Water‐use efficiency in response to climate change: from leaf to ecosystem in a temperate steppe

Water‐use efficiency (WUE) has been recognized as an important characteristic of ecosystem productivity, which links carbon (C) and water cycling. However, little is known about how WUE responds to climate change at different scales. Here, we investigated WUE at leaf, canopy, and ecosystem levels under increased precipitation and warming from 2005 to 2008 in a temperate steppe in Northern China. We measured gross ecosystem productivity (GEP), net ecosystem CO₂ exchange (NEE), evapotranspiration (ET), evaporation (E), canopy transpiration (T_c), as well as leaf photosynthesis (P_max) and transpiration (T_l) of a dominant species to calculate canopy WUE (WUE_c=GEP/T), ecosystem WUE (WUE_gep=GEP/ET or WUE_nee=NEE/ET) and leaf WUE (WUE_l=P_max/T_l). The results showed that increased precipitation stimulated WUE_c, WUE_gep and WUE_nee by 17.1%, 10.2% and 12.6%, respectively, but decreased WUE_l by 27.4%. Climate warming reduced canopy and ecosystem WUE over the 4 years but did not affect leaf level WUE. Across the 4 years and the measured plots, canopy and ecosystem WUE linearly increased, but leaf level WUE of the dominant species linearly decreased with increasing precipitation. The differential responses of canopy/ecosystem WUE and leaf WUE to climate change suggest that caution should be taken when upscaling WUE from leaf to larger scales. Our findings will also facilitate mechanistic understanding of the C–water relationships across different organism levels and in projecting the effects of climate warming and shifting precipitation regimes on productivity in arid and semiarid ecosystems.     

Water use efficiency, transpiration and net CO2 exchange of four alfalfa genotypes submitted to progressive drought and subsequent recovery

The predicted worldwide increase in arid areas and water stress episodes will strongly affect crop production. Plants have developed a wide diversity of physiological mechanisms for drought tolerance. A decline in photosynthesis and thus yield production is a common response to drought, as are increases in the water use efficiency of photosynthesis (WUE_ph) and productivity (WUE_p). The aim of our study was to determine the physiological effects (especially WUE_ph and WUE_p) of progressive drought and subsequent recovery in three cultivars adapted to a Mediterranean climate [Tafilalet (TA), Tierra de Campos (TC), and Moapa (MO)], and another representative from an oceanic climate (Europe (EU)). The accuracy of the relationships between WUE_ph or WUE_p and carbon isotope discrimination (Δ ¹³C) in shoots was also investigated as a function of water stress intensity. Mild drought (7 days of water withholding) decreased the net CO₂ exchange (A), leaf conductance to water (g) and transpiration in EU leading to an increased WUE_ph. Δ ¹³C was correlated with WUE_p but not with WUE_ph, probably due to a late decrease in g. After moderate drought (14 days), A and g decreased in all cultivars, increasing WUE_ph. In this period WUE_p also increased. Both WUE parameters were correlated with Δ ¹³C, which may indicate that the g value at the end of moderate water stress was representative of the growing period. After 21 days, TA was the most productive cultivar but under severe drought conditions there was no difference in DM accumulation among cultivars. After the recovery period, leaf area was increased but not total DM, showing that leaves were the most responsive organs to rewatering. Severe water stress did not decrease WUE_ph or WUE_p, and Δ ¹³C did not increase after recovery. This absence of a response to severe drought may indicate significant effects on the photosynthetic apparatus after 21 days of withholding water. As for mild drought, WUE_p but not WUE_ph was correlated with Δ ¹³C, supporting the view that WUE_p is a more integrative parameter than WUE_ph.     

