The influence of reducing the chlorophyll concentration by photobleaching on energy transfer to artificial traps within Photosystem II antenna systems

The influence of reducing the chlorophyll concentration by photobleaching on the relative excitation-energy-transfer rates to the artificial quinone trap dibromothymoquinone was examined in both a Photosystem II-grana preparation and the isolated lightharvesting chlorophyll a/b protein complex. Energy transfer as a function of decreasing chlorophyll was biphasic. Photobleaching of up to 30% of the total chlorophyll produced no detectable decrease in the energy-transfer rate. For photobleaching in excess of 30% the transfer rate decreased as a linear function which extrapolates to zero at 100% photobleaching, in agreement with a diffusive Forster-type transfer model in a bidimensional pigment matrix (Rubin, L.B., Braginskaya, O.V., Isakova, M.L. and Efremov, N.A. (1985) Photochem. Photobiol. 42, 77-87). The initial photobleaching-insensitive phase may therefore suggest that in the non-bleached antenna systems energy transfer could have a significant non-Förster component. It is also concluded that the physiological effects of photobleaching on photosynthetic activity via a reduced energy transfer efficiency are not likely to be significant. © 1989 Elsevier Science Publishers B.V. (Biomedical Division).