GRANA STACKING AND PROTECTION OF PHOTOSYSTEM-II IN THYLAKOID MEMBRANES OF HIGHER-PLANT LEAVES UNDER SUSTAINED HIGH IRRADIANCE - AN HYPOTHESIS

We propose yet another function for the unique appressed thylakoids of grana stacks of higher plants, namely that during prolonged high ligh t, the non-functional, photoinhibited PS II centres accumulate as D1 p rotein degradation is prevented and may act as dissipative conduits to protect other functional PS II centres. The need for this photoprotec tive mechanism to prevent high D1 protein turnover under excess photon s in higher plants, especially those grown in shade, is due to conflic ting demands between efficient use of low irradiance and protection fr om periodic exposure to excessive irradiance.