Water use efficiency in C3 cereals under Mediterranean conditions: a review of physiological aspects

In this review, we will discuss physiological traits of C₃ cereals related to water use efficiency (WUE) in Mediterranean environments, from leaf (WUE_{instantaneous}) to crop level (WUE_yield or ‘water productivity’). First, we analyse the WUE_{instantaneous} and the possible trade‐off between improving this parameter and growth/yield performance. Ways to ameliorate WUE without penalties are discussed. We also analyse in what cases breeding by high or low WUE_{instantaneous} is a suitable criterion to maintain grain yield under drought (Mediterranean) conditions. This question is approached in the framework of carbon isotope discrimination, (Δ¹³C), the main indirect parameter used to integrate (at time and space scale) the WUE_{instantaneous} in C₃ plants. A negative correlation between these two parameters has been confirmed by several studies. The relationship between Δ¹³C and grain yield, however, is more complex, and may differ from one environment to another. In Mediterranean conditions with moderate or no water stress, a positive correlation between Δ¹³C and grain yield is found in barley and wheat, whereas in ‘stored‐water’ crops (such as in some regions of Australia), lower Δ¹³C (i.e. higher WUE_{instantaneous}) is associated with higher grain yield, particularly in more stressful conditions. These apparent inconsistencies and their possible implications for plant breeding are discussed. One physiological trait that has received minor attention in attempts to improve WUE_{instantaneous} is the role of ear photosynthesis. Ears of barley and durum wheat have a higher WUE_{instantaneous} than the flag leaf, both in well‐watered and in drought conditions. The underlying causes of the higher WUE_{instantaneous} of ears are not fully understood, but their refixation capacity (i.e. the capacity to re‐assimilate respired carbon dioxide) could be important. Although the genotypic variability of this trait has not been extensively studied, some data support the idea that variation in refixation capacity may be attributable to genetic factors. At the crop level, decreasing soil evaporation is a crucial factor in efforts to improve the WUE_yield in Mediterranean conditions, and fast initial growth of the crop (i.e. early vigour) seems to be relevant. In wheat, modern varieties with dwarfing genes (giberellic acid – insensitive) have higher yields but, concomitantly, they have lower initial growth performance. Recently, semi‐dwarf cultivars (giberellic acid – sensitive) with high grain yield and simultaneously high early vigour were found, opening new avenues to increase WUE_yield in wheat. The negative effects of futile water loss by cuticular and nocturnal transpiration are also commented. Finally, we discuss some agronomic practices (in particular, ‘deficit irrigation’ systems) linked to physiological traits that confer higher WUE_yield,, in particular, in the cases of Mediterranean regions.     

Water-use responses of ��living fossil�� conifers to CO2 enrichment in a simulated Cretaceous polar environment

Background and Aims

During the Mesozoic, the polar regions supported coniferous forests that experienced warm climates, a CO₂-rich atmosphere and extreme seasonal variations in daylight. How the interaction between the last two factors might have influenced water use of these conifers was investigated. An experimental approach was used to test the following hypotheses: (1) the expected beneficial effects of elevated [CO₂] on water-use efficiency (WUE) are reduced or lost during the 24-h light of the high-latitude summer; and (2) elevated [CO₂] reduces plant water use over the growing season.

Methods

Measurements of leaf and whole-plant gas exchange, and leaf-stable carbon isotope composition were made on one evergreen (Sequoia sempervirens) and two deciduous (Metasequoia glyptostroboides and Taxodium distichum) ‘living fossil’ coniferous species after 3 years'' growth in controlled-environment simulated Cretaceous Arctic (69°N) conditions at either ambient (400 µmol mol⁻¹) or elevated (800 µmol mol⁻¹) [CO₂].

Key Results

Stimulation of whole-plant WUE (WUE_P) by CO₂ enrichment was maintained over the growing season for the three studied species but this pattern was not reflected in patterns of WUE inferred from leaf-scale gas exchange measurements (iWUE_L) and δ¹³C of foliage (tWUE_L). This response was driven largely by increased rates of carbon uptake, because there was no overall CO₂ effect on daily whole-plant transpiration or whole-plant water loss integrated over the study period. Seasonal patterns of tWUE_L differed from those measured for iWUE_L. The results suggest caution against over simplistic interpretations of WUE_P based on leaf isotopic composition.

