Improve Water Use Efficiency Research Articles

Improved Water Use Efficiency Associated with Cultivars and Agronomic Management in the North China Plain

Both winter wheat (Triticum aestivum L.) and maize (Zea mays L.) are the two staple crops of the North China Plain (NCP) that are combined in a single‐year rotation. While annual evapotranspiration increased slightly, field studies conducted at Luancheng Station indicated that crop yield improved by 50% and resulted in significant water use efficiency (WUE) increases from 1982 to 2002. Water use efficiency has improved from 10 to 15 kg mm−1 ha−1 for winter wheat and from 14 to 20 kg mm−1 ha−1 for maize in the Piedmont of Mt. Taihang in the NCP. Yield increase was associated with the increase in kernel numbers per unit area without alteration of the weight of the kernels for both winter wheat and maize. Number of kernels per spike of winter wheat was increased from about 22 for cultivars used in 1980s to about 28 for cultivars used presently. Number of kernels per ear of maize was increased from about 300 for cultivars used in 1980s to about 450 presently. From the early 1990s, combine had been used to harvest winter wheat, allowing straw mulch to be applied to maize. Measurements of WUE from 1987 to 1992 and again from 1997 to 2002 showed that WUE of maize under mulch was significantly higher than that without mulch. Mulching reduced soil evaporation loss by 40 to 50 mm per annum measured by microlysimeters, and WUE was averagely improved 8 to 10% for the 12 seasons. An improvement in irrigation scheduling had also improved WUE. Irrigation applications to winter wheat were reduced from about eight times in 1980s to about four times presently. Field tests from 1999 to 2004 still showed that reducing the present number of seasonal wheat irrigations to either three, two, or one depending on seasonal rainfall would benefit both grain production and WUE of winter wheat.

Silicon Improves Water Use Efficiency in Maize Plants

The influence of silicon (Si) on water use efficiency (WUE) in maize plants (Zea mays L. cv. Nongda108) was investigated and the results showed that plants treated with 2 mmol L−1 silicic acid (Si) had 20% higher WUE than that of plants without Si application. The WUE was increased up to 35% when the plants were exposed to water stress and this was accounted for by reductions in leaf transpiration and water flow rate in xylem vessels. To examine the effect of silicon on transpiration, changes in stomata opening were compared between Si-treated and nontreated leaves by measuring transpiration rate and leaf resistance. The results showed that the reduction in transpiration following the application of silicon was largely due to a reduction in transpiration rate through stomata, indicating that silicon influences stomata movement. In xylem sap of plants treated with 2 mmol L−1 silicic acid, the Si concentration was 200-fold higher, while the Ca concentration which is mainly determined by the transpiration rate, was 2.5-fold lower than that of plants grown without Si. Furthermore, the water flow rate in xylem vessels of plants with and without Si was compared. Flow rate in plants with 2 mmol L−1 Si was 20% lower than that without Si, which was accounted for by the increased affinity for water in xylem vessels induced by silica deposits. These results demonstrated the role of Si in improving WUE in maize plants.

Control of stomatal aperture and carbon uptake by deficit irrigation in two grapevine cultivars

