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Mechanisms involving ammonium toxicity, excess light, and photosynthesis are scarcely known in plants. We tested the hypothesis that high NH 4 + supply in presence of high light decreases photosynthetic efficiency of rice plants, an allegedly tolerant species. Mature rice plants were previously supplied with 10 mM NH 4 + or 10 mM NO 3 − and subsequently exposed to 400 µmol m −2 s −1 (moderate light—ML) or 2000 µmol m −2 s −1 (high light—HL) for 8 h. HL greatly stimulated NH 4 + accumulation in roots and in a minor extent in leaves. These plants displayed significant delay in D1 protein recovery in the dark, compared to nitrate-supplied plants. These responses were related to reduction of both PSII and PSI quantum efficiencies and induction of non-photochemical quenching. These changes were also associated with higher limitation in the donor side and lower restriction in the acceptor side of PSI. This later response was closely related to prominent decrease in stomatal conductance and net CO 2 assimilation that could have strongly affected the energy balance in chloroplast, favoring ATP accumulation and NPQ induction. In parallel, NH 4 + induced a strong increase in the electron flux to photorespiration and, inversely, it decreased the flux to Rubisco carboxylation. Overall, ammonium supply negatively interacts with excess light, possibly by enhancing ammonium transport towards leaves, causing negative effects on some photosynthetic steps. We propose that high ammonium supply to rice combined with excess light is capable to induce strong delay in D1 protein turnover and restriction in stomatal conductance, which might have contributed to generalized disturbances on photosynthetic efficiency.