NONLINEAR FLUORESCENCE YIELD OF PHOTOSYSTEM-2 SUBUNITS OF HIGHER-PLANTS

Using intense ns-laser pulses up to 2 x 10(18) photons per cm(2) per p ulse the intensity dependence of fluorescence yield (Phi) of different photosystem 2 (PS2) subunits in spinach on the intensity of excitatio n pulse (I) was measured. A strong drop of Phi was observed with incre asing laser pulse I. Measurements of the light-harvesting complex of P S2, LHC2, were taken at excitation wavelengths of 645-700 nm. A shift of the Phi vs. I curves to lower I with increasing absorption cross se ction was observed. Measurements of PS2 and thylakoid membrane fragmen ts taken at a fixed excitation wavelength of 645 nn showed the same Ph i vs. I dependences as LHC2. This suggests that LHC2-pigment proteins dominate the exciton concentration in the photosynthetic apparatus. A small drop of Phi measured at CP29 and CC2 particles was possibly caus ed by smaller domains. Smaller drop of Phi measured at D1D2 particles was similar to that of free chlorophyll (Chl), indicating that bimolec ular annihilation was absent. Experimental results could be described using a simple two-level model including exciton-exciton-annihilation and depletion of Chl ground states. Good fits of measured Phi-intensit y curves were possible for LHC2 with all the used excitation wavelengt hs. Outputs of these approximations are statements about excited state concentration and time dependence of their decay as a function of exc itation irradiance.