Journal of Photoscience (2002), Vol. 9(3), pp. 65−70Decrease of Photochemical Efficiency Induced by Methyl Viologen in Rice (Oryza sativa L.) Leaves is Partly due to the Down-Regulation of PSII

In the rice leaves treated with methyl viologen (MV), the photochemical efficiency of PSII (or Fv/Fm) was significantly decreased, and significant portion of the photoinactivation process was reversible during the dark-recovery. The dark-reactivation process was relatively slow, reaching its plateau after 2-2.5 h of dark incubation. The damaged portion of functional PSII was 13%, based on the value of 1/F0-1/Fm after this dark-recovery period. The reversible photoinactivation process of PSII function in the MV-treated leaves consisted of a xanthophyll cycle-dependent development of NPQ and a xanthophyll cycle-independent process. The latter process was reversible in the presence of nigericin. As well as the increase in the values of Chl fluorescence parameters, the epoxidation process during the dark-recovery after the MV-induced photooxidation was very slow. These results suggest that the photooxidative effect of MV is partly protected by the down-regulation of PSII before inducing physical damages in core proteins of PSII. key words: methyl viologen, chlorophyll fluorescence, down-regulation of PSII, reversible photoinactivation, dark-recovery, photosynthesi