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 increased physiological activity of lower, remaining leaves. The objective of this study was to test the ability of these and other mechanisms of defoliation tolerance to function in drought-stressed plants. Controlled irrigation rates and artificial defoliation treatments were used in a 2-yr study, Defoliation significantly increased soil-moisture percentage in both years, showing that defoliated stressed plants conserve more water than nondefoliated stressed plants. Compensatory regrowth was observed following defoliation, resulting in more leaves in defoliated plants than predicted. Removal of expanding leaves, however, reduced leaf area and also reduced growth potential relative to nondefoliated plants. Senescence of lower leaves was delayed in defoliated plants, as has been found in previous studies. The responses of leaf physiological rates to defoliation were similar in drought-stressed and well-irrigated plots. These data suggest that crop models integrating physiological parameters and defoliation rates could add or subtract the effect of drought stress as a constant or simple function.