Reduction in N2 fixation by soybean in response to insect-induced defoliation

Nitrogen-fixing ability (measured by C2H2-C2H4 assay) of greenhouse-grown, vegetative soybean, Glycine max (L.) Merrill, decreased linearly in response to increased levels of defoliation by the soybean looper, Pseudoplusia includens (Walker). Acetylene reduction rate of control plants was 5-fold greater than that of plants with 77% defoliation. This decrease was the result of reduced number, weight, and efficiency (nodule-specific activity) of nodules. Nodule-specific activity was unaffected by defoliation of less than 47%, but decreased sharply as defoliation exceeded 47%