Improvements in water use efficiency of crops are essential under the scenarios of water scarcity predicted by global change models for the Mediterranean region. In recent years, deficit irrigation, including partial root drying (PRD), has been proposed as an irrigation technique to improve water use efficiency and standardize grapevine yield and quality. The objective of this study was to evaluate the impact of deficit irrigation on photosynthetic responses of field grown grapevines of cv. Moscatel and Castelão. The treatments were: full irrigation (FI), corresponding to 100% ETc; non-irrigated, but rain fed (NI) and partial root zone drying (PRD) and deficit irrigation (DI), both corresponding to an irrigation of 50% ETc. While in the DI treatment water was applied to both sides of the root system, in the PRD treatment, water was supplied to only one side of the root system, alternating sides periodically. In both cultivars, PRD and DI vines showed intermediate pre-dawn leaf water potential (ψpd) values (around −0.4 MPa) while FI vines, showed ψpd around −0.2 MPa during the growing season. NI showed the lowest ψpd, reaching −0.6 MPa in Moscatel and −0.8 MPa in Castelão, at the end of growing season. Water status of PRD vines remained closer to FI than DI, especially at midday. Photosynthetic rates and fluorescence parameters of the deficit irrigation treatments (PRD and DI) did not show significant differences from FI vines, for most of the season. In Moscatel, although PRD generally showed gs, relative stomatal limitation (RSL) and intrinsic water use efficiency (A/gs) values closer to NI than DI, most of the differences between PRD and DI were not statistically significant. Our results showed a stability of the photosynthetic machinery in grapevines under low-to-moderate water availabilities, as demonstrated by the maintenance of the activity of three Calvin Cycle enzymes and of the Vcmax values. However, a decline in Jmax was observed in NI vines, that can be a result either of a decrease in ATP production or, alternatively, of decreased mesophyll conductance to CO2 diffusion. In general terms, stomatal limitation of photosynthesis is likely to be dominant in non-irrigated plants. Deficit irrigation had no negative impact on CO2 assimilation, despite less water application than in full-irrigation. Differences among varieties may be related to differences in sensitivity of stomata, shoot growth and/or the interaction between rootstock and cultivar to soil water availability.

Water use efficiency and profitability on an irrigated dairy farm innorthern Victoria: a case study

A study was conducted to look at water use efficiency and profitability on an irrigated dairy farm in northern Victoria. For this case study farm, an economic evaluation was performed for various development options that could result in higher water use efficiency, increased profit, and meet the farm owners' objectives. Water use efficiency was defined as the amount of milk (kg milk fat plus protein) produced from pasture per megalitre of water (irrigation plus effective rainfall).The case study farm data indicated that between 1995–96 and 1998–99 there was no simple, direct association between water use efficiency and profitability. The development options considered included building a new dairy and increasing herd size and either increasing the area of irrigated pasture or intensifying on the existing irrigated area. The likely water use efficiency and economic efficiency were estimated for the various development options. The development budgets suggested that intensifying on the existing irrigated land was the most attractive option, if the predicted improvements in water use efficiency were achieved. This option had an internal rate of return of 64%, broke even after 5 years and was compatible with the objectives of the farm owners.Options that resulted in simultaneous increases in water use efficiency, profitability and labour efficiency appear to be more likely to be adopted than options that focus solely on increasing water use efficiency.

Estimating and mapping deep drainage risk at the district level in the lower Gwydir and Macquarie valleys, Australia

In the Murray–Darling Basin, irrigated agriculture, which produces rice, dairy, cotton and citrus, is a large consumer of water resources. Effective management of the water resource is therefore important to ensure sustainability of irrigated agriculture. In the lower Gwydir and Macquarie valleys, respectively located in northern and central New South Wales of Australia, extensive irrigated-cotton production is an important contributor to the nation’s export earnings. However, there are problems of excessive deep drainage (DD) in these regions. To address them requires soil and water quality information, but there is little quantitative information to plan for and implement improved water use efficiency. In this paper, we explore methods that could efficiently generate data on natural resources. First, we carried out an electromagnetic induction (EM38) survey to characterise broad soil profile types in the Ashley (lower Gwydir valley) and Trangie (lower Macquarie valley) districts. From the resulting apparent electrical conductivity (ECa, mS/m) data collected using an EM 38 (vertical mode of operation), soil profile sites were selected and sampled, followed by laboratory analysis carried to determine exchangeable cations and clay content. The soil data collected were analysed with a salt and leaching fraction (SaLF) model, based on specific water quality and quantity parameters, such as electrical conductivity of irrigation water (ECiw, dS/m) and rainfall (R, mm/year). Various water application rates (I) were also considered, to simulate irrigated cotton (I = 600 mm/year) and rice production (I = 1200 mm/year) as well as shallow water reservoirs (I = 1800 mm/year). For each irrigation scenario, DD values (mm/year) were estimated. An exponential function was used to describe the relationships between ECa values obtained with the EM38 and estimated DD. These relationships were then used to estimate DD at each of the EM38 survey sites, whereupon cut-off (zc) values were used for indicator transforms of the data. Using indicator kriging (IK) and various irrigation scenarios, we demonstrate the usefulness of this approach in identifying areas of high risk of DD exceeding various cut-off values (zc = 50, 75, 100 and 200 mm/year). Thus, we show where improvements in water-use efficiency could be achieved in the irrigated cotton growing districts of Ashley and Trangie.

