Photosynthetic apparatus in chilling-sensitive plants : VI. Cold and dark-induced changes in chloroplast superoxide dismutase activity in relation to loosely-bound manganese content.

Both total and CN-insensitive superoxide dismutase (SOD) activities in relation to the level of manganese were studied in chloroplasts and digitonin subchloroplast particles obtained from fresh, cold and dark-stored, and illuminated leaves of Licopersicon esculentum, Mill. Following cold and dark treatment of detached leaves and intact plants, both total SOD activity and Mn content were greatly diminished while illumination of the leaves (2 h, 8,000 lx, 25° C) resulted in a large increase of Mn content and a partial restoration, of SOD activity. When growing plants were kept for 3-4 days in the dark, either at 0° C or 25° C, a decrease in chloroplast SOD activity, Mn content, and Hill reaction was also observed. However, with a prolonged dark treatment either at 0° C or at 25° C, there was a slight increase in chloroplast Mn content accompanied by an enhancement of both SOD and Hill reaction activities, suggesting that even in the dark some translocation of Mn from cytosol into chloroplasts may occur. Hypotonically treated chloroplasts and digitonin subchloroplast particles contained almost exclusively CN-insensitive, SOD activity, while Tris-washed chloroplast preparations were completely deprived of SOD activity. All these data seem to indicate that a chloroplast loosely-bound Mn pool is involved in CN-insensitive SOD activity.