MEHLER-PEROXIDASE REACTION MEDIATES ZEAXANTHIN FORMATION AND ZEAXANTHIN-RELATED FLUORESCENCE QUENCHING IN INTACT CHLOROPLASTS

Induction of zeaxanthin formation and the associated nonphotochemical quenching in iodoacetamide-treated, non-CO2-fixing intact chloroplasts of Lactuca sativa L. cv Romaine is reported. The electron transport needed to generate the required DELTA-pH for zeaxanthin formation and nonphotochemical quenching are ascribed to the Mehler-ascorbate peroxidase reaction. KCN, an inhibitor of ascorbate peroxidase, significantly affected these activities without affecting linear electron transport to methyl viologen or violaxanthin deepoxidase activity. At 1 millimolar KCN, zeaxanthin formation and DELTA-pH were inhibited 60 and 55%, respectively, whereas ascorbate peroxidase activity was inhibited almost totally. The KCN-resistant activity, which apparently was due to electron transport mediated by the Mehler reaction alone, however, was insufficient to support a high level of nonphotochemical quenching. We suggest that in vivo, as CO2 fixation becomes limiting, the Mehler-peroxidase reaction protects photosystem II against the excess light by supporting the electron transport needed for zeaxanthin-dependent nonphotochemical quenching and concomitantly scavenging H2O2. Ascorbate is essential for this process to occur.