Illumination of chloroplasts may cause inactivation of PS II centers called photoinhibition. It is generally considered that in PS II preparations with functional oxygen evolving complex, photoinhibition by visible light arises due to back recombination of charges P680⁺Pheo^- (SnQa^-) producing triplet chlorophyll and then harmful singlet oxygen [1]. There are indications however that this is not the only mechanism contributing to PS II inactivation by visible light under aerobic conditions. For example, formation of plastosemiquinone anion radical, which has a shoulder of absorption at 400-450 nm, as well as illumination with far red light, which is not absorbed by PS II can lead to photoinhibition. Interaction of light with FeS-centers present in thylakoid membranes was also reported to produce singlet oxygen. In order to test for possible contribution of these additional processes to the total yield of PS II photoinhibition, we have determined the relative quantum yields of photoinhibition in isolated barley thylakoids upon their illumination with light of different wavelength (violet light: 405 nm 383 and 448 nm; blue light, 472 nm 446 and 525 nm; green light, 520 nm 504 and 554 nm; orange light, 565 nm =546 and 608 nm; red light, 653 nm 628 and 750 nm]), obtained by passing the light of a slide projector through a combination of glass filters. PS II activity in photoinhibited thylakoids was estimated from the rate of DCPIP reduction.

To quantify the amount of light of different wavelength, specifically absorbed by PSII centers under the photoinhibitory conditions, registration of delayed luminescence emitted from DCMU-poisoned thylakoids was used. In doing this, we first measured the light-response curves of the delayed luminescence emitted upon exposure of thylakoids to white light of defined intensities. Then the the delayed luminescence signals were recorded in responce to excitation with the light of different wavelengths and compared to the signals obtained upon the illumination of thylakoids with the white light of the known intensities.

The measured kinetics of PS II photoinhibition with the light of different wavelengths were strictly exponential and therefore could be characterized by the first-order rate-constants K_pi. The calculated values of K_pi appeared to be strictly proportional to the efficiency of light absorption by PS II calculated in the above experiments, which indicates that the quantum yield of PSII photoinhibition is independent of the wavelength of the photoinhibitory light. Thus,the obtained data shows that contribution of reactions other than charge recombination in PS II should be of neglegible importance for PS II photoinhibition.

1. Jung, J., Kim, H.J., and Cho, M. Action spectra for the generation of singlet oxygen from mitochondrial membranes from Soybean (Glycine max) hypocotyls. Photochem.Photobiol. 52:561-566, 1990.