Drought-stress effects on branch and mainstem seed yield and yield components of determinate soybean

A better understanding of how drought stress affects soybean [Glycine mar (L.) Merr.] seed-yield determination would aid in the development of improved cultivars for the southeastern Coastal Plain and better production systems aimed at ameliorating the effects of drought stress. The objective of this field study was to examine the effects of drought stress on both soybean branch vegetative growth and the distribution of seed yield and yield components between the main stem and branches. Soybean was grown on an Eunola loamy sand in 1998 and 1999 with three levels of drought-stress treatment: (i) irrigation and no deep tillage, (ii) deep tillage but no irrigation, and (iii) no deep tillage or irrigation. Total seed yield, branch seed yield, and the percentage of total seed yield on the branches were highest with irrigation, followed by the in-row subsoiled/no deep tillage treatment and the no irrigation/no deep tillage treatment. Drought-stress treatment had no effect on mainstem seed yield. Branch seed number per square meter was highly correlated with branch seed yield (r = 0.994; P < 0.0001) and total seed yield (r = 0.989; P < 0.01) over both years and all levels of drought-stress treatment. A close relationship was found between branch seed number per square meter and branch dry weight at harvest maturity (r = 0.963; P < 0.05), final branch length per square meter (r = 0.994; P < 0.05), and final branch number per square meter (r = 0.995; P < 0.05). Most branch growth occurred between initial flowering and the beginning of seed fill. Less association was found between individual seed weight and seed yield from the mainstem or branch fractions. These data indicate that drought stress occurring between initial flowering and seed fill decreases total seed yield primarily by reducing branch vegetative growth, which reduces branch seed number and branch seed yield.