Maize (Zea mays) seedlings were exposed for 6 h to strong irradiance (1 000 μmol 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 (PN), 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 ΦPS2 (relative quantum efficiency of PS2 electron transport) and the amount of O2-⋅. Treatment with exogenous O2-⋅ reduced the PN and ΦPS2 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, PN and ΦPS2 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. and D. Ke, G. Sun, Y. Jiang.
The effect of heat stress (35 to 50 °C) on photosynthesis was investigated in heat tolerant (N 22) and heat sensitive (IR 8) cultivars of rice {Oryza sativa L.). The net photosynthetic rate showed greater thermal stability in N 22 than in IR 8. The relative dechne of the rate of whole chain electron transport and photosystem 2 (PS2) activity was more pronounced in IR 8 than N 22. In both cultivars photosystem (PSI) activity was stimulated by thermal treatment. Chlorophyll (Chl) a fluorescence transient arising ffom PS2 showed inhibition in both cultivars at 45 and 50 °C. Maximum fluorescence decreased more in IR 8 than in N 22 by high temperature treatment.