Are grana necessary for regulation of light harvesting?

The importance of the functional flexibility of the light-harvesting complexes of photosystem II (LHCII) in accommodating the fluctuation in the balance between light input and metabolic capacity in plants is emphasised. This flexibility is provided for by a relatively complex assembly of protein subunits, the interactions between them being controlled by protonation, xanthophyll deepoxidation and phosphorylation. It is suggested that the 3-dimensional order imposed upon this assembly of proteins by the grana is a vital aspect of the modulation of LHCII function. Grana establish the LHCII conformation needed for efficient light harvesting and help prevent the dense array of proteins from collapsing into a highly dissipative state. The grana then allow a controlled development of nonphotochemical quenching under the driving force of violaxanthin de-epoxidation and protonation. In plants grown under different irradiances the different grana content and xanthophyll cycle pool size together allow maximum quantum yield in limiting light and an appropriate level of non-photochemical quenching in excess light.