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 improve d cultivars for the southeastern Coastal Plain and better production system s aimed at ameliorating the effects of drought stress. The objective of thi s field study was to examine the effects of drought stress on both soybean branch vegetative growth and the distribution of seed yield and yield compo nents between the main stem and branches. Soybean was grown on an Eunola lo amy 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, a nd the percentage of total seed yield on the branches were highest with irr igation, followed by the in-row subsoiled/no deep tillage treatment and the no irrigation/no deep tillage treatment. Drought-stress treatment had no e ffect on mainstem seed yield. Branch seed number per square meter was highl y 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-stres s 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 br anch number per square meter (r = 0.995; P < 0.05). Most branch growth occu rred between initial flowering and the beginning of seed fill. Less associa tion was found between individual seed weight and seed yield from the mains tem or branch fractions. These data indicate that drought stress occurring between initial flowering and seed fill decreases total seed yield primaril y by reducing branch vegetative growth, which reduces branch seed number an d branch seed yield.