Absorption at 505 nm and high-performance liquid chromatography showed that light-induced conversion of violaxanthin to antheraxanthin and zeaxanthin was much stronger in leaves of nitrate- than in leaves of ammonium-grown Phaseolus vulgaris. Feeding ascorbate via the petiole increased zeaxanthin formation in ammonium-grown plants. However, there was no difference in energy quenching, qE, or photoinhibition (measured as FV/FM), as determined by chlorophyll fluorescence in nitrate- and ammonium-grown plants. Dithiothreitol decreased the light-induced stimulation of zeaxanthin formation and increased photoinhibition in nitrate-grown plants, suggesting that these plants utilize zeaxanthin for the protection of photosystem II (PSII). Ammonium-grown plants seem to have established an alternative way to protect PSII.