Effects of exogenous spermine on activities of antioxidant enzymes and photosynthesis in cucumber seedlings under NO-3 stress Resistance to Sulfur Dioxide of Four Colored Leaf Species in Prunus Responses of Poplar (Populus×euramericana cv. Distribution of Ca2+ at the Tip of Phyllostachys edulis Root under Drought Stress and Physiological Functions of Exogenous Ca2+ Effects of Hg2+,Cr3+ and Pb2+ Stress on the Membrane Peroxidation Content and Activities of Antioxidant Enzymes