INFLUENCE OF PHOTON FLUX-DENSITY AND FLUCTUATION ON THE NITROGEN-FIXING GLYCINE-MAX (L MERR)-BRADYRHIZOBIUM-JAPONICUM SYMBIOSIS IN A CONTROLLED ENVIRONMENT

Nodulated soybeans, Glycine max (L Merr), were grown under 6 photon flux densities (PFD) from 0.35-1.03 mmol photons m-2.s-1, in order to determine the optimal light intensity for symbiotic nitrogen fixation in controlled environment. The nitrogenase activity (C2H2 reduction) was highest at 0.68 mmol photon m-2.s-1 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 m-2.s-1, where it was much lower. Decreasing the PFD had no effect of notrogenase activity during the following 24 h. By contrast in an increase in PFD rapidly induced a significant decline of nitorgenase 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.