Conclusions

The data suggest that the efficiency of whole-tree water use may be improved by CO₂ enrichment in a simulated high-latitude environment, but that transpiration is relatively insensitive to atmospheric CO₂ in the living fossil species investigated.Key words: Water-use efficiency, elevated CO₂, living fossil plants, conifers, paleoecology, ancient polar forests, stable carbon isotopes, stomatal conductance, canopy transpiration     

Effects of elevated CO2 and temperature on photosynthesis and leaf traits of an understory dwarf bamboo in subalpine forest zone,China

The dwarf bamboo (Fargesia rufa Yi), growing understory in subalpine dark coniferous forest, is one of the main foods for giant panda, and it influences the regeneration of subalpine coniferous forests in southwestern China. To investigate the effects of elevated CO₂, temperature and their combination, the dwarf bamboo plantlets were exposed to two CO₂ regimes (ambient and double ambient CO₂ concentration) and two temperatures (ambient and +2.2°C) in growth chambers. Gas exchange, leaf traits and carbohydrates concentration were measured after the 150‐day experiment. Elevated CO₂ significantly increased the net photosynthetic rate (A_net), intrinsic water‐use efficiency (WUE_i) and carbon isotope composition (δ¹³C) and decreased stomatal conductance (g_s) and total chlorophyll concentration based on mass (Chl_m) and area (Chl_a). On the other hand, elevated CO₂ decreased specific leaf area (SLA), which was increased by elevated temperature. Elevated CO₂ also increased foliar carbon concentration based on mass (C_m) and area (C_a), nitrogen concentration based on area (N_a), carbohydrates concentration (i.e. sucrose, sugar, starch and non‐structural carbohydrates) and the slope of the A_net–N_a relationship. However, elevated temperature decreased C_m, C_a and N_a. The combination of elevated CO₂ and temperature hardly affected SLA, C_m, C_a, N_m, N_a, Chl_m and Chl_a. Variables A_net and N_a had positive linear relationships in all treatments. Our results showed that photosynthetic acclimation did not occur in dwarf bamboo at elevated CO₂ and it could adjust physiology and morphology to enable the capture of more light, to increase WUE and improve nutritional conditions.     

Seasonal photosynthetic gas exchange and water-use efficiency in a constitutive CAM plant, the giant saguaro cactus (Carnegiea gigantea)

Crassulacean acid metabolism (CAM) and the capacity to store large quantities of water are thought to confer high water use efficiency (WUE) and survival of succulent plants in warm desert environments. Yet the highly variable precipitation, temperature and humidity conditions in these environments likely have unique impacts on underlying processes regulating photosynthetic gas exchange and WUE, limiting our ability to predict growth and survival responses of desert CAM plants to climate change. We monitored net CO₂ assimilation (A _net), stomatal conductance (g _s), and transpiration (E) rates periodically over 2 years in a natural population of the giant columnar cactus Carnegiea gigantea (saguaro) near Tucson, Arizona USA to investigate environmental and physiological controls over carbon gain and water loss in this ecologically important plant. We hypothesized that seasonal changes in daily integrated water use efficiency (WUE_day) in this constitutive CAM species would be driven largely by stomatal regulation of nighttime transpiration and CO₂ uptake responding to shifts in nighttime air temperature and humidity. The lowest WUE_day occurred during time periods with extreme high and low air vapor pressure deficit (D _a). The diurnal with the highest D _a had low WUE_day due to minimal net carbon gain across the 24 h period. Low WUE_day was also observed under conditions of low D _a; however, it was due to significant transpiration losses. Gas exchange measurements on potted saguaro plants exposed to experimental changes in D _a confirmed the relationship between D _a and g _s. Our results suggest that climatic changes involving shifts in air temperature and humidity will have large impacts on the water and carbon economy of the giant saguaro and potentially other succulent CAM plants of warm desert environments.     

