Salt-induced changes in antioxidative enzyme activities in root tissues do not account for the differential salt tolerance of two cowpea cultivars

The salt stress effect in root growth and antioxidative response were investigated in two cowpea cultivars which differ in salt tolerance in terms of plant growth and leaf oxidative response. Four-day-old seedlings (establishment stage) were exposed to 100 mM NaCl for two days. The roots of the two cultivars presented distinct response in terms of salt-induced changes in elongation and dry weight. Root dry weight was only decreased in Perola (sensitive) cultivar while root elongation was mainly hampered in Pitiuba (tolerant). Root relative water content remained unchanged under salinity, but root Na+ content achieved toxic levels as revealed by the K+/Na+ ratio in both cultivars. Then, root growth inhibition might be due to ionic toxicity rather than by salt-induced water deficit. Although electrolyte leakage markedly increased mainly in the Perola genotype, lipid peroxidation decreased similarly in both salt-stressed cultivars. APX and SOD activities were reduced by salinity in both cultivars reaching similar values despite the decrease in Pitiuba had been higher compared to respective controls. CAT decreased significantly in Pitiuba but did not change in Perola, while POX increased in both cultivars. The salt-induced decrease in the CAT activity of Pitiuba root is, at principle, incompatible to allow a more effective oxidative protection. Our results support the idea that the activities of SOD, APX, CAT and POX and lipid peroxidation in cowpea seedling roots were not associated with differential salt tolerance as previously characterized in terms of growth rate and oxidative response in plant leaves.