Abstract
The mechanism of salt tolerance was investigated by comparing the differences in leaf senescence and scavenging system of activated oxygen in two rice cultivars. Furthermore, we tried to determine which enzyme of the scavenging system is mainly involved in this function in rice plants. Elongation of the 3rd leaves of seedlings treated with NaCl at the 2nd leaf stage was more retarded in the Annapurna than in the Nipponbare. The amount of absorbed Na+ in the 2nd leaves was also larger in Annapurna than in Nipponbare. Decrease of chlorophyll fluorescence (ΔFv'/Fm') by photooxidation of chlorophyll of photosystem II and increase of malondialdehyde production by oxidation of unsaturated fatty acids after NaCl treatment were more conspicuous in Annapurna as a reflection of effect of salt on growth retardation of the shoot and the induction of injurious symptoms on the second leaves of seedlings. These findings suggest that the degree of peroxidation of the lipids of the membrane in the leaves of the salt-sensitive cultivar is higher. Superoxide dismutase activity increased to a higher level by the NaCl treatment in Annapurna than in Nipponbare. Even though both ascorbate peroxidase and glutathione reductase were also induced by the NaCl treatment, only the former showed varietal differences resulting in a higher induction in Nipponbare. On the other hand, catalase activity was reduced drastically by the NaCl treatment unlike the above enzymes for the scavenging of activated oxygen, particularly in Annapurna. Therefore the above results suggested that salt damage in rice seedlings might be caused by activated oxygen and that the difference in salt tolerance among cultivars might depend, at least partially, on the difference in their ability to produce activated oxygen and its scavenging, especially in the process of detoxification of H2O2 which seemed to be controlled mainly by ascorbate peroxidase and catalase activities in the chloroplast and the cytosol outside of the chloroplast in the cell, respectively.