Sugarcane water use from shallow water tables: implications for improving irrigation water use efficiency

The resource potential of shallow water tables for cropping systems has been investigated using the Australian sugar industry as a case study. Literature concerning shallow water table contributions to sugarcane crops has been summarised, and an assessment of required irrigation for water tables to depths of 2 m investigated using the SWIMv2.1 soil water balance model for three different soils. The study was undertaken because water availability is a major limitation for sugarcane and other crop production systems in Australia and knowledge on how best to incorporate upflow from water tables in irrigation scheduling is limited. Our results showed that for the three soils studied (representing a range of permeabilities as defined by near-saturated hydraulic conductivities), no irrigation would be required for static water tables within 1 m of the soil surface. Irrigation requirements when static water tables exceeded 1 m depth were dependent on the soil type and rooting characteristics (root depth and density). Our results also show that the near-saturated hydraulic conductivities are a better indicator of the ability of water tables below 1 m to supply sufficient upflow as opposed to soil textural classifications. We conclude that there is potential for reductions in irrigation and hence improvements in irrigation water use efficiency in areas where shallow water tables are a low salinity risk: either fresh, or the local hydrology results in net recharge.

Global estimates of water withdrawals and availability under current and future “business-as-usual” conditions

New global models provide the opportunity to generate quantitative information about the world water situation. Here the WaterGAP 2 model is used to compute globally comprehensive estimates about water availability, water withdrawals, and other indicators on the river-basin scale. In applying the model to the current global water situation, it was found that about 24% of world river basin area has a withdrawal to availability ratio greater than 0.4, which some experts consider to be a rough indication of “severe water stress”; the impacts of this stress are expected to be stronger in developing countries than in industrialized ones. Under a “business-as-usual” scenario of continuing demographic, economic and technological trends up to 2025, water withdrawals are expected to stabilize or decrease in 41% of world river basin areas because of the saturation of water needs and improvement in water-use efficiency. Withdrawals grow elsewhere because population and economic growth will lead to rising demand for water, and this outweighs the assumed improvements in water-use efficiency. An uncertainty analysis showed that the uncertainty of these estimates is likely to have a strong geographic variability.

Water resources and their use in food production systems

National scale assessments of global water scarcity predict that by 2050 as much as 66% of the world's future population will experience some water stress and that water requirements for food production will be a major issue. However, practical planning and management of water resources requires models with greater spatial and temporal resolution. Such models give improved (higher) estimates of the number of people at risk and how these might change as the population grows. Further improvements to these models require local/basin scale water resource data and, ultimately, the integration of relevant social, economic, institutional and legal information. An example of how progress might be made with this complex task of integration is given in the paper.¶ Although the above models are important for the identification and management of water resources issues, the crux of the food production issue lies in improving the efficiency with which food is produced from existing water and land resources. Water use efficiency1 can be increased in irrigated and rainfed systems using both physical and non-physical means. Substantial improvements are possible by maximizing transpiration and/or increasing the yield per unit of water transpired. The former is most practicable in systems with low water use efficiency and can be achieved mainly via managerial and/or institutional improvements, while the latter is more appropriate in systems with higher water use efficiency, via introducing agronomic and/or other technical improvements.¶There is also scope for improving water use efficiency and food production by considering larger catchment scales, where water can be recycled within irrigation schemes or moved between different rainfed parts of the landscape. This holistic approach to catchment water management allows the upstream/downstream impacts of improvements in agricultural water use efficiency to be taken into account and further emphasises the need for integration of both physical information and socio-economic factors if truly sustainable solutions are to be achieved. Future global change research, in both the physical and social sciences, has a major role to play in addressing this real and immediate challenge.¶¶ 1 Defined here as yield per unit area/water used to produce yield.

