Photoinhibition of Chloroplast Reactions. II. Multiple Effects

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Abstract
Ultraviolet light inhibits the photoreduction of 2,6-dichloro-phenolindophenol or nicotinamide adenine dinucleotide phosphate with water as the electron donor (evolution of oxygen) but not the photoreduction of nicotinamide adenine dinucleotide phosphate with ascorbate as the electron donor. It inhibits photophosphorylation associated with either system. Experiments undertaken to test whether plastoquinone is the site of UV inhibition yielded inconclusive results. Visible light ( 420 m[mu]) causes the loss of all chloroplast activities, photosystem I being more sensitive than system n. The data suggests 2 modes of action for visible light. The one sensitized by system n results in damage resembling that of UV light. The other, sensitized by system I, results in the destruction of the reaction center of this system.