Growth and yield responses of two soybean cultivars to defoliation and water stress

While the effects of defoliation on soybean (Glycine max L.) growth and yield have been studied closely, the defoliation tolerance of soybean under drought conditions is not well understood. This study was aimed at determining the responses of determinate (Enrei) and indeterminate (Tousan 69) soybean cultivars to the combined stresses of defoliation and drought. Controlled irrigation rates and simulated insect defoliation were used on soybean grown in the greenhouse. Defoliation significantly reduced the decrease in soil water contents to the extent that defoliated plants had higher leaf water potentials than non-defoliated plants under drought conditions. The average seed yield of irrigated Enrei (197 g/sguare m) exceeded that of non-irrigated plants by 19%, while defoliation reduced seed yield by 50%. Also, the average seed yield of irrigated Tousan 69 (284.8 g/sguare m) exceeded that of non-irrigated plants by 39% and defoliation reduced yield by 14%. The responses of yields of the two cultivars to irrigation and defoliation indicated that the determinate cultivar, Enrei had higher drought tolerance than the indeterminate cultivar, Tousan 69. However, Tousan 69 was more tolerant to defoliation than Enrei. For the cultivar Enrei, defoliation caused a significant yield reduction with or without irrigation. Drought did not significantly affect the yield of defoliated Enrei, indicating that water application is not of prime importance to defoliated determinate soybean cultivars. On the other hand, defoliation did not affect the yield of Tousan 69 under the irrigated treatment, because of compensation for the defoliation effect via delayed leaf senescence and new leaf area expansion, which enhanced the light interception capacity of defoliated plant canopies. However, under drought condition, expansion in the area of new leaves after defoliation was markedly restricted and defoliation caused a significant yield reduction. This result indicates that water management is very important for increasing defoliation tolerance of indeterminate soybean cultivars.