Effect of Multiple Factor Source-Sink Manipulation on Nitrogen and Carbon Assimilation by Soybean

Abstract

The effect of light enhancement and hastened reproductive development on N and dry matter accumulation by field-grown soybean (G. max [L.] Merr.) was studied. The impacts of photosynthate supply and reproductive development on change in the season-long profiles of in vivo leaf nitrate reductase (NR) activity and root nodule acetylene reduction (AR) activity were evaluated. Light enhancement resulted in significant increases in dry matter accumulation, root nodule fresh weight and AR activity. Seed yield was increased in both light enhanced treatments in 1978 and in one in 1979. Hastened flowering and seed development was accomplished through photoperiod manipulation within a single genotype. Seasonal decline in leaf NR activity was most rapid in plants entering reproductive development early. An early increase in root nodule fresh weight and AR activity was also observed in response to this treatment and was followed similarly by early decline. The addition of high levels of soil-applied N increased leaf NR activity and delayed late season decline in NR activity for both control and early reproductive plants. NO3- supply was therefore implicated as limiting to leaf NR activity during the decline associated with flowering and early seed development. A limited additional increase in leaf NR activity was observed in response to light enhancement plus soil-applied N. As no significant increase in leaf NR activity was observed in response to light enhancement alone, leaf NO3- supply was further implicated as more limiting to leaf NR activity than was photosynthate supply during flowering and early seed development.