Effect of salinity stress on the antioxidant defence systems of two varieties of cowpea (Vigna unguiculata L.)

Osmotic stress, oxidative stress and oxidation of essential macromolecules are common consequences of salinity stress that limit plant growth and productivity. Plants are known to evolve several strategies such as upsurge of antioxidant defence systems (ADS) and accumulation of osmolytes, so as to thrive under such conditions. In the present study, the effect of salinity stress (using irrigation method) on ADS of two cultivars (IT-99 and IT-288) of cowpea was examined. Plant samples (roots, young leaves and matured leaves) were harvested on day 21 of treatment with saline solution (100 – 400 mM NaCl). Antioxidant markers and osmolytes levels were quantified and compared with the controls (0.0 mM NaCl). The activities of superoxide dismutase, catalase, peroxidase and ascorbate peroxidase significantly increased (p<0.05) in the leaves, except for IT-288 where catalase activity significantly decreased (p<0.05) when compared to the control. On the contrary, catalase and peroxidase activities significantly decreased (p<0.05) in the roots of both cultivars. Largely, ascorbate, glutathione (GSH) and tocopherols levels increased as salinity increases, except for GSH in roots of IT-99, and leaves of IT-288. The amount of flavonoids detected in the same tissue were not significantly (p>0.05) different in all the salinity levels investigated. The level of proline increased at moderate salinity levels in all samples and at high salinity in roots of IT-99 and mature leaves of IT-288. For IT-99, levels of glycinebetaine significantly increased (p<0.05) at high salinity, but significantly decreased at similar levels in IT-288. H2O2 levels significantly increased in the roots but decreased (p<0.05) in leaves samples. Malondialdehyde concentration generally increased significantly (p<0.05) when compared with control. The findings of these study suggest that both cultivars were induced to express higher antioxidant activity and to a certain extent synthesis of more osmolytes.