Chilling-Enhanced Photooxidation: Evidence for the Role of Singlet Oxygen and Superoxide in the Breakdown of Pigments and Endogenous Antioxidants

Abstract

Chilling temperatures (5 degrees C) and high irradiance (1000 microeinsteins per square meter per second) were used to induce photooxidation in detached leaves of cucumber (Cucumis sativus L.), a chilling-sensitive plant. Chlorophyll a, chlorophyll b, beta carotene, and three xanthophylls were degraded in a light-dependent fashion at essentially the same rate. Lipid peroxidation (measured as ethane evolution) showed an O(2) dependency. The levels of three endogenous antioxidants, ascorbate, reduced glutathione, and alpha tocopherol, all showed an irradiance-dependent decline. alpha-Tocopherol was the first antioxidant affected and appeared to be the only antioxidant that could be implicated in long-term protection of the photosynthetic pigments. Results from the application of antioxidants having relative selectivity for (1)O(2), O(2) (-), or OH indicated that both (1)O(2) and O(2) (-) were involved in the chilling- and light-induced lipid peroxidation which accompanied photooxidation. Application of D(2)O (which enhances the lifetime of (1)O(2)) corroborated these results. Chilling under high light produced no evidence of photooxidative damage in detached leaves of chilling-resistant pea (Pisum sativum L.). Our results suggest a fundamental difference in the ability of pea to reduce the destructive effects of free-radical and (1)O(2) production in chloroplasts during chilling in high light.