The Sustainable Agriculture Farming System Project in California's Sacramento Valley

The Sustainable Agriculture Farming Systems project (SAFS) was established in 1988 to study the transition from conventional to low-input and organic crop production practices. The project includes four-year crop rotations under conventional (conv-4), low-input, and organic management and a conventionally managed, two-year rotation (conv-2). Positive effects on soil quality resulting from low-input and organic management include increased soil organic matter, a reduction in soil-borne diseases, increased pools of P and K, higher microbial biomass and activity, an increase in mobile humic acids and increased water infiltration rates and soil water-holding capacity. Pesticide use in the low-input cropping system is about 25% of that used in the conventional systems. The most profitable farming system continues to be the conv-2 system due to the greater frequency of tomato in that rotation. Among the four-year rotations, the organic system, in which the produce commands premium prices, is the most profitable, although least profitable if premium prices are not applied. Information generated from SAFS research has been disseminated via videotape, workshops, annual field days, field tours, educational materials, peer-reviewed articles and an Internet homepage. Future challenges for the SAFS project include development of reduced-tillage and cover crop management strategies to optimize N availability following cash crops, weed management in organic and low-input systems, improvement of water-use efficiency in alternative systems and sequestration of C in the soil.

The effect of exogenous abscisic acid on stomatal development, stomatal mechanics, and leaf gas exchange in Tradescantia virginiana.

Gas exchange parameters and stomatal physical properties were measured in Tradescantia virginiana plants grown under well-watered conditions and treated daily with either distilled water (control) or 3.0 mM abscisic acid (ABA). Photosynthetic capacity (CO(2) assimilation rate for any given leaf intercellular CO(2) concentration [c(i)]) and relative stomatal sensitivity to leaf-to-air vapor-pressure difference were unaffected by the ABA treatment. However, at an ambient CO(2) concentration (c(a)) of 350 micromol mol(-1), ABA-treated plants operated with significantly lower c(i). ABA-treated plants had significantly smaller stomata and higher stomatal density in their lower epidermis. Stomatal aperture versus guard cell pressure (P(g)) characteristics measured with a cell pressure probe showed that although the form of the relationship was similar in control and ABA-treated plants, stomata of ABA-treated plants exhibited more complete closure at P(g) = 0 MPa and less than half the aperture of stomata in control plants at any given P(g). Scaling from stomatal aperture versus P(g) to stomatal conductance versus P(g) showed that plants grown under ABA treatment would have had significantly lower maximum stomatal conductance and would have operated with lower stomatal conductance for any given guard cell turgor. This is consistent with the observation of lower c(i)/c(a) in ABA-treated plants with a c(a) of 350 micromol mol(-1). It is proposed that the ABA-induced changes in stomatal mechanics and stomatal conductance versus P(g) characteristics constitute an improvement in water-use efficiency that may be invoked under prolonged drought conditions.

エブ＆フローかん水と培養液へのＮａＣｌ添加によるセル成型育苗の省力化とキャベツ苗品質の向上

To address the problem of uneven moisture conditions of the rootballs of cabbage plug seedlings grown with conventional mist irrigation, we trialed the use of ebb and flow irrigation. We also attempted to improve the quality of the seedlings by adding NaCl to the culture solution. We obtained the following results.1. With the ebb and flow irrigation system, 20% less working time was required than with conventional mist irrigation.2. Ebb and flow irrigation promoted growth uniformity of seedlings and improved growth after transplanting.3. Adding NaCl at a density of 0.3% to the culture solution encouraged greening of leaves and a decrease in the flat rate of plant appearance.4. When NaCl was added early, seedling growth was greatly depressed.5. Addition of NaCl 5 days before transplanting did not decrease yield.6. Plug seedlings exposed to added NaCl acquired drought tolerance, as shown by both improvement of water-use efficiency and the results of electrolyte leakage testing after drought treatment.7. Yield was higher with ebb and flow irrigation than with conventional mist irrigation.Use of an ebb and flow irrigation system reduced labor costs, improved the uniformity of seedlings, and enhanced seedling growth after transplanting. Furthermore, adding NaCl at 0.3% 5 days before transplanting improved the adaptability of seedlings to the automatic transplanter and improved their drought tolerance with-out decreasing cabbage yield.

