Photoreduction of C-550 and oxidation of cytochrome b559 in chloroplasts: Dependence on the state of photosystem II

Abstract

The photoreduction of C-550 and the photooxidation of cytochrome b 559 were studied by illumination at −55 °C (by continuous white light or by 2-μs flashes) of spinach chloroplasts, which had been previously dark adapted or preilluminated at 21 °C by a variable number ( n) of saturating short flashes. Differential absorption spectra were then recorded at −196 °C. 1. 1. A saturating illumination by continuous light reduces all the C-550, but the fraction of cytochrome b 559 which is oxidized varies with n: 32% for n = 0; 100% for n = 2; 36% for n = 4. Percentages are given in respect to the amount of cytochrome b 559 photooxidizable by continuous light at −196 °C. An oscillatory pattern is observed, which parallels the state of Photosystem II, as defined by the number of oxidizing equivalents accumulated during preillumination. 2. 2. Excitation by a series of 10–12 saturating flashes produces the same result as continuous light. However, the effect of the first flash oscillates also with n, with a period of four. For n = 0 (and approximately for n = 4) 75% of the C-550 is reduced, with no oxidation of cytochrome b 559. For n = 2, only about 30% of the C-550 is reduced although the flash is saturating, and some cytochrome b 559 is concomitantly oxidized. 3. 3. In the presence of ferricyanide which oxidizes cytochrome b 559 in the dark, no change in the redox state of that cytochrome occurs following any illumination. For C-550, the pattern previously described is unchanged, except that not all the C-550 can be trapped in the reduced form. The present results are discussed in relation to the properties of the reaction centre of Photosystem II, where two types of photochemical behaviour are believed to occur.