EFFECT OF LEVEL AND TIMING OF MOISTURE STRESS ON SOYBEAN PLANT DEVELOPMENT AND YIELD COMPONENTS

Field studies were conducted in 1984 and 1985 using non-weighing lysimeters of 1 m2 to determine the effect of level of soil moisture stress (L) and timing of moisture stress (T) on reproductive phenology, plant characteristics, and yield components of Maple Amber soybean [Glycine max (L.) Merr.]. The study incorporated the most probable combinations of moisture stress in order to provide a basis for planning and managing irrigation for optimum soybean production. The effect of L was consistent over T and Year. Neither L nor T affected days to full bloom stage (R2). Increases in L linearly (r2 = 0.71) increased days to beginning seed stage (R5), but decreased (r2 = 0.93) days to full maturity stage (R8). L was related quadratically to plant height (r2 = 0.88) and number of pod-bearing nodes (r2 = 0.98), and linearly to number of pods (r2 = 0.75) and total dry matter (r2 = 0.91). No consistent effect of T on the timing of R5 to R8 stages was found, mainly because of the presence of a Year X T interaction (P = 0.05). Withholding irrigation during the R5 stage from previously adequately watered crops caused significant reduction in plant height (7%), number of pod-bearing nodes on the main stem (13%), number of pods (18%), number of seeds (20%), total seed weight (25%), and total dry matter (23%), indicating the importance of irrigation during this stage. It was concluded that, if irrigation water is limiting, withholding irrigation at R2 may be acceptable, but irrigation at R5 is essential for optimizing yield components and thus yield.