Effects of Light Stress on Photosystem I in Chilling-Sensitive Plants

Abstract

Light stress at chilling temperatures (0–10°C) enhances photoinhibition of photosynthesis because of reduced photosynthetic energy utilization that results from lowered rates of carbon metabolism (1). Photosystem (PS) II is known as a primary site of photoinhibition. Previously, PSI was assumed to be only marginally affected by excess light. However, a preferential inactivation of PSI by illumination of chilling-sensitive cucumber (Cucumis sativus) leaves at low temperature has been reported recently (2, 3). A similar effect was described for leaves of potato (Solanum tuberosum) (4). Little is known about mechanisms protecting PSI. The xanthophyll cycle, which is a protective mechanism for PSII might also protect PSI, as the latter contains approximately the same amount of xanthophylls as does PSII (5). Zeaxanthin (Z) and antheraxanthin (A) formed via the xanthophyll cycle from violaxanthin (V) by violaxanthin de-epoxidase (VDE) are supposed to facilitate thermal dissipation of excess light energy. Furthermore, the role of the antioxidative system (Halliwell-Asada pathway) in protecting PSI has not been clarified. In the present study, we investigated photoinhibition of PSI in comparison with PSII in two chilling-sensitive plant species, Cucumis sativus and Cucurbita maxima, under different conditions. We tested two inhibitors to elucidate the role of protective systems in PSI: Dithiothreitol (DTT) was used to inhibit VDE. Methyl viologen (MV), an efficient electron acceptor of PSI, was applied to promote formation of superoxide anion radicals (•O₂⁻) and other active oxygen species.