The effect of ultraviolet light on photophosphorylation and the hill reaction

Abstract

Abstract 1. 1. Photophosphorylation and Hill-reaction activity of swiss-chard chloroplasts or chloroplast fragments were decreased by irradiation with short-wavelength ultraviolet light. 2. 2. Photophosphorylation was more resistant to irradiation than the Hill reaction. 3. 3. The presence of ascorbate during irradiation prevented the decrease in Hill reaction and photophosphorylative activity. Protection was more pronounced for the Hill reaction than for photophosphorylative activity. Compounds containing SH-groups could not replace ascorbate; it is suggested that ascorbate protects mostly by reducing the effective ultraviolet intensity, and in addition by exerting a specific effect on the chloroplasts themselves. 4. 4. By ultraviolet irradiation, it was possible to obtain chloroplast preparations which had lost the ability to perform the Hill reaction, but still possessed high photophosphorylative activity. Such preparations may be useful in further research. 5. 5. The endogeneous plastoquinone of chloroplasts was shown to be destroyed by irradiation. 6. 6. The activity of irradiated chloroplasts was stimulated by the addition of plastoquinone. However, a similar stimulation was obtained on addition of plastoquinone to non-irradiated chloroplasts.