Studies on soybean nodule senescence.

Soybean Glycine max. L. Merr. nodule senescence was studied using the loss of acetylene reduction by intact tap root nodules as its indication. Tap root nodules from two varieties (Calland and Beeson) of field-grown soybeans were used. The specific activities of nitrogenase (micromoles/minute gram fresh weight of nodules) as measured by the acetylene reduction assay decreased abruptly between 58 to 65 and 68 to 75 days after planting the Beeson and Calland soybeans, respectively. Major changes were not detected in dry weight, total nitrogen, and leghemoglobin levels during the period when in vivo nitrogenase activity declined. Ammonium levels in the cytosol of nodules and poly-beta-hydroxybutyrate increased moderately just prior to or coincidental with the loss of nitrogenase activity. Neither enzymes that have been postulated to be involved in ammonium assimilation nor NADP(+)-specific isocitrate dehydrogenase exhibited any large changes in specific activities during the initial period when nitrogenase activity declined.