Dual Role of Superoxide Radicals in the Chilling-Induced photoinhibition in Maize Seedlings

Maize (Zea mays) seedlings were exposed for 6 h to strong irradiance (1 000 mol m^–1 s^–1 of PPFD) at 5, 12, 17, or 25 °C, followed by an exposure to the darkness for 6 h at 22 °C. Leaf chlorophyll fluorescence, net photosynthetic rate (P _N), and the amount of superoxide radicals (O₂ ^–) in relation to chilling-induced photoinhibition were investigated. During the photophase, a good correlation (r=–0.879) was observed between _PS2 (relative quantum efficiency of PS2 electron transport) and the amount of O₂ ^–. Treatment with exogenous O₂ ^– reduced the P _N and _PS2 as the chilling stress did, that was inhibited by specific scavenger of O₂ ^–. Hence chilling-induced photoinhibition might be due to the production of O₂ ^–. In contrast, in the dark period, P _N and _PS2 of the seedlings treated with the exogenous O₂ ^– were enhanced, but they were inhibited by the specific scavenger of O₂ ^–, showing the photoprotective role of O₂ ^– in the recovery phase. Furthermore, in terms of the effect of exogenous O₂ ^– on the xanthophyll cycle, the O₂ ^– production suggested a promotion effect for the de-epoxidation of violaxanthin during the photophase, the epoxidation of zeaxanthin at the dark stage, and the increase of the xanthophyll pool both in the photophase and dark phase, resulting in an enhancement of the ability of non-photochemical quenching to avoid or alleviate the damage to photosynthetic apparatus.