Photodynamic Action by Endogenous Non-Chlorophyll Sensitizer As a Cause of Photoinhibition

As sunlight is not always optimized for every terrestrial plant in terms of light quality, quantity and duration, some plants suffer detrimental effects of sunlight exposure under certain conditions. Photoinhibition of photosynthesis is a typical phenomenon representing harmful light effects, commonly observed in many photosynthetic organisms. It is generally accepted that functional, structural loss of photosystem II complex (PS II) is the primary event of photoinhibition. Accumulating data also suggest that singlet oxygen ( 1 O2) is the main toxic species directly involved in it. There are two different views on the specific site and mechanism of 1 O 2 production in the photosynthetic membrane. One of them favors the PS II reaction center, where the primary charge pairs recombination occurs as a prerequisite for the generation of 1 O2, and the other inclines to photosensitized 1 O2 formation by a substance located outside PS II. This article describes how we, as the advocators of the latter concept, have arrived at the conclusion that 1 O2 immediately involved in PS II photodamage is largely generated from the Rieske center of the cytochrome b6/f complex and diffuses into PS II, attacking the reaction center subunits