Reproductive Development and Nitrogen Fixation in Soybean (Glycine max[L.] Merril)

Gomes, M. A. F. and Sodek, L. 1987. Reproductive development and nitrogen fixation in soybean (Glycine max (L.) Merril).—J. exp. Bot. 38: 1982–1987. Nitrogenase activity (acetylene reduction) was measured during the growth cycle of soybean plants induced to flower at two different ages. The decline in nitrogenase activity towards the end of the cycle was clearly associated with pod-filling for both flowering dates when plants were cultivated under lower light and temperature conditions (out of season). Under higher light and temperature conditions (normal growing season) the decline was independent of the flowering date. Furthermore, the timing of the decline was not altered when plants were maintained under long-day (vegetative) conditions nor when flowers were removed. It is suggested that under more favourable growth conditions the diversion of assimilates by the fruits is not the primary cause of the decline in nodule activity, but competition by the fruits may be important when the production of photo-assimilates is more limited.