Mesophyll CO2 conductance and leakiness are not responsive to short‐ and long‐term soil water limitations in the C4 plant Sorghum bicolor

Breeding economically important C₄ crops for enhanced whole‐plant water‐use efficiency (WUE_plant) is needed for sustainable agriculture. WUE_plant is a complex trait and an efficient phenotyping method that reports on components of WUE_plant, such as intrinsic water‐use efficiency (WUE_i, the rate of leaf CO₂ assimilation relative to water loss via stomatal conductance), is needed. In C₄ plants, theoretical models suggest that leaf carbon isotope composition (δ¹³C), when the efficiency of the CO₂‐concentrating mechanism (leakiness, ?) remains constant, can be used to screen for WUE_i. The limited information about how ? responds to water limitations confines the application of δ¹³C for WUE_i screening of C₄ crops. The current research aimed to test the response of ? to short‐ or long‐term moderate water limitations, and the relationship of δ¹³C with WUE_i and WUE_plant, by addressing potential mesophyll CO₂ conductance (g_m) and biochemical limitations in the C₄ plant Sorghum bicolor. We demonstrate that g_m and ? are not responsive to short‐ or long‐term water limitations. Additionally, δ¹³C was not correlated with gas‐exchange estimates of WUE_i under short‐ and long‐term water limitations, but showed a significant negative relationship with WUE_plant. The observed association between the δ¹³C and WUE_plant suggests an intrinsic link of δ¹³C with WUE_i in this C₄ plant, and can potentially be used as a screening tool for WUE_plant in sorghum.     

A genetic link between leaf carbon isotope composition and whole‐plant water use efficiency in the C4 grass Setaria

Genetic selection for whole‐plant water use efficiency (yield per transpiration; WUE_plant) in any crop‐breeding programme requires high‐throughput phenotyping of component traits of WUE_plant such as intrinsic water use efficiency (WUE_i; CO₂ assimilation rate per stomatal conductance). Measuring WUE_i by gas exchange measurements is laborious and time consuming and may not reflect an integrated WUE_i over the life of the leaf. Alternatively, leaf carbon stable isotope composition (δ¹³C_leaf) has been suggested as a potential time‐integrated proxy for WUE_i that may provide a tool to screen for WUE_plant. However, a genetic link between δ¹³C_leaf and WUE_plant in a C₄ species has not been well established. Therefore, to determine if there is a genetic relationship in a C₄ plant between δ¹³C_leaf and WUE_plant under well watered and water‐limited growth conditions, a high‐throughput phenotyping facility was used to measure WUE_plant in a recombinant inbred line (RIL) population created between the C₄ grasses Setaria viridis and S. italica. Three quantitative trait loci (QTL) for δ¹³C_leaf were found and co‐localized with transpiration, biomass accumulation, and WUE_plant. Additionally, WUE_plant for each of the δ¹³C_leaf QTL allele classes was negatively correlated with δ¹³C_leaf, as would be predicted when WUE_i influences WUE_plant. These results demonstrate that δ¹³C_leaf is genetically linked to WUE_plant, likely to be through their relationship with WUE_i, and can be used as a high‐throughput proxy to screen for WUE_plant in these C₄ species.     

水盐梯度下克里雅河流域芦苇光合响应特征

选择新疆克里雅河流域地下水埋深不同的样点芦苇为材料,采用Li-6400便携式光合系统仪对各样点芦苇的光合响应特征进行实地测定,探讨其适应干旱胁迫和盐胁迫环境的光合机理。结果显示:(1)在干旱区,当地下水位大于2m时,芦苇的光饱和点、表观量子效率、光呼吸速率随地下水位的下降显著降低,而水分利用效率却随着地下水位的下降呈上升趋势;随着地下水位的下降,芦苇的羧化效率、最大净光合速率、CO2饱和点显著下降,水分利用效率随之增大;1.6m地下水位的芦苇相对于2.7m地下水位下芦苇的最大净光合速率、光饱和点、光补偿点、暗呼吸速率分别下降19.98%、40.61%、37%、74.56%。(2)在盐胁迫下芦苇的光合参数整体低于受干旱胁迫的参数;盐胁迫下的芦苇水分利用效率大于受干旱胁迫环境。(3)与陆生环境下的芦苇相比,渍水条件下芦苇的最大净光合速率、光饱和点、光补偿点分别下降41.32%、14.56%、55.55%。研究表明,提高空气CO2供应量可降低水分利用效率从而缓解芦苇盐胁迫伤害,并且可通过调节气孔开闭程度显著增加水生芦苇最大净光合速率,这种变化是植物自身对环境因素变化的生理性反馈和适应性选择对策的结果。     

