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 light, the non-functional, photoinhibited PS II centres accumulate as D1 protein degradation is prevented and may act as dissipative conduits to protect other functional PS II centres. The need for this photoprotective mechanism to prevent high D1 protein turnover under excess photons in higher plants, especially those grown in shade, is due to conflicting demands between efficient use of low irradiance and protection from periodic exposure to excessive irradiance.