Projecting U.S. freshwater withdrawals

As past attempts to forecast water use have shown, predicting the future is fraught with difficulty. Yet a lesser objective, to extend past trends into the future, can offer a useful look at what may lie ahead. Relying on U.S. Geological Survey water use data for the period 1960–1995, this paper projects U.S. water use based on trends in basic water use factors. Those trends are largely encouraging. Over the past 35 years, withdrawals in industry and at thermoelectric plants have steadily dropped per unit of output, withdrawals per acre have dropped in some irrigated regions, and per capita domestic withdrawals, after rising steadily from 1960 to 1990, dropped in 1995. If these trends continue, aggregate withdrawals in the United States over the next 40 years will stay within 10% of the 1995 level, despite economic growth and an expected 41% increase in population. This projection is in contrast to most previous projections of U.S. water use, which did not adequately account for future improvements in water use efficiency. Important qualifications to these projections are that some regions of the United States, especially the southeast, will experience above average increases and that the projections are for the average year and thus underestimate demands during dry years.

Morphological, phenological and water-use dynamics of sorghum varieties ( Sorghum bicolor) under Striga hermonthica infestation

Leaf number was unaffected due to Striga infestation in the course of crop development in susceptible and resistant sorghum cultivars. Striga effect on stem height of IS 9302 was significant particularly during flowering and grain filling periods. No significant reduction in stem height was observed in SRN 39. Leaf area index (LAI) of IS 9302 Striga infested plants was significantly lower during panicle initiation and flowering. In SRN 39 reduction in LAI started relatively late and was significant only at peak flowering and at final harvest. Specific leaf area (SLA) in both sorghum cultivars was unaffected by Striga infestation. However, the leaf weight ratio (LWR) in the Striga infested plants of both cultivars was increased. Both SLA and LWR indicate that the Striga effect on the assimilatory surface and on the portion of dry matter allocated to leaves was small. The change in allometric coefficient was not high in both Striga infested sorghum cultivars. Only about 14% more dry matter was allocated to roots in IS 9302 Striga infested plants. Both the dynamic changes in leaf morphology and allometric coefficient indicate that the Striga effect may not be mainly through the interference with water balance of the infested host plants. Under Striga infestation there was a delay of about 120 degree-days to reach booting stage in both sorghum cultivars. The delay in developmental shift from booting to flowering under Striga infestation was also more pronounced. Unlike SRN 39, most of IS 9302 Striga infested plants failed to flower. Possible reasons for this delay are discussed. Water use efficiency (WUE) of Striga infected IS 9302 sorghum plants were reduced by about 72% compared to the control. The reduction in WUE of SRN 39 was about 24%. However, the evapotranspiration of Striga infected IS 9302 plants was even larger than the control and almost equal in the SRN 39 infected plants. This implies that dry matter accumulation was reduced, thereby reducing WUE significantly. In this case, the resistant cultivar SRN 39 showed less reduction in LAI, no more dry matter partitioning to roots and a significant improvement in WUE compared to IS 9302 under Striga infestation.

Water use efficiency and uptake patterns in a runoff agroforestry system in an arid environment

Water is the most limiting factor for plant production in arid to semiarid regions. In order to overcome this limitation surface runoff water can be used to supplement seasonal rainfall. During 1996 we conducted a runoff irrigated agroforestry field trial in the Turkana district of Northern Kenya. The effects of two different Acacia saligna (Labill.) H. Wendl. tree planting densities (2500 and 833 trees per ha), tree pruning (no pruning vs. pruning) and annual intercrops (no intercrop vs. intercrop: Sorghum bicolor (L.) Moench during the first season and Vigna unguiculata (L.) Walp. during the second season) on water use were investigated. The annual crops were also grown as monocrops. Water consumption ranged from 585 to 840 mm during the first season (only treatments including trees). During the second season, which was shorter and the plants relied solely on stored water in the soil profile, water consumption was less than half of that during the first season. Highest water consumptions were found for non-pruned trees at high density and the lowest were found for the annual crops grown as monocrops. Tree pruning decreased water uptake compared to non-pruned trees but soil moisture depletion pattern showed complementarity in water uptake between pruned trees and annual intercrops. The highest values of water use efficiency for an individual treatment were achieved when the pruned trees at high density were intercropped with sorghum (1.59 kg m−3) and cowpea (1.21 kg m−3). Intercropping and high tree density increased water use efficiency in our runoff agroforestry trial. We ascribe the observed improvement in water use efficiency to the reduction of unproductive water loss from the bare soil.

