Acclimation of Foliar Antioxidant Systems to Growth Irradiance in Three Broad-Leaved Evergreen Species

The protective role of leaf antioxidant systems in the mechanism of plant acclimation to growth irradiance was studied in Vinca major, Schefflera arboricola, and Mahonia repens, which were grown for several months at 20, 100, and 1200 [mu]mol photons m-2 s-1. As growth irradiance increased, several constituents of the "Mehler-peroxidase" pathway also increased: superoxide dismutase, ascorbate peroxidase, glutathione reductase, ascorbate, and glutathione. This occurred concomitantly with increases in the xanthophyll cycle pool size and in the rate of nonphotochemical energy dissipation under steady-state conditions. There was no evidence for photosystem II overreduction in plants grown at high irradiance, although the reduction state of the stromal NADP pool, estimated from measurements of NADP-malate dehydrogenase activity, was greater than 60% in V. major and S. arboricola. Ascorbate, which removes reactive O2 species generated by O2 photoreduction in the chloroplast and serves as a reductant for the conversion of the xanthophyll cycle pigments to the de-epoxidized forms A plus Z, generally exhibited the most dramatic increases in response to growth irradiance. We conclude from these results that O2 photoreduction occurs at higher rates in leaves acclimated to high irradiance, despite increases in xanthophyll cycle-dependent energy dissipation, and that increases in leaf antioxidants protect against this potential oxidative stress.