Influence of photon flux density and fluctuation on the nitrogen fixing Glycine max (L Merr)- Bradyrhizobium japonicum symbiosis in a controlled environment

Summary — Nodulated soybeans, Glycine max (L Merr), were grown under 6 photon flux densities (PFD) from 0.351.03 mmol photons min order to determine the optimal light intensity for symbiotic nitrogen fixation in controlled environment. The nitrogenase activity (C reduction) was highest at 0.68 mmol photons m which also corresponded to the highest growth of plants and the biggest mass of nodules. The specific acetylene reducing activity was not affected by PFD, except at 1.03 mmol photons mwhere it was much lower. Decreasing the PFD had no effect on nitrogenase activity during the following 24 h. By contrast, an increase in PFD rapidly induced a significant decline of nitrogenase activity. At a similar PFD, the periodic light fluctuation was less beneficial than steady illumination for growth and nitrogen fixation. It is concluded that plant photosynthesis determines nodule mass which in turn determines the rate of nitrogen fixation. Nodule nitrogenase activity is not directly regulated by concurrent photosynthesis however, probably because of available photosynthate reserves and other regulatory mechanisms. Excess of light inhibits nitrogenase activity. nitrogenase / nodules / photosynthesis / Rhizobium / Glycine max = soybean