Nutrient Management, Cultivar Development and Selection Strategies to Optimize Water Use Efficiency

Summary Cultivars can be selected for traits that improve water use efficiency, and fertilizers and other nutrient sources also can be managed to optimize water use efficiency of a crop. Both cultivar selection and nutrient management impact water use efficiency by altering photosynthetic rate, yield, rooting characteristics, transpiration, or soil evaporation. In order to optimize water use efficiency, cultivar and nutrient decisions should be made jointly. This integrated approach will lead to improvements in water use efficiency, with an increase in productivity and profitability per unit water.

Evaluation of Sorghum Stover and Tree Leaf Mulches for Sustainable Maize, Sorghum and Sunflower Cropping in a Semi-Arid Region of Zimbabwe

ABSTRACT The effect of sorghum stover and tree leaf mulches on seed yields and yield components of maize (Zea mays L.), sorghum (Sorghum bicolor L.), and sunflower (Helianthus annus L.) crops in tied ridges were studied under rainfed conditions at Chiredzi Research Station in 1993/94 and 1994/95 seasons. Mulching significantly increased maize seed and stover yields by 26% in 1993/94 and showed some improvement in total soil water content and water use efficiency over the control treatment in 1994/95 season. Seed yield of sunflower was significantly correlated (r = 0.55) to cumulative soil water content in 1994/95. Stover yield for sunflower was significantly correlated (r = 0.60) to cumulative soil water content in 1994/95. The results from this study suggest that sorghum stover and dry tree leaf mulches had significant beneficial effects on maize performance in a year with good and well distributed rainfall whereas for sorghum and sunflower, the benefits were higher in the drier year.

Water Requirement of Subsurface Drip-irrigated Corn in Northwest Kansas

Irrigation development during the last 50 years has led to overdraft in many areas of the large Ogallala aquifer in the central United States. Faced with the decline in irrigated acres, irrigators and water resource personnel are examining many new techniques to conserve this valuable resource. A three-year study (1989 to 1991) was conducted on a Keith silt loam soil (Aridic Argiustoll) in northwest Kansas to determine the water requirement of corn (Zea mays L.) grown using a subsurface drip irrigation (SDI) system. A dryland control and five irrigation treatments, designed to meet from 25 to 125% of calculated evapotranspiration (ET) needs of the crop were examined. Although cumulative evapotranspiration and precipitation were near normal for the three growing seasons, irrigation requirements were higher than normal due to the timing of precipitation and high evapotranspiration periods. Analysis of the seasonal progression of soil water revealed the well-watered treatments (75 to 125% of ET treatments) maintained stable soil water levels above approximately 55 to 60% of field capacity for the 2.4-m soil profile, while the deficit-irrigated treatments (no irrigation to 50% of ET treatments) mined the soil water. Corn yields were highly linearly related to calculated crop water use, producing 0.048 Mg/ha of grain for each millimeter of water used above a threshold of 328 mm. Analysis of the calculated water balance components indicated that careful management of SDI systems can reduce net irrigation needs by nearly 25%, while still maintaining top yields of 12.5 Mg/ha. Most of these water savings can be attributable to minimizing nonbeneficial water balance components such as soil evaporation and long-term drainage. The SDI system is one technology that can make significant improvements in water use efficiency by better managing the water balance components.