Net CO2 assimilation of cacao seedlings during periods of plant water deficit

The effect of leaf water potential () on net CO₂ assimilation rate (A), stomatal conductance (g), transpiration (E) and water-use efficiency (WUE) was measured for three cultivars of cacao (Theobroma cacao L.) seedlings during three recurrent drought cycles. Net assimilation varied greatly at high water potentials, but as dropped below approximately -0.8 and -1.0 MPa, A was reduced to less than 1.5 mol CO₂ m^-2 s^-1. The relation between g and A was highly significant and conformed to an asymptotic exponential model, with A approaching maximal values at stomatal conductances of 55–65 mmol H₂O m^-2 s^-1. Net assimilation varied linearly (r=0.95) with transpiration, and the slope of the A-E relation (WUE) was approximately 3.0 mol CO₂ mmol^-1 H₂O throughout the range of stomatal conductances observed. C _i was insensitive to water stress, even though both g and A were strongly affected. Under the experimental conditions used here, mesophyll photosynthesis did not appear to control g through changes in C _i. As stress intensified within each drying cycle, WUE of nonirrigated seedlings did not decline relative to that of controls even though CO₂ and water vapor exchange rates underwent large displacements. The effect of seed source was highly significant for WUE, and the basis for observed differences among genotypes is discussed.Abbreviations ABA Abscisic Acid     

Cool-season whole-plant gas exchange of exotic and native semiarid bunchgrasses

The success of invasive aridland plants may depend on their utilization of precipitation not fully exploited by native species, which could lead to seasonally altered ecosystem carbon and water fluxes. We measured volumetric soil water across 25-cm profiles (??_25cm) and springtime whole-plant water- and carbon-fluxes of the exotic Lehmann lovegrass (Eragrostis lehmanniana) and a native bunchgrass, bush muhly (Muhlenbergia porteri), following typical (55?mm in 2009) and El Ni?o-enhanced accumulations (154?mm in 2010) in a SE Arizona savanna. Across both years, ??_25cm was higher under lovegrass plots, with similar evapotranspiration (ET) between lovegrass and bush muhly plots. However, in 2010 transpiration (T) was higher in bush muhly than lovegrass, implying higher soil evaporation in lovegrass plots maintained similar ET. Net ecosystem carbon dioxide exchange (NEE) was similar between lovegrass and bush muhly plots in 2009, but was more negative in bush muhly plots following El Ni?o, indicating greater CO₂ assimilation. Ecosystem respiration (R _eco) and gross ecosystem photosynthesis (GEP) were similar between lovegrass and bush muhly plots in 2009, but were higher in bush muhly plots in 2010. As a result, lovegrass plots reduced ecosystem water-use efficiency (WUE_e?=?NEE/ET), while bush muhly WUE_e remained constant between 2009 and 2010. Concurrent whole-plant WUE (WUE_p?=?GEP/T) did not change in lovegrass plots, but increased in bush muhly plots between these years. We concluded that cool-season precipitation use is not a component of Lehmann lovegrass invasive success, but that the change in ET partitioning and attendant shifts in cool-season WUE_e may increase interannual variation in ecosystem water- and carbon-exchange dynamics in the water-limited systems it dominates.     

