Addition of nitrate to a suspension of NO3--depleted Chlorella vulgaris cells raised the O2-evolving capacity of the organism by 60%. The rate of O2-evolution under flash irradiation of the depleted cells was drastically reduced, which could be restored by addition of NO3-. The 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB)-insensitive O2-evolution, i.e., photosystem (PS) 2 activity of NO3--depleted cells, showed a 75% stimulation by addition of NO3-. PS1-mediated electron transport was also stimulated (50%) by addition of NO3-. Fluorescence yields of the NO3--depleted cells were significantly reduced. A normal fluorescence response was restored by the addition of NO3-. The fluorescence yield of the NO3--depleted and DCMU-treated-cells increased significantly after addition of NO3- ions, indicating a further reduction of the primary acceptor of PS2 (Q). In addition, the low temperature fluorescence emission spectra showed that energy transfer to PS2 and PS1 was much higher when nitrate was present. Hence nitrate accelerates the light-induced charge transfer from the intact O2-evolving system to the primary electron acceptor of PS2 and stimulates the PS1-mediated electron transport. and M. El-Anwar H. Osman, A. H. El-Naggar.