Physiological Response to Salinity in Rice Plant : III. A possible mechanism for Na+ exclusion in rice root under NaCl-stress conditions

The mechanism and varietal difference of Na+ exclusion in rice roots exposed to NaCl stress was studied in relation to reverse osmosis as the principle of ion exclusion. Exudates from the cut-surface of a root exposed to 20 mmoll-1 NaCl solution with hydrostatic pressure of 294 and 686 kPa were sampled. Na+ concentration of the exudate from the whole cut-surface was at most 60% lower than that of the root medium. However, Na+ concentration of the exudate from the cut-surface of the stele was about 85% lower than that of the root medium. These results indicated that Na+ exclusion might occur at the exodermis and endodermis. The decrease of Na+ concentration of the exudate with hydrostatic pressure indicated that Na+ exclusion might be a non-metabolic process which could be driven by negative pressure induced by transpiration, and seemed to be based on reverse osmosis. From the results of X-ray micro-analysis, Na+ movement across the endodermis was less in IR28 than in Kala-Rata 1-24 (KR1). However, transpiration rate under saline conditions was higher in KR1, and it was considered to be supported by morphological characteristics of KR1 root, i.e. more roots per plant, longer and thicker roots, high ratio of stele area to the area of cross-section and more vessels per cross-section. It is concluded that the varietal difference in salt tolerance between KR1 and IR28 is related to the difference in the ability to maintain the negative pressure for ion exclusion by keeping the transpiration rate under saline conditions.