Influence of NaCl Salinity on the Behaviour of Protease, Aminopeptidase and Carboxypeptidase in Rice Seedlings in Relation to Salt Tolerance

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Activities of the enzymes protease, aminopeptidase and carboxypeptidase were determined in seedlings of rice cultivars with different salt tolerances raised under increasing levels of NaCl salinity. Salinity caused a marked increase in protease activity in roots as well as shoots, though activity was higher in roots than in shoots. Salt-tolerant cultivars possessed higher levels of protease activity in control as well as salt-stressed seedlings compared with salt-susceptible cultivars. During a growth period of 5-20 days, leucine aminopeptidase (LAP) activity increased up to days 10-15 and decreased thereafter. Salt treatment caused a sharp increase in LAP activity in roots of both sets of cultivars. The increase was larger in tolerant than in susceptible cultivars. In shoots, unlike roots, higher salinity suppressed LAP activity, and suppression was more marked in susceptible cultivars than in tolerant ones. Carboxypeptidase activity was higher in susceptible cultivars than in tolerant ones under both control as well as salt treatments. Roots maintained higher levels of carboxypeptidase activity than shoots. Results suggest an increased rate of proteolysis in salt-stressed rice seedlings and an association of salt-tolerance ability with higher protease and aminopeptidase activities and lower carboxypeptidase activity under salinisation.