Responses of young ‘Pink lady’ apple to alternate deficit irrigation following long-term drought: growth, photosynthetic capacity, water-use efficiency, and sap flow

We studied photosynthetic capacity, growth, sap flow, and water-use efficiency in young trees of ‘Pink Lady’ apple (Malus domestica) that were exposed to 60 d of moisture stress. Three irrigation schemes were tested in the greenhouse: well-watered control; drought; or alternate deficit irrigation (ADI). Compared with the drought-stressed plants, those treated via ADI showed better height growth, larger scion diameters, and greater total leaf area, as well as significantly increased gains in dry biomass and rootstock diameters. However, their performance was still significantly lower than that demonstrated by continuously well-watered plants. Sap flow was greater under ADI than under drought, but less than under control conditions. The average rate of net photosynthesis, total amount of irrigation water applied, and dry biomass gain had highly significant and positive linear correlations with long-term water-use efficiency (WUE_L). The same was true between average stomatal conductance and WUE_L. By contrast, instantaneous water-use efficiency (WUE_I) was very significantly and negatively correlated with WUE_L. In addition, values for WUE_L were much higher from well-watered plants when compared with either drought-stressed trees or those treated per ADI. Therefore, our results indicate that, although ‘Pink Lady’ apple normally has high WUE, it still consumes a large amount of water. Therefore, the practice of ADI following a period of long-term drought could be used to improve growth and WUEL by this cultivar.     

The relationship between the soil water storage and water-use efficiency of seven energy crops

The aim of this work was to determine two types of photosynthetic water-use efficiency in order to examine their utility as selection criteria for tolerance of energy crops to soil water deficit. Furthermore, effects of crop cultivation on soil water content and storage were investigated. Seven energy crops were examined: miscanthus, prairie cordgrass, willow, thorn-free rose, Virginia mallow, Bohemian knotweed, and topinambour. The highest values of instantaneous (WUE) and intrinsic (WUE_i) water-use efficiencies were found for miscanthus and prairie cordgrass. The reduction of WUE and/or WUE_i was caused mainly by a rapid rise in the transpiration rate and a greater stomatal conductance, respectively. Principal component analysis showed that neither WUE nor WUE_i could be recommended as universal selection criteria for the drought tolerance in different energy crops. The proper localization of soil with a good supply of water is most the important condition for energy crop plantations.     

Photosynthesis Parameters in Two Cultivars of Mulberry Differing in Salt Tolerance

Three-month-old mulberry (Morus alba L.) cultivars (salt tolerant cv. S1 and salt sensitive cv. ATP) were subjected to different concentrations of NaCl for 12 d. Leaf area, dry mass accumulation, total chlorophyll (Chl) content, net CO₂ assimilation rate (P _N), stomatal conductance (g _s), and transpiration rate (E) declined, and intercellular CO₂ concentration (C _i) increased. The changes in these parameters were dependent on stress severity and duration, and differed between the two cultivars. The tolerant cultivar showed a lesser reduction in P _N and g _s coupled with a better C _i and water use efficiency (WUE) than the sensitive cultivar.     

Canopy and leaf photosynthetic characteristics and water use efficiency of sweet sorghum under drought stress

The objective of this paper was to examine the relationship between Water Use Efficiency (WUE) at the canopy and leaf levels, to determine soil moisture conditions, which can optimize yield, and WUE of sweet sorghum (Sorghum bicolor (Linn.) Moench), thus providing some theoretical foundation for using marginal land effectively and developing production of sweet sorghum. Three levels of soil moisture conditions were established, and photosynthetic characteristics and yield were measured. The canopy apparent photo-synthetic rate (CAP) and leaf photosynthetic rate (P _N) were reduced gradually with increased drought stress, and the CAP was lower than the P _N under every soil moisture conditions. The P _N had a midday depression phenomenon, but the CAP did not exhibit this midday depression phenomenon under severe drought stress. The linear regression relationship of CAP and P _N was CAP = 1.5945 + 0.1496 P _N. The canopy apparent WUE_C and leaf WUE_L were the highest under moderate drought stress. The first was 5.3 and 5.8 times higher than the WUE_L in mid-July and late August, respectively. The stem fresh biomass yield was 77 tons/ha under moderate drought stress and WUE of aboveground biomass yield (WUE_B) was also the highest. Our results showed that moderate drought stress did not result in a significant reduction in biomass yield but increased WUE significantly.     

