SYMBIOTIC NITROGEN-FIXATION AND YIELD OF SOYBEAN FOLLOWING DEFOLIATION BY SOYBEAN LOOPER (LEPIDOPTERA, NOCTUIDAE) DURING POD OR SEED DEVELOPMENT

Acetylene reduction and nitrogen difference assays were used to evalua te effects of defoliation by soybean looper, Pseudoplusia includens (W alker), during reproductive growth stages on symbiotic nitrogen fixati on by soybean, Glycine max (L.) Merrill. Defoliation (74-94%) of group IV 'Clark' caused 80-100% reduction of ethylene produced per gram nod ule dry weight (nodule specific activity), regardless of whether defol iation occurred during bloom and pod development or during seed develo pment Defoliation (50-55%) of group VI cultivar 'Lee' from full bloom into pod development stages caused up to 69% reduction of nodule speci fic activity, whereas lower defoliation levels (32%) during seed devel opment did not affect this parameter. The nitrogen difference assay di d not identify reductions in the amount of nitrogen acquired from symb iotically fixed sources in response to defoliation except in 1 experim ent when 55% defoliation of Lee occurred during a period from full blo om into early seed development. However, this assay was useful for det ermining the influence of N acquired from soil and fixed sources on pl ant N status. Up to 68% of the N in nondefoliated nondulating isoline plants was acquired from fixed sources. Nitrogen deficiency symptoms ( chlorosis) were evident by early seed development, indicating that soi l N was becoming depleted that growth stage. In general, when averaged across cultivars and years, greater amounts N were acquired from fixe d sources by the early seed development stage (54%) than by bloom and pod development stages (35%). Yield of Clark was reduced by 800-1,359 kg/ha (74-94% defoliation, full bloom through pod development) and by 750 kg/ha (94% defoliation, early seed development). Yield of Lee was reduced by 1,492 kg/ha (50% defoliation, full bloom through pod develo pment) and by 971 kg/ha (32% defoliation, early-seed development).