RAPID PROTEIN-TURNOVER IN PHOTOSYSTEM-2 - FUNDAMENTAL FLAW OR REGULATORY MECHANISM

Pea (Pisum sativum L.) leaves fully developed under either high or low irradiance were exposed for up to 30 h to dark or limiting saturating or excessive irradiance. The time courses of degradation of a populat ion of radioactively prelabelled D1 protein and the fluorescence param eter F(V)/F(M), measured on intact leaf discs at room temperature were studied. The D1 protein degradation was shown to be dependent on irra diance but not directly correlated with changes in photosynthetic capa city. The irradiance dependence of D1 protein degradation was differen t in the two types of leaves and it occurred at a slow but significant rate in the dark. The irradiance dependence of D1 degradation may be associated with the 3-dimensional structure of the thylakoid membrane and is suggested to play a key role in the regulation of radiant energ y absorption and photosynthetic capacity of the leaves.