Photosynthetic Control and Its Role in Protection of Photosystem I against Photoinhibition

Abstract

This review addresses photosynthetic control as a protective mechanism that prevents photoinhibition of photosystem I under conditions of imbalance between CO₂ assimilation during the Calvin–Benson–Bassham cycle and light reactions in the thylakoid photosynthetic apparatus. We discuss the pathways of photosystem I photoinhibition and describe protective mechanisms that prevent photodamage of photosystem I. We propose a hypothesis regarding the influence of photosynthetic control on formation of reactive oxygen species in photosystem I. pH-sensitivity of plastoquinol oxidation at the quinol-oxidizing (Qo) site of the cytochrome b₆f complex is analyzed, and function of two proton-conducting channels that release protons into the thylakoid lumen from the cytochrome b₆f complex is described. We examine impact of photosynthetic control on the functioning of the cytochrome b₆f complex itself, and propose a hypothesis regarding the preferential activation of photosynthetic control in the thylakoid grana, which ensures operation of the cyclic electron transport around photosystem I as a main protective mechanism.