Photoinhibition of PSII occurs at the same quantum efficiency from very low to very high light, which raises a question about how important is the rate of photosynthetic electron transfer in photoinhibition. We modulated electron transfer rate and light intensity independently of each other in lincomycin-treated pea leaves and in isolated thylakoids, in order to elucidate the specific effects of light and PSII electron transport on photoinhibition. Major changes in the rate of electron transport caused only small changes in the rate of photoinhibition, suggesting the existence of a significant photoinhibitory pathway that contains an electron-transfer-independent phase. We compared the action spectrum of photoinhibition with absorption spectra of PSII components that could function as photoreceptors of the electron-transfer-independent phase of photoinhibition and found that the absorption spectra of Mn(III) and Mn(IV) compounds resemble the action spectrum of photoinhibition, showing a steep decrease from UV-C to blue light and a low visible-light tail. Our results show that the release of a Mn ion to the thylakoid lumen is the earliest detectable step of both UV- and visible-light-induced photoinhibition. After Mn release from the oxygen-evolving complex, oxidative damage to the PSII reaction center occurs because the Mn-depleted oxygen-evolving complex cannot reduce P680+ normally.

Photoinhibition is light-induced inactivation of PSII. Hypotheses about the photoreceptor(s) of photoinhibition include the Chl antenna of PSII, manganese of the oxygen-evolving complex (OEC), uncoupled Chl and iron-sulfur centres. We measured the action spectrum of photoinhibition in vivo from wild-type Arabidopsis thaliana L. and from the npq1-2 and npq4-1 mutants defective in non-photochemical quenching (NPQ) of excitations of the PSII antenna. The in vivo action spectrum was found to resemble closely the in vitro action spectra published for photoinhibition. We compared the action spectrum with absorbance spectra of model compounds of the OEC complex and other potential photoreceptors of photoinhibition. The comparison suggests that both manganese and Chl function as photoreceptors in photoinhibition. In accordance with the function of two types of photoreceptors in photoinhibition, NPQ was found to offer only partial protection against photoinhibition at visible wavelengths. The low protective efficiency of NPQ supports the conclusion that the Chl antenna of PSII is not the only photoreceptor of photoinhibition. Comparison of the action spectrum of photoinhibition with the emission spectrum of sunlight shows that the UV part of sunlight is responsible for the major part of photoinhibition under natural conditions.

Evidence has recently been presented that photoinhibition of photosystem II (PSII) is triggered by absorption of light by the oxygen-evolving manganese cluster. To get insight into the effects of light on enzymes containing manganese or other transition metal cofactors, the photosensitivities of Mn catalase, Mn superoxide dismutase, the haem (Fe)-containing bovine liver catalase, and CuZn superoxide dismutase were investigated. Glucose oxidase was studied as an example of an enzyme that does not have a metal cofactor. Sensitivities of these five enzymes to UVC, UVA, and visible light were compared in anaerobic conditions. The Mn(III)-oxo-Mn(III)-containing Mn catalase was found to be more sensitive to both visible and UV light than bovine liver catalase. Furthermore, the action spectrum of photoinhibition of Mn catalase was found to be fairly similar to that of photoinhibition of PSII. The Mn(II)-containing Mn superoxide dismutase was sensitive to UVC light and somewhat sensitive to UVA light, while only UVC light caused some inhibition of CuZn superoxide dismutase. Glucose oxidase was the least photosensitive of the enzymes studied. The photosensitivity of Mn enzymes supports the hypothesis that the oxygen-evolving manganese complex of PSII can be damaged by UV and visible light absorbed by its Mn(III) or Mn(IV) ions.

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The light response curves of the acceptor and donor side mechanisms of photoinhibition of Photosystem II were calculated, using Arabidopsis as a model organism. Acceptor-side photoinhibition was modelled as double reduction of QA, noting that non-photochemical quenching has the same effect on the quantum yield of QA double reduction in closed PSII centres as it has on the quantum yield of electron transport in open centres. The light response curve of acceptor-side photoinhibition in Arabidopsis shows very low efficiency under low intensity light and a relatively constant quantum yield above light saturation of photosynthesis. To calculate the light response curve of donor-side photoinhibition, we built a model describing the concentration of the oxidized primary donor P680 + during steady-state photosynthesis. The model is based on literature values of rate constants of electron transfer reactions of PSII, and it was fitted with fluorescence parameters measured in the steady state. The modelling analysis showed that the quantum yield of donor-side photoinhibition peaks under moderate light. The deviation of the acceptor and donor side mechanisms from the direct proportionality between photoinhibition and photon flux density suggests that these mechanisms cannot solely account for photoinhibition in vivo, but contribution of a reaction whose quantum yield is independent of light intensity is needed. Furthermore, a simple kinetic calculation suggests that the acceptor-side mechanism may not explain singlet oxygen production by photoinhibited leaves. The theoretical framework described here can be used to estimate the yields of different photoinhibition mechanisms under different wavelengths or light intensities.