Superoxide dismutase, catalase and peroxidase activities do not confer protection against oxidative damage in salt-stressed cowpea leaves

The aim of this study was to determine whether guaiacol peroxidase (POX), superoxide dismutase (SOD) and catalase (CAT) activities are effective in the protection and recovery of cowpea (Vigna unguiculata (L.) Walp.) leaves exposed to a salt-induced oxidative stress. The salt treatment (200 mM NaCl) was imposed during six consecutive days and the salt withdrawal after 3 d (recovery treatment). Control plants received no NaCl treatment. The salt treatment caused almost complete cessation of leaf relative growth rate in parallel with the transpiration rate. The restriction in leaf growth was associated with a progressive increase in membrane damage, lipid peroxidation and proline content. Salt withdrawal induced a significant recovery in both leaf growth rate and transpiration. Surprisingly, these prestressed/recovered plants showed only a slight recovery in leaf lipid peroxidation and membrane damage. Leaf CAT activity experienced a twofold decrease only after 1 d NaCl treatment, and salt withdrawal had no effect on its recovery. SOD activity did not change compared with control plants. By contrast, POX activity significantly increased after 1 d NaCl treatment and showed a significant recovery to levels near to those of control. In conclusion, it appears that the ability of cowpea plants to survive under high levels of salinity is not caused by an operating antioxidant system involving SOD, POX and CAT activities in mature leaves.