Protection against the photo-induced inactivation of the photosystem II complex by abscisic acid

To examine the effect of abscisic acid (ABA) on the photoinduced inactivation of the photosystem II (PSII) complex, a suspension culture of Chlamydomonas reinhardtii was treated with ABA for 24 h in darkness and then, after removal of ABA, the cells were exposed to strong light at a photon flux density of 2000 micromol m(-2) s(-1) at various temperatures. The activity of PSII, as estimated in terms of chlorophyll fluorescence and the evolution of oxygen, decreased significantly during the exposure of cells to the strong fight, and the extent of the photo-induced decrease in PSII activity was much greater at lower temperatures. Irrespective of temperature, the decrease in PSII activity in ABA-treated cells was significantly smaller than that in control cells. Moreover, the recovery of PSII activity from the photo-inactivated state in ABA-treated cells was significantly faster than that in control cells. The recovery of PSII activity in both ABA-treated and control cells was almost entirely prevented by the presence of chloramphenicol. These results indicate that ABA protects the PSII complex in C. reinhardtii against photo-induced inactivation by accelerating the recovery of this complex.