STUDIES ON THE SALT TOLERANCE IN KOREAN RICE CULTIVARS .1. MECHANISM OF SALT TOLERANCE IN DRY-MATTER PRODUCTION AND LEAF PHOTOSYNTHESIS

Reduction of relative growth rate (RGR) due to the NaCl treatment was examined in the seedlings of six Korean rice cultivars which had been reported to be different in salt tolerance on the level of grain yield. On the level of RGR, three of the cultivars were identified as salt tolerant, and the remaining three were identified as the sensitive cultivars. Further analysis of RGR indicated that the reduction of RGR by NaCl treatment was attributed in large part to the reduction of the net assimilation rate (NAR) rather than to that of the leaf area ratio (LAR). Leaf photosynthesis which is closely related with NAR, was also reduced by NaCl treatment in salt sensitive cultivars to a greater extent than tolerant cultivars. Ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) content was determined in a tolerant cultivar, Han-Kang-Chal, and a sensitive one, Dong-Hae. Han-Kang-Chal showed a smaller decrease of Rubisco content per absorbed Na in the NaCl-treated leaves. This indicates that degradation of Rubisco by absorbed Na is slower in tolerant cultivars than in sensitive cultivars. Moreover, Han-Kang-Chal showed a larger decrease in osmotic potential in the NaCl treated leaves, suggesting that osmotic adjustment was developed under salt stress conditions in a tolerant cultivar.