Effects of Water-Deficit Stress on Photosynthesis, Its Components and Component Limitations, and on Water Use Efficiency in Wheat (Triticum aestivum L.)

It is of theoretical as well as practical interest to identify the components of the photosynthetic machinery that govern variability in photosynthesis rate (A) and water-use efficiency (WUE), and to define the extent by which the component processes limit A and WUE during developing water-deficit stress. For that purpose, leaf exchange of CO₂ and H₂O was determined in two growth-chamber-grown wheat cultivars (Triticum aestivum L. cv TAM W-101 and cv Sturdy), and the capacity of A was determined and broken down into carboxylation efficiency (c.e.), light- and CO₂-saturated A, and stomatal conductance (g_s) components. The limitations on A measured at ambient CO₂ concentration (A₃₅₀) were estimated. No cultivar difference was observed when A₃₅₀ was plotted versus leaf water potential (Ψ_w). Light- and CO₂-saturated A, c.e., and g_s decreased with decreasing leaf Ψ_w, but of the corresponding photosynthesis limitations only those caused by insufficient c.e. and g_s increased. Thus, reduced stomatal aperture and Calvin cycle activity, but not electron transport/photophosphorylation, appeared to be major reasons for drought stress-induced inhibition of A₃₅₀. WUE measured as A₃₅₀/g_s first increased with stomatal closure down to a g_s of about 0.25 mol H₂O m⁻² s⁻¹ (Ψ_w = −1.6 MPa). However, it was predicted that A₃₅₀/g_s would decrease with more severe stress due to inhibition of c.e.     

科尔沁草甸生态系统水分利用效率及影响因素

生态系统水分利用效率(WUE)是衡量碳水循环耦合程度的重要指标。利用科尔沁温带草甸草地碳水通量观测数据,对该生态系统总初级生产力水分利用效率(WUEGPP)的日季变化规律及对环境和生理因子的响应进行分析。结果表明:(1)WUEGPP日变化呈下降-稳定-上升的变化趋势,最大值出现在日出后1—2 h,阴天条件下WUEGPP高于晴天,生长中期WUEGPP高于生长初期和末期;(2)总初级生产力、总蒸散和WUEGPP季节变化均呈夏季高、春秋低的形式,生长季平均值分别为0.57 mg m-2s-1、0.08 g m-2s-1和5.97 mg/g,最大值分别为1.49 mg m-2s-1、0.16 g m-2s1和13.62 mg/g;(3)总初级生产力与饱和差、气温和叶面积指数均呈二次曲线关系,与冠层导度呈对数曲线关系;总蒸散与气温呈二次曲线关系,与饱和差、叶面积指数和冠层导度相关性均不显著;(4)WUEGPP与饱和差、气温和叶面积指数均呈二次曲线关系,与冠层导度呈对数曲线关系,饱和差、冠层导度和叶面积指数分别为2.0 k Pa、0.0015 m/s和4.2是控制WUEGPP增加的阈值;(5)净生态系统生产力水分利用效率(WUENEP)和净初级生产力水分利用效率(WUENPP)季节变化规律与WUEGPP一致,均值分别为3.47和5.47 mg/g。     

Photosynthesis, water use efficiency and stable carbon isotope composition are associated with anatomical properties of leaf and xylem in six poplar species

