Effect of Cold Hardening on SOD and Glutathion Reductase Activities and on the Contents of the Reduced Form of Glutathion and Ascorbic Acid in Rice and Cucumber Seedlings Effect of Cold-Hardening on Thylakoid Membrane Lipids and Proteins of Spring Wheat and Winter Wheat Calcium Effect on the Process of Cold-Hardening of Rice Seedlings Increment of Chilling Tolerance and Its Physiological Basis in Chilling-sensitive Corn Sprouts and Tomato Seedlings after Cold-hardening at Optimum Temperatures Effect of cold hardening on SOD,POD activities and on contents of MDA in different ploidy watermelon seedling