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In Argentina, wheat (Triticum aestivum L.) is cropped over a wide range of climatic conditions. Considerable variability in the ratio of dry weight produced per unit of transpired water, usually referred to as water‐use efficiency (WUE), is expected as variation in climatic factors affects photosynthesis and transpiration in different ways. Also, previous studies have shown that water supply limitations may affect WUE in wheat. The objective of this study was to quantify the effects of climatic environment and water availability on WUE in wheat crops. Six experiments were conducted at different locations of the Argentine wheat belt and crop dry weight and water use were measured in periods when water use was dominated by transpiration. Three of the experiments included both irrigated and rainfed treatments. Mean daily values of (i) pan evaporation, (ii) relative humidity, (iii) potential water use, and (iv) vapor pressure deficit, were used to find a general relationship that explained effects of the climatic environment on WUE. For experiments with high water availability, daytime vapor pressure deficit was better related to WUE than the other climatic factors. WUE was greater for experiments with water limitation, probably because stomatal closure to restrict transpiration rate occurred around midday when vapor pressure deficit was highest. As a consequence, relative dry weight under water limitation was not linearly related to relative water use as proposed in previous studies. A quadratic relationship that better represented this response was derived.

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Summary — The objective of this study was to determine the effect of water deficits during the reproductive period of an indeterminate soybean (Glycine max (L) Merr) crop on growth, seed yield and its components, and to establish whether the R 1 -R 4 or the R 4 -R 6.3 period was the most sensitive to drought. Group maturity III cultivar Asgrow 3127 was planted in the 1986—1987 and 1987-1988 growing seasons on a loamy soil (typic argiudol). The trial consisted of 3 deficiency treatments starting with 50% of soil-available water at the soil depth explored by the roots at the time of initiation, and with a control (II) constantly maintained at > 50% of soil available water. The treatment drought periods were: 0I) from R 1 to R 4 ; 10) from R 4 to R 6.3 ; and 00) from R 1 to R 4 and from R 4 to R 6.3 . Plots were individually irrigated and during the drought periods protected from rainfall with plastic covers. The control treatment (II) had greater dry matter production than 0I and 10, and these 2 treatments produced more dry matter than 00. Drought from R 1 to R 4 decreased leaf area production and drought from R 4 to R 6.3 accelerated leaf senescence. Leaf area index was related to percent solar radiation interception showing a critical LAI of = 5.5. Consequently, water deficits affected solar radiation interception. The reduction in PAR intercepted by the crop due to water deficits was no greater than 12%. The utilization efficiency of the intercepted PAR that ranged from 1.5-1.9 g per MJ was clearly affected by the drought. Drought during the first period affected the vegetative growth with little effect on seed yield. This effect was reflected on harvest index and on dry matter remobilization efficiency. On the other hand, the reduction in crop growth rate induced by drought during the R 4 -R 6.3 period was associated with important decreases in the number of reproductive structures per unit area as well as in total vegetative dry matter. The number of pods/ha was the yield component most affected by the treatments. For 0I, the number of seeds/pod and the weight of the seeds compensated the reduction in number of pods, relative to the control. water deficit / Glycine mex (L) Merr

purposes and to adopt management practices that allow WUE to be maximized. In Argentina, wheat (Triticum aestivum L.) is cropped over a wideIn Argentina, wheat is mainly cropped from 32 to range of climatic conditions. Considerable variability in the ratio of 39ЊS and 57 to 63ЊW (Hall et al., 1992). Considerable dry weight produced per unit of transpired water, usually referred to variability in climatic conditions and WUE is expected as water-use efficiency (WUE), is expected as variation in climatic factors affects photosynthesis and transpiration in different ways. throughout this zone. Several authors have proposed Also, previous studies have shown that water supply limitations may the use of environmental indexes to explain changes in affect WUE in wheat. The objective of this study was to quantify the WUE across environments. Some of the most frequently effects of climatic environment and water availability on WUE in used indexes were mean pan evaporation (de Wit, 1958), wheat crops. Six experiments were conducted at different locations mean relative humidity (Arkley, 1963), mean vapor of the Argentine wheat belt and crop dry weight and water use were pressure deficit (VPD) (Bierhuizen and Slatyer, 1965), measured in periods when water use was dominated by transpiration. and mean potential water use (Doorenbos and Kassam, Three of the experiments included both irrigated and rainfed treat-1979). Vapor pressure deficit is a widely used and conments. Mean daily values of (i) pan evaporation, (ii) relative humidity, cise index that explains changes in the ratio yield/water (iii) potential water use, and (iv) vapor pressure deficit, were used use (Angus and van Herwaarden, 2001). The effect of to find a general relationship that explained effects of the climatic VPD on WUE has been reported for corn (Zea mays environment on WUE. For experiments with high water availability, L.), sorghum [Sorghum bicolor (L.) Moench], potato daytime vapor pressure deficit was better related to WUE than the (Solanum tuberosum L.), alfalfa (Medicago sativa L.), other climatic factors. WUE was greater for experiments with water soybean [Glycine max (L.) Merr.] (Tanner and Sinclair, limitation, probably because stomatal closure to restrict transpiration rate occurred around midday when vapor pressure deficit was highest. 1983), and barley (Hordeum vulgare L.) (Monteith, As a consequence, relative dry weight under water limitation was not 1986). Comparisons of the abilities of different meteorolinearly related to relative water use as proposed in previous studies.