Abstract: The changes of the membrane protective system in rice ( Oryza sativa L. ) seedlings during the enhancement of chilling resistance with salt, cold and heat shock pretreatment were compared to explore the mechanism of cross adaptation of the plants. The different stress pretreatments maintained the cell membrane stability and enhanced the chilling resistance of the seedlings at the chilling stress. By comparison with nonpretreated seedlings, there were similarities and specificities among the three stress pretreated seedlings on the changes of enzymic and nonenzymic protective system as well as isoenzymes zymograms. The SOD activities in all the three pretreated seedlings were very similarly higher than those of the nonpretreated seedlings at any tested time. The catalase (CAT) activities of heat shock pretreated seedlings were always higher than those of the nonpretreated seedlings at the three stages. In contrast, the CAT activities of salt pretreated seedlings were lower. Those of the cold pretreated seedlings were lower after the pretreatment and chillings injury, however were higher after the rescovery for 2 days. The peroxidase (POD) activities of the salt and cold pretreated seedlings were lower or identical with those of the nonpretreated seedlings after the pretreatment and lower than those of the nonpretreated seedlings after chilling injury; those of the heat shock pretreated seedlings were egual to the nonpretreated seedlings. However, the activities in all the three pretreated seedlings were higher after the rescovery for 2 days. The contents of ascorbic acid (ASA) and the reduced form of glutathione (GSH) in cold pretreated seedlings were markedly higher than those of nonpretreated seedlings at the three tested time. In salt and heat shock pretreated seedlings, the contents of AsA and GSH were lower or slightly higher after the treatment and chilling injury, but were higher after the reseovery for 2 days. Using polyacrylamide gel eleetrophoresis analysis, the zymogram of SOD isoenzymes had similar four bands in the salt, cold and heat shock pretreated and nonpretreated seedlings at the three tested time. The intensities of isoenzyme bands 1, 2 in three pretreated seedlings were higher than those of nonpretreated seedlings. As compared with nonpretreated seedlings, a new isoenzyme band of POD appeared in the salt, cold and heat shock pretreated seedlings after treatmem and chi!ling injury. A hand 4 appeared in the heat shock pretreated seedlings after pretreatment and a 5th band appeared in the salt and cold pretreated seedlings after pretreatment and was also induced in all the three different pretreated seedlings after chilling injury, but disappeared after reseovery for 2 days. Band 8 of salt pretreated seedlings was lacking in all three periods. These results indicated that the changes of membrane protective system in rice seedlings during the enhancement of chilling resistance by the salt, cold and heat shock pretreatments exhibited some common characters and their speeifieities. It seems that even ff the plants were exposed to the same environmental stress, they may not only use a single resistance mechanism for adaptation.

Key words: Rice, Cross adaptation, Stress, Membrane protective system, Chilling resistance