Improving the efficiency of water use by plant breeding and molecular biology

Plant breeding over the last century has indirectly increased water use efficiency of the major grain crops because yield has increased with no additional water use. These increases have been substantial in all major food crops. Improved varieties have come from conventional breeding programs where selection has been for yield. Although these increases are likely to continue, they may not occur at the same rate as before. Most of the increases have been due to improvements in harvest index which must now be approaching its theoretical limit in many of our major crops. There has been little increase in water use efficiency for biomass and this must be increased if improvements in water-use efficiency are to continue. Possibilities are numerous. They will depend on the crop and the environment in which it is grown. Modification of phenology to better suit the environment, which has been responsible for many of the increases in the past, will continue to be important in most crops. Genetic increases in water-use efficiency should also come from the manipulation of the crop canopy to reduce wasteful evaporation of water from the soil surface and from an improvement in transpiration efficiency. The most likely avenues for success are discussed.

Alfalfa leaf senescence induced by drought stress: photosynthesis, hydrogen peroxide metabolism, lipid peroxidation and ethylene evolution

Exchange rates of CO2 and H2O and metabolism of hydrogen peroxide have been measured in leaves of alfalfa ev. Aragón) under drought stress. The inhibitory effect of drought upon photosynthesis depended on the severity of the stress treatment. Leaf water potential (Ψleaf) down to,‐2.8 MPa reduced CO2 availability due to stomatal closure and inhibited the rate of photosynthesis. Leaf water potential lower than,‐2.8 MPa directly affected CO2 fixation, although CO2 was not limiting. Transpiration was more affected by stornatal closure than photosynthesis, which led to am apparent improvement in WUE (water use efficiency). Alfalfa leaves with Ψleaf lower than,‐2.0 MPa had an increased quantum requirement, probably due to the severe stress effect on photoenergetic reactions.Ethylene evolution from alfalfa leaves increased when they were subjected to Ψleaf of,‐ 1.6 MPa. Under more severe stress, the leaves showed low or almost no ethylene production. In parallel with the increase in ethyiene production, alfalfa leaves exhibited an increased membrane lipid peroxidation index (maloridialdehyde content) and an increased peroxide content. Superoxide disinutase activity (SOD; EC 1.15.1.1) was not affected by drought stress. Catalase (EC 1.11.1.6) was inhibited at slight stress, but significantly increased at a Ψleaf of ‐2.0 MPa. Peroxidase (EC 1.11.1.7) was progressively inhibited as drought stress developed. The possible implication of reactive O2 intermediates in drought stress‐induced senescence of alfalfa leaves is discussed in the light of the pattern of enzymatic scavenging systems.

Water‐Use Efficiency and Carbon Isotope Discrimination in Wheat

Drought stress is one of the most limiting factors to crop production in semiarid regions. Improved water‐use efficiency (WUE) could provide breeding programs with a means to improve adaptation to drought‐prone environments. Carbon isotope discrimination (Δ) has been proposed as a criterion to select for improved WUE. This study evaluates differences in total dry matter (TDM), WUE (TDM/transpiration), and Δ of flag leaves of landrace and modern wheat (Triticum aestivum L.) genotypes grown in pots in a well‐watered (wet) and in a drought‐stressed (dry) experiment for 2 yr. The wheat genotypes were also grown in wet and dry field conditions. In the pot experiments, average WUE of the genotypes varied from 2.85 to 4.41 g dry matter kg−1 water in the wet experiment and from 3.08 to 4.53 in the dry experiment. The mean values of Δ were 21.7 ✕ 1O−10 for the wet pot experiment and 21.6 ✕ 10−3 for the dry pot experiment. The tall landrace genotypes had greater TDM and WUE, but were later in maturity than the modern dwarf and semi‐dwarf genotypes. Values of Δ associated with different genotypes were negatively correlated with WUE. Broad‐sense heritabilities for WUE and Δ were 93 and 90%, respectively, in the pot experiments. In the dry field experiments, Δ was positively associated with above‐ground dry matter and grain yield in both years. In the wet field experiments, there was a positive correlation between Δ and grain yield only in 1988. Broad‐sense heritabilities for Δ were 72% for wet field conditions and 74% for dry field conditions. These observations indicate that selection for C isotope discrimination, under either wet or dry conditions, can improve water use efficiency in wheat.