SOYBEAN PHYSIOLOGY, REGROWTH, AND SENESCENCE IN RESPONSE TO DEFOLIATION

Recent studies have shown that one component of soybean [Glycine max ( L.) Merr.] response to insect defoliation is delayed senescence and in creased physiological activity of lower, remaining leaves. The objecti ve of this study was to test the ability of these and other mechanisms of defoliation tolerance to function in drought-stressed plants. Cont rolled irrigation rates and artificial defoliation treatments were use d in a 2-yr study, Defoliation significantly increased soil-moisture p ercentage in both years, showing that defoliated stressed plants conse rve more water than nondefoliated stressed plants. Compensatory regrow th was observed following defoliation, resulting in more leaves in def oliated plants than predicted. Removal of expanding leaves, however, r educed leaf area and also reduced growth potential relative to nondefo liated plants. Senescence of lower leaves was delayed in defoliated pl ants, as has been found in previous studies. The responses of leaf phy siological rates to defoliation were similar in drought-stressed and w ell-irrigated plots. These data suggest that crop models integrating p hysiological parameters and defoliation rates could add or subtract th e effect of drought stress as a constant or simple function.