Photosystem II and photoinhibition

Abstract

Plants harvest light energy and convert it into chemical energy. Light absorption by photosystems I and II (PSI and PSII) results in charge separations in their reaction centers (RCs), initiating a chain of redox reactions with PSI generating the reducing power for CO2 assimilation into sugars, and PSII oxidizing the ultimate electron donor, H2O, to O2. The rates of PSI and II must be balanced. If PSII releases more electrons than PSI can use, the intermediary pool of plastoquinone molecules and the primary acceptor quinone QA become fully reduced. PSII charge separation then leads to recombination, whereby the RC triplet state and hence singlet oxygen can be formed, causing damage to PSII that can decrease the photosynthetic yield. Photoinhibition results. Charge recombination to the triplet state may probably be avoided to some extent by a cyclic electron transfer (CET) pathway, short-circuiting the charge separation, which is somehow switched on when needed by cytochrome b559, an intrinsic subunit of PSII. Mechanism, quantitative significance, and even the very existence of CET continue to be subject to debate, however. This thesis describes studies on triplet generation, cytochrome b559, CET induction conditions, and other aspects of this elusive protection mechanism against photoinhibition.