Although fast‐growing Populus species consume a large amount of water for biomass production, there are considerable variations in water use efficiency (WUE) across different poplar species. To compare differences in growth, WUE and anatomical properties of leaf and xylem and to examine the relationship between photosynthesis/WUE and anatomical properties of leaf and xylem, cuttings of six poplar species were grown in a botanical garden. The growth performance, photosynthesis, intrinsic WUE (WUE_i), stable carbon isotope composition (δ¹³C) and anatomical properties of leaf and xylem were analysed in these poplar plants. Significant differences were found in growth, photosynthesis, WUE_i and anatomical properties among the examined species. Populus cathayana was the clone with the fastest growth and the lowest WUE_i/δ¹³C, whereas P. × euramericana had a considerable growth increment and the highest WUE_i/δ¹³C. Among the analysed poplar species, the highest total stomatal density in P. cathayana was correlated with its highest stomatal conductance (g_s) and lowest WUE_i/δ¹³C. Moreover, significant correlations were observed between WUE_i and abaxial stomatal density and stem vessel lumen area. These data suggest that photosynthesis, WUE_i and δ¹³C are associated with leaf and xylem anatomy and there are tradeoffs between growth and WUE_i. It is anticipated that some poplar species, e.g. P. × euramericana, are better candidates for water‐limited regions and others, e.g. P. cathayana, may be better for water‐abundant areas.     

Water use efficiency of a maize/cowpea intercrop on a highly acidic tropical soil as affected by liming and fertilizer application

Due to global warming, there is a need to increase the water use efficiency of crops under rainfed agriculture, particularly in semi-arid regions. Therefore, the effect of NPK fertilizer application (with or without liming) on the water use efficiency of a maize/cowpea intercropping system was investigated in the semi-arid part of Brazil. The crops were grown on a strongly acidic, sandy soil with three treatments: (i) Complete NPK fertilizer application with lime (Compl), (ii) Complete NPK fertilizer application without lime (Compl-L) and (iii) Control. On the average, dry matter production was 2.6 times higher with the Compl treatment than in the Control and 1.6 times higher than in the Compl-L treatment. The soil water balance was calculated with two different model approaches (HILLFLOW and EPICSEAR). When checked against measured soil water content during the growing period, both models produced accurate results, but only EPICSEAR was sensitive to the effects of liming and fertilizer application on soil water balance and dry matter production at this site. Comparison between the Compl and the Compl-L treatments shows that the increase in transpirational water use efficiency (WUE_T) (+63 and +80%, respectively) is mainly due to the application of NPK. Although the site is highly acid, liming was of minor importance for increasing the WUE_T. However, observations and simulations demonstrate that, through the additional application of lime, the gross water use efficiency (WUE_C) in a maize/cowpea intercropping system can be increased by 60% compared to sole application of NPK and by more than 160% compared to the control. Abbreviations: EPIC – erosion productivity impact calculator; EPICSEAR – erosion productivity impact calculator for semi-arid regions; TDR – time domain reflectometry; WUE – water use efficiency.     

Chloride as a macronutrient increases water‐use efficiency by anatomically driven reduced stomatal conductance and increased mesophyll diffusion to CO2

Chloride (Cl^?) has been recently described as a beneficial macronutrient, playing specific roles in promoting plant growth and water‐use efficiency (WUE). However, it is still unclear how Cl^? could be beneficial, especially in comparison with nitrate (NO₃^?), an essential source of nitrogen that shares with Cl^? similar physical and osmotic properties, as well as common transport mechanisms. In tobacco plants, macronutrient levels of Cl^? specifically reduce stomatal conductance (g_s) without a concomitant reduction in the net photosynthesis rate (A_N). As stomata‐mediated water loss through transpiration is inherent in the need of C₃ plants to capture CO₂, simultaneous increase in photosynthesis and WUE is of great relevance to achieve a sustainable increase in C₃ crop productivity. Our results showed that Cl^?‐mediated stimulation of larger leaf cells leads to a reduction in stomatal density, which in turn reduces g_s and water consumption. Conversely, Cl^? improves mesophyll diffusion conductance to CO₂ (g_m) and photosynthetic performance due to a higher surface area of chloroplasts exposed to the intercellular airspace of mesophyll cells, possibly as a consequence of the stimulation of chloroplast biogenesis. A key finding of this study is the simultaneous improvement of A_N and WUE due to macronutrient Cl^? nutrition. This work identifies relevant and specific functions in which Cl^? participates as a beneficial macronutrient for higher plants, uncovering a sustainable approach to improve crop yield.