THE EFFECT OF LOW TEMPERATURE ON SUPEROXIDE DISMUTASE IN VARIOUS ORGANELLES OF CUCUMBER SEEDLING COTYLEDON WITH DIFFERENT COLD TOLERANCE
Liu Hong-xian, Zeng Shao-xi, Wang Yi-rou, Li Ping, Chen De-feng and Guo Jun-yan(Kuo Chun-yen)(South China Institute of Botany, Academia Sinica)
The localization and isoenzyme pattern of superoxide dismutase(SOD) in various organelles of cotyledons of etiolated cucumber seedlings were characterized. The changes in electrolyte leakage, SOD activity and electropherogram in cotyledons of cucumber seedlings with different cold tolerance during cold treatment were also studied, The experimental results are as follows: (1) Approximately 4% of the total SOD activity was present in the 1000×g precipitate fraction (chloroplast); about 9～10% of the activity was in the 11000×g pellet fraction (mitochondria); and nearly 86% was in the 11000×g supernatant (cell solute). There were three isoenzyme bands of SOD in these three organelles among which the one with relatively slow migrating rate was Mn-SOD isoenzyme and the others were Cu-Zn-SOD isoenzymes. (2) The sensitivities of SOD to low temperature in various organelles were different, with the highest being in chloroplast, then in mitochondria and in cell solute. The activity of Cu-Zn-SOD zymogram in chloroplasts and mitochondria decreased or disappeared as the temperature decreased. (3) As temperature decreased, the activity of SOD in cucumber seedling cotyledons decreased and the leakage of electrolytes out of the cell increased; subsequently, the injury symptoms of cucumber seedlings gradually appeared. However, a significant reduction of SOD activity appeared before the leakage of electrolytes from the cell began. The seedlings of two cucumber cultivars with different cold tolerance exhibited some differences in both SOD activity and leakage of electrolytes. Finally, the correlation between the SOD activity and the breakdown of the structure and function of the cell membrane system under low temperature stress in plant has been discussed.