A new mechanism for adaptation to changes in light intensity and quality in the red alga, Porphyra perforata. II. Characteristics of state II-state III transitions.

Characteristics of state II-state III transitions in the red alga, Porphyra perforata, were studied by measuring the fluorescence time course at room temperature and fluorescence spectra at 77 K. The state II to III transition was induced by system II light and was sensitive to uncouplers of photophosphorylation. This state II to III transition has a dark step(s) that could be easily separated from the light process. A state III to II transition occurred in the dark, but system I light accelerated the transition. The accelerating effect of system I light was not sensitive to uncouplers of photophosphorylation, but was inhibited by the addition of valinomycin + KCl or antimycin A. Compared to state I-state II transitions, the state II-state III transitions occurred more rapidly. The state II to state III transitions are different from the state I to state II transitions in that in state III the activity of photosystem II is changed without having any effect on photosystem I activity (Satoh and Fork, Biochim. Biophys, Acta, in press, 1982). It is suggested that the state II-state III transition represents a mechanism by which the alga can avoid photodamage resulting from absorption of excess light energy by photosystem II.