增加, 叶片蒸腾速率降低, 叶温升高; 同一环境下, 具有低辐射吸收率的叶片蒸腾速率和叶温更低, 并且气孔阻力越大, 这种差异越明显。另外, 叶片宽度及风速是影响叶片蒸腾及叶温的重要因子。干旱地区植物生长季节, 风速小于0.1 m·s ^-1、气孔阻力接近1000 s·m ^-1时, 降低叶片宽度不仅有利于降低叶片温度, 而且能够降低叶片蒸腾速率, 从而实现保持水分, 增强植物适应高温、干旱的能力。     

Water relations and photosynthetic water use efficiency as indicators of slow climate change effects on trees in a tropical mountain forest in South Ecuador

The effects of an increasing moisture on trees of the tropical species-rich mountain rain forest in the South Ecuadorian Andes was investigated, using the daily total water consumption (TWC) and the instantaneous water use efficiency (WUE, ratio of photosynthetic CO₂ uptake per water loss by transpiration) as ecophysiological indicators. Two canopy and one sub-canopy tree species, (Vismia tomentosa, Clusiaceae, an as of yet unknown Lauracee, and Spirotheca rosea, Bombacaceae) were the experimental objects. Seasonal changes as well as a long-term (18 months) trend of increasing precipitation caused an inverse reaction of the TWC of the trees. Because of a rather unlimited water supply to the trees from a permanently high water content of the soil, transpiration followed mainly the atmospheric demand of water vapor, and increasing moisture hence reduced water loss by transpiration. It was hypothesized that in spite of the reduction in transpiratory water loss photosynthetic carbon acquisition would be not or less affected due to an increase in water use efficiency. Concomitant measurements of photosynthetic net CO₂ uptake showed the expected increase of WUE in V. tomentosa and S. rosea, but no clear reaction of the Lauracee. Accompanying measurements of stem extension growth confirmed an undiminished growth of V. tomentosa and S. rosea but showed also suspended growth of the Lauracee during the wettest months. While TWC can be continuously monitored with the heat dissipation technique, WUE is determined by leaf porometry in campaigns for which access to the canopy is required. Simultaneous recordings of the gas exchange of leaves at 4 different positions in the crown of one of the experimental trees (V. tomentosa) showed the usability of the trait WUE in combination with the total daily water consumption as indicator set for assessing the response of trees to a subtly changing climate. However, not all tree species appear as likewise useful indicator trees.     

The combined effect of constant water deficit and nitrogen supply on WUE, NUE and Δ13C in durum wheat potted plants

Water scarcity and nitrogen shortage are the main constraints on durum wheat productivity. This paper examines the combined effects of a constant water deficit and nitrogen supply (NS) on growth, photosynthesis, stomatal conductance (g_s) and transpiration, instantaneous and time‐integrated water use efficiency (WUE) and nitrogen use efficiency (NUE) and carbon isotope discrimination (Δ¹³C) in durum wheat genotypes grown in pots under greenhouse conditions. Three water levels (40%, 70% and 100% container capacity), two nitrogen doses (high and low N) and four genotypes were assayed in a total of 24 experimental treatments. Water and nitrogen treatments were imposed 2 weeks after plant emergence. The growth, nitrogen content and Δ¹³C of the shoot and the gas exchange in the flag leaf were determined about 2 weeks after anthesis. As expected, both water and NS had a strong positive effect on growth. However, a reduction in water supply had low effect decreasing photosynthesis and transpiration, Δ¹³C and NUE and increasing WUE. On the contrary, increasing the level of nitrogen supplied had a significant negative effect on g_s, which decreased significantly the ratio of intercellular to ambient CO₂ concentrations and Δ¹³C, and increased both instantaneous and time‐integrated WUE. In addition, a higher N level also negatively affected the instantaneous and time‐integrated NUE. The Δ¹³C of shoots correlated significantly and negatively with either instantaneous or time‐integrated measurements of WUE. Moreover, within each NS, Δ¹³C also correlated negatively with the integrated NUE. We concluded that under our experimental conditions, Δ¹³C gives information about the efficiency with which not just water but also nitrogen are used by the plant. In addition, this study illustrates that a steady water limitation may strongly affect biomass without consistent changes in WUE. The lack of effect of the different water regimes on gas exchange, WUE and Δ¹³C illustrate the importance of how stress is imposed during growth.     

Nitrogen fertilization enhances water‐use efficiency in a saline environment

Effects of salinity and nutrients on carbon gain in relation to water use were studied in the grey mangrove, Avicennia marina, growing along a natural salinity gradient in south‐eastern Australia. Tall trees characterized areas of seawater salinities (fringe zone) and stunted trees dominated landward hypersaline areas (scrub zone). Trees were fertilized with nitrogen (+N) or phosphorus (+P) or unfertilized. There was no significant effect of +P on shoot growth, whereas +N enhanced canopy development, particularly in scrub trees. Scrub trees maintained greater CO₂ assimilation per unit water transpired (water‐use efficiency, WUE) and had lower nitrogen‐use efficiency (NUE; CO₂ assimilation rate per unit leaf nitrogen) than fringe trees. The CO₂ assimilation rates of +N trees were similar to those in other treatments, but were achieved at lower transpiration rates, stomatal conductance and intercellular CO₂ concentrations. Maintaining comparable assimilation rates at lower stomatal conductance requires greater ribulose 1·5‐bisphosphate carboxylase/oxygenase activity, consistent with greater N content per unit leaf area in +N trees. Hence, +N enhanced WUE at the expense of NUE. Instantaneous WUE estimates were supported by less negative foliar δ¹³C values for +N trees and scrub control trees. Thus, nutrient enrichment may alter the structure and function of mangrove forests along salinity gradients.     

The efficiency of water use in water stressed plants is increased due to ABA induced stomatal closure

Gas exchange and abscisic acid content of Digitalis lanata EHRH. have been examined at different levels of plant water stress. Net photosynthesis, transpiration and conductance of attached leaves declined rapidly at first, then more slowly following the withholding of irrigation. The intercellular partial pressure of CO₂ decreased slightly. The concentration of 2-cis(S)ABA increased about eight-fold in the leaves of non-irrigated plants as compared with well-watered controls. A close linear correlation was found between the ABA content of the leaves and their conductance on a leaf area basis. In contrast, the plot of net assimilation versus ABA concentration was curvilinear, leading to an increased efficiency of water use during stress. After rewatering, photosynthesis reached control values earlier than transpiration, leaf conductance and ABA content. From these data it is concluded that transpiration through the stomata is directly controlled by the ABA content, whereas net photosynthesis is influenced additionally by other factors.Possible reasons for the responses of photosynthesis and water use efficiency to different stress and ABA levels are discussed.Abbreviations A net CO₂ assimilation - ABA abscisic acid - C_i intercellular CO₂ concentration - g stomatal conductance - T transpiration - WUE water use efficiency