Comparative EPR and thermoluminescence study of anoxic photoinhibition in Photosystem II particles

Photosystem II particles were exposed to 800 W m(-2) white light at 20 degrees C under anoxic conditions. The F-O level of fluorescence was considerably enhanced indicating formation of stable-reduced forms of the primary quinone electron acceptor, Q(A). The F-m level of fluorescence declined only a little. The g = 1.9 and g = 1.82 EPR forms characteristic of the bicarbonate-bound and bicarbonate-depleted semiquinone-iron complex, Q(A)(-)Fe(2+), respectively, exhibited differential sensitivity against photoinhibition. The large g = 1.9 signal was rapidly diminished but the small g = 1.82 signal decreased more slowly. The S-2-state multiline signal, the oxygen evolution and photooxidation of the high potential form of cytochrome b-559 were inhibited approximately with the same kinetics as the g 1.9 signal. The low potential form of oxidized cytochrome b-559 and Signal IIslow arising from Tyr(D)(+) decreased considerably slower than the g = 1.9 semiquinone-iron signal. The high potential form. of oxidized cytochrome b-559 was diminished faster than the low potential form. Photoinhibition of the g = 1.9 and g = 1.82 forms of Q(A) was accompanied with the appearance and gradual saturation of the spin-polarized triplet signal of P 680. The amplitude of the radical signal from photoreducible pheophytin remained constant during the 3 hour illumination period. In the thermoluminescence glow curves of particles the Q band (S(2)Q(A)(-) charge recombination) was almost completely abolished. To the contrary, the C band (TyrD(+)Q(A)(-) charge recombination) increased a little upon illumination. The EPR and thermoluminescence observations suggest that the Photosystem II reaction centers can be classified into two groups with different susceptibility against photoinhibition.