Physiological and biochemical mechanisms of salt tolerance in rice; 1: sensitivity thresholds to salinity of some physiological processes in rice (Oryza sativa L.)

The sensitivity thresholds for some physiological processes against NaCl-salinity wine mapped in both salt-tolerant IR9884-54-3-1 and salt-sensitive IR29. Mapping was carried out over 9 concentrations of NaCl: 0, 15, 30, 50, 70, 90, 120, 150, and 200 mM, during the vegetative stage. Occurrence of symptoms and degree of stress were highly associated with increasing concentration of NaCl. Salt concentration and the measured values for fresh and dry weights of the leaf blade, leaf sheath, root and whole plant appeared linear. The differences in treatment means between the two varieties were highly significant. Interaction, however, was absent between the varieties and salt treatments. Biomass was significantly reduced at 50 mM. The salt-sensitive IR29 decreased by 30 percent in plant day weight whereas the salt-tolerant IR9884 by 25 percent; above this concentration, the decrease was noted at 50 percent to 85 percent and 25 percent to 70 percent, respectively. Significant reduction in the functional leaf area by about 37 percent was also observed at 50 mM in IR29; IR9884, however, was affected at 30 mM with a corresponding reduction of 22 percent from the control. Shoot fresh: dry weight ratio, photosynthesis and transpiration rates, water use efficiency, stomatal conductance, chlorophyll synthesis, and total N were not significantly affected by NaCl-salinity at 50 mM, however, above this concentration, the measured values decreased. Salinity did not bring about significant changes on the temperature of the rice leaf. N was distributed in the rice plant as follows, blade - 53 percent, sheath - 36 percent, root - 11 percent. NaCl enhanced synthesis of N-containing compounds in the blade and sheath. The minimum concentration of NaCl that stimulated significant increases in total N for both varieties was at 50 mM. Maximum N content for IR9884 was attained at 50 mM whereas for IR29, it was reached at higher level of NaCl (50 mM)