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 mumol m(-1) s(-1) of PPFD) at 5, 12, 17, or 25 degreesC, followed by an exposure to the darkness for 6 h at 22 degreesC. Leaf chlorophyll fluorescence, net photosynthetic rate (P-N), and the amount of superoxide radicals (O-2(-)) in relation to chilling-induced photoinhibition were investigated. During the photophase, a good correlation (r = -0.879) was observed between Phi(PS2) (relative quantum efficiency of PS2 electron transport) and the amount of O-2(-). Treatment with exogenous O-2(-) reduced the P-N and Phi(PS2) as the chilling stress did, that was inhibited by specific scavenger of O-2(-). Hence chilling-induced photoinhibition might be due to the production of O-2(-). In contrast, in the dark period, P-N and Phi(PS2) of the seedlings treated with the exogenous O-2(-) were enhanced, but they were inhibited by the specific scavenger of O-2(-) showing the photoprotective role of O-2(-) in the recovery phase. Furthermore, in terms of the effect of exogenous O-2(-) on the xanthophyll cycle, the O-2(-) 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.