Is non photochemical quenching protective

Abstract

Plants exposed to high photon fluence are susceptible to photooxidative damage (photoinhibition). As photoinhibition is linear with the number of absorbed photons (reciprocity rule) it is generally thought that the antenna-based, non-photochemical quenching (qNP) represents an important protective strategy by lowering the excited singlet state levels in PSII antenna. We have investigated the relation between the lowering of the singlet excited state levels (quenching) and photoinhibition in thylakoids isolated from a variety of species (spinach, barley, Arabidopsis) and intact cells of the unicellular alga Chlamydomonas reinhardii. The results show that antenna quenching affords only a minimal protection against photoinhibition which is, at the most no more than 35% of that expected based on the reciprocity rule. Moreover the light-induced degradation of the reaction centre protein D1, generally considered to constitute an initial phase of the photoinhibition repair mechanism, is demonstrated to be insensitive to the excited state level in PSII antenna. The low level of protection afforded by antenna quenching is probably due to both photoinhibition and D1 degradation being caused by uncoupled or weakly coupled chlorophylls, an interpretation supported by the action spectra.