Differences in mechanisms of salt tolerance between rice and barley plants

Mechanisms of salt tolerance were estimated in terms of the ATPase activity in roots, the Na excluding ability of roots, and the regulation of Na translocation from roots to shoot by comparing Na tolerant plants (barley) and Na sensitive plants (rice) grown at high NaCl concentrations. The results obtained were as follows. 1) As the relative dry weight (dry weight at high NaCl concentration in the medium to the dry weight at 2 mM NaCl concentration in medium) of rice decreased remarkably with the increase of NaCl concentration in the medium compared to that of barley, salt tolerance of barley was higher than that of rice. High salt tolerance of barley was ascribed to the fact that Na translocation from roots to shoot was prevented at high NaCl concentration in the growth medium. 2) ATPase activity in roots of rice decreased remarkably with the increase of NaCl concentration (50 to 200 mM NaCl), while that of barley grown at a high NaCl concentration remained high. Since the effect of the NaCl concentration in the reaction solution on the ATPase activity in vitro was negligible, high NaCl concentration in medium may not inhibit directly ATPase but may inhibit indirectly ATPase synthesis or denature membranes, etc. 3) ATPase activities in the plasma membrane and tonoplast of barley roots were higher than those of rice when roots were exposed to NaCl concentrations in the range from 0 to 25 mM in the medium. 4) When Na-22 tracer experiment was conducted over a short period of time, both Na exclusion from roots and Na retranslocation from shoot to roots were higher in rice than in barley, regardless of Na concentration (2 to 150 mM Na) in medium because ATPase activity of the plasma membrane and tonoplast in rice roots remained high under short Na treatment. In rice other factors were involved, such as Na sensitivity of leaf tissues after prolonged exposure. Thus, immediately after the exposure of roots to NaCl, roots of rice plants actively excluded Na from roots to medium, and a large amount of Na was retranslocated from shoots to roots in rice plant compared to barley. However when rice plants were exposed to high NaCl concentrations for a long period of time, the functions of Na exclusion and Na retranslocation from shoots to roots could not be maintained because the ATPase activity of roots of rice plants decreased due to the high Na concentration in the medium, and the metabolism of leaf tissues was more sensitive to Na in rice than in barley.