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Acetylene reduction and nitrogen difference assays were used to evaluate effects of defoliation by soybean looper, Pseudoplusia includens (Walker), during reproductive growth stages on symbiotic nitrogen fixation by soybean, Glycine max (L.) Merrill. Defoliation (74-94%) of group IV 'Clark' caused 80-100% reduction of ethylene produced per gram nodule dry weight (nodule specific activity), regardless of whether defoliation occurred during bloom and pod development or during seed development. Defoliation (50-55%) of group VI cultivar 'Lee' from full bloom into pod development stages caused up to 69% reduction of nodule specific activity, whereas lower defoliation levels (32%) during seed development did not affect this parameter. The nitrogen difference assay did not identify reductions in the amount of nitrogen acquired from symbioticalIy fixed sources in response to defoliation except in I experiment when 55% defoliation of Lee occurred during a period from full bloom into early seed development. However, this assay was useful for determining the influence of N acquired from soil and fixed sources on plant N status. Up to 68% of the N in non defoliated nodulating isoline plants was acquired from fixed sources. Nitrogen deficiency symptoms (chlorosis) were evident by early seed development, indicating that soil N was becoming depleted by that growth stage. In general, when averaged across cultivars and years, greater amounts of N were acquired from fixed 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 development) and by 971 kgl1m (32% defoliation, early seed development).
Document Type: Research Article
Publication date: April 1, 1996
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Journal of Economic Entomology is published bimonthly in February, April, June, August, October, and December. The journal publishes articles on the economic significance of insects and is divided into the following sections: apiculture & social insects; arthropods in relation to plant disease; forum; insecticide resistance and resistance management; ecotoxicology; biological and microbial control; ecology and behavior; sampling and biostatistics; household and structural insects; medical entomology; molecular entomology; veterinary entomology; forest entomology; horticultural entomology; field and forage crops, and small grains; stored-product; commodity treatment and quarantine entomology; and plant resistance. In addition to research papers, Journal of Economic Entomology publishes Letters to the Editor, interpretive articles in a Forum section, Short Communications, Rapid Communications, and Book Reviews.