WATER DEFICITS DURING REPRODUCTIVE GROWTH OF SOYBEANS .1. THEIR EFFECTS ON DRY-MATTER ACCUMULATION, SEED YIELD AND ITS COMPONENTS

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 R1-R4 or the R4-R6.3 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: 01) from R1 to R4; l0) from R4 to R6.3; and 00) from R1 to R4 and from R4 to R6.3. 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 0l and l0, and these 2 treatments produced more dry matter than 00. Drought from R1 to R4 decreased leaf area production and drought from R4 to R6.3 accelerated leaf senescence. Leaf area index was related to percent solar radiation interception showing a critical LAI of almost-equal-to 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 R4-R6.3 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 0l, the number of seeds/pod and the weight of the seeds compensated the reduction in number of pods, relative to the control.