Low threshold levels of ultraviolet‐B in a background of photosynthetically active radiation trigger rapid degradation of the D2 protein of photosystem‐II

Summary The photosystem II reaction centre has at its core a heterodimer made up of two proteins, D1 and D2. The D1 protein is known to be rapidly degraded by photosyn‐thetically active radiation while the D2 protein is relatively stable. This paper reports that when the aquatic higher plant, Spirodela was exposed to ultraviolet‐B radiation, D2 degradation accelerated markedly and half life times approached those of the D1 protein. Moreover, in the presence of an environmentally relevant background of photosynthetically active radiation, low fluxes of ultraviolet‐B (but not ultraviolet‐A) radiation synergistically stimulated degradation of the D2 protein within functional reaction centres. Thus, above a critical threshold, ultraviolet‐B specifically targets the D1–D2 heterodimer for accelerated degradation.