Effects of NaCl stress on germination, antioxidant responses, and proline content in two rice cultivars

We investigated the physiological and biochemical bases for salt tolerance in two rice (Otyza sativa L.) cultivars -- relatively salt-tolerant 'Dongjin' and salt-sensitive 'Kumnam'. Salinized hydroponic cultures were studied at the germination and seedling stages. NaCl inhibited germination more severely in 'Kumnam' than in 'Dongjin. Increasing the salt concentration also deterred growth to a larger extent in the former. Moreover, the leaves of 'Kumnam' exhibited greater increases in lipid peroxidation and Na+ accumulation than those of 'Dongjin' under stress. The activities of constitutive and salt-induced superoxide dismutase (SOD, EC 1.15.1.1) and ascorbate peroxidase (AP, EC 1.11.1.11) were also higher in 'Kumnam', while only catalase (CAT, EC 1.11.1.6) activity was slightly higher in stressed plants of 'Dongjin'. The positive correlation between leaf proline levels and NaCl concentration was more evident in 'Kumnam'. However, 'Dongjin' seeds, which had higher germinability in the presence of NaCl, also contained more proline. These results suggest that the higher salt tolerance in 'Dongjin' seedlings could be ascribed to their lower NaCl accumulations in the leaves. This presumably is due to reductions in the uptake or transport rates of saline ions to the shoots from the roots. Finally, we believe that the higher germination rate by 'Dongjin' is caused by its higher seed proline content.