Analysis of Photosynthetic Responses of PSⅡ Photochemistry Efficiency in Flag Leaf of Three Native Spring Wheat Varieties on the Qinghai-Tibetan Plateau Effects of short term enhanced UV-B radiation on the PSⅡ photochemical efficiency of alpine plant Saussurea superba. Photochemical Efficiency of PSⅡand Membrane Lipid Peroxidation in Leaves of indica and japonica Rice (Oryza sativa) Under Chilling Temperature and Strong Light Stress Conditions