PHOTOINACTIVATION AND PHOTOPROTECTION OF PHOTOSYSTEM-II IN NATURE

Photosystem II plays a central role not only in energy transduction, b ut also in monitoring the molecular redox mechanisms involved in signa l transduction for acclimation to environmental stresses. Central to t he regulation of photosystem II (PSII) function as a light-driven mole cular machine in higher plant leaves. is an inevitable photoinactivati on of one PSII after 10(6)-10(7) photons have been delivered to the le af. although the act of photoinactivation per se requires only one pho ton. PSII function in acclimated pea leaves shows a reciprocity betwee n irradiance and the time of illumination. demonstrating that the phot oinactivation of PSII is a light dosage effect, depending on the numbe r of photons absorbed rather than the rate of photon absorption. Hence , PSII photoinactivation will occur at low as well as high irradiance. There is a heterogeneity of PSII functional stability, possibly with less stable PSII monomers being located in grana margins and more stab le PSII dimers in appressed granal domains. Matching the inevitable ph otoinactivation of PSII, green plants have an intrinsic capacity for D 1 protein synthesis to restore PSII function which is saturated at ver y low light. Photoinhibition of PSII in vivo is often a photoprotectiv e strategy rather than a damaging process.