Triplet State and Chlorophylls. In: Fong FK, editor. Light Reaction Path of Photosynthesis. Berlin: Springer Berlin Heidelberg; 1982. pp. 152-95. [DOI: 10.1007/978-3-642-81795-3_5]

Mechanism of photosystem II photoinactivation and D1 protein degradation at low light: the role of back electron flow

Light intensities that limit electron flow induce rapid degradation of the photosystem II (PSII) reaction center D1 protein. The mechanism of this phenomenon is not known. We propose that at low excitation rates back electron flow and charge recombination between the QB*- or QA*- semiquinone acceptors and the oxidized S(2,3) states of the PSII donor side may cause oxidative damage via generation of active oxygen species. Therefore, damage per photochemical event should increase with decreasing rates of PSII excitation. To test this hypothesis, the effect of the dark interval between single turnover flashes on the inactivation of water oxidation, charge separation and recombination, and the degradation of D1 protein were determined in spinach thylakoids. PSII inactivation per flash increases as the dark interval between the flashes increases, and a plateau is reached at dark intervals, allowing complete charge recombination of the QB*-/S2,3 or QA*-/S2 states (about 200 and 40 s, respectively). At these excitation rates: (i) 0.7% and 0.4% of PSII is inactivated and 0.4% and 0.2% of the D1 protein is degraded per flash, respectively, and (ii) the damage per flash is about 2 orders of magnitude higher than that induced by equal amount of energy delivered by excess continuous light. No PSII damage occurs if flashes are given in anaerobic conditions. These results demonstrate that charge recombination in active PSII is promoted by low rates of excitation and may account for a the high quantum efficiency of the rapid turnover of the D1 protein induced by limiting light.

Oscillations of reaction center II-D1 protein degradation in vivo induced by repetitive light flashes. Correlation between the level of RCII-QB- and protein degradation in low light

The D1 protein subunit of the photochemical reaction center II (RCII) turns over rapidly in oxygenic photosynthetic organisms exposed to the light. At high photon flux densities (PFD), photoinactivation of RCII precedes the degradation of the D1 protein. We found that the apparent quantum yield for the D1 protein degradation in Chlamydomonas cells is severalfold higher at low PFDs (10-100 mumol m-2 s-1) as compared to that observed at PFDs which induce photoinactivation of RCII (1.5-3 x 10(3) mumol m-2 s-1). Relative high levels of reduced RCII secondary plastoquinone acceptor, QB-, are induced in cells exposed to low PFDs as determined by thermoluminescence measurements. The probability of generating elevated levels of QB- which may recombine with the S2,3 oxidized states of the oxygen evolving complex decreases with increase in the light intensities at which consecutive double reduction of QB and exchange with the plastoquinone pool prevail. We have used light flashes to test if a correlation exists between the degradation of D1 protein and the relative level of QB-. D1 protein degradation could be induced in dark-incubated cells exposed to a series of 1.4 x 10(3) single light flashes given at intervals compatible with generation of elevated levels of QB- and its decay by charge recombination. Oscillations of the QB- level in cells exposed to 960-1440 series of 1 to several flashes correlated with oscillations of the D1 protein degradation in Chlamydomonas cells and in the Scenedesmus wild type but not in the LF-1 mutant lacking photosystem II donor side activity. In this mutant the "S state cycle" and QB- oscillations are abolished. We propose that the process of recombination of long lived RCII-QB- with the S2,3 states may involve damaging events related to the D1 protein degradation induced by light flashes or continuous low light in vivo.