Effects of Supplementary Ultraviolet‐B Radiation on Photosynthetic Transcripts at Different Stages of Leaf Development and Light Levels in Pea (Pisum sativum L.): Role of Active Oxygen Species and Antioxidant Enzymes.

Supplementary UVB radiation caused the down‐regulation of transcripts of the photosynthetic genes Lhcb and psbA in fully expanded pea leaves. This response was less marked in apical buds and was ameliorated by high light. The effect of UVB on leaves was mimicked by free radical generators and prevented by antioxidant feeding. Under UVB, active oxygen species levels, determined by level of lipid peroxides, was highest in leaves under low light and lower under high light and in buds. Activities of ascorbate peroxidase, glutathione reductase and superoxide dismutase were increased in response to UVB irradiation and were higher under high light and in buds. These results indicate that active oxygen species are involved in the UVB signal pathway leading to the down‐regulation of photosynthetic genes. Furthermore, they suggest that the antioxidant capacity of plant tissue determines the susceptibility of these transcripts to UVB radiation.