PHOTOINHIHITION OF PHOTOSYSTEM-I - ITS PHYSIOLOGICAL SIGNIFICANCE IN THE CHILLING SENSITIVITY OF PLANTS

Photoinhibition was defined originally as the decrease in photosynthet ic activity that occurs upon excess illumination. The site of photoinh ibition has generally been considered to be located in PSII. However, a novel type of photoinhibition has recently been characterized in chi lling-sensitive plants. This photoinhibition occurs under relatively w eak illumination at chilling temperatures and the main site of damage is in PSI. The photoinhibition of PST is initiated by the inactivation of the acceptor side, with the subsequent destruction of the reaction center and the degradation of the product of the psaB gene, which is one of the two major subunit polypeptides of the PSI reaction center c omplex. Chilling and oxidative stress (the presence of reactive specie s of oxygen) are characteristic requirements for the photoinhibition o f PSI in vivo.