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The role of nitric oxide (NO) in photosynthesis is poorly understood as indicated by a number of studies in this field with often conflicting results. As various NO donors may be the primary source of discrepancies, the aim of this study was to apply a set of NO donors and its scavengers, and examine the effect of exogenous NO on photosynthetic electron transport in vivo as determined by chlorophyll fluorescence of pea (Pisum sativum) leaves. Sodium nitroprusside-induced changes were shown to be mediated partly by cyanide, and S-nitroso-N-acetylpenicillinamine provided low yields of NO. However, the effects of S-nitrosoglutathione are inferred exclusively by NO, which made it an ideal choice for this study. Q(A)(-) reoxidation kinetics show that NO slows down electron transfer between Q(A) and Q(B), and inhibits charge recombination reactions of Q(A)(-) with the S(2) state of the water-oxidizing complex in photosystem II. Consistent with these results, chlorophyll fluorescence induction suggests that NO also inhibits steady-state photochemical and nonphotochemical quenching processes. NO also appears to modulate reaction-center-associated nonphotochemical quenching.