Salt tolerance in aquatic macrophytes: possible involvement of the antioxidative enzymes

Possible involvement of the antioxidative enzymes in salt tolerance was investigated in three submerged aquatic macrophytes, viz. Hydrilla verticillata, Najas indica and Najas gramenia, which differed in their tolerance to salinity. NaCl in light resulted in significant decrease in the chlorophyll a/b ratio in H. verticillata, which was significantly neutralized by the presence of DABCO (diazabicyclo[2.2.2]octane), sodium benzoate or mannitol along with the NaCl in the exposure medium indicating induction of oxidative stress upon salt treatment. All the three test species showed enhanced activity of superoxide dismutase (SOD; EC 1.15.1.1) and peroxidases (EC 1.11.1.7) upon exposure to either NaCl or SWS (seawater salinity); the increases in their activity in response to NaCl were positively correlated with the salt tolerance of the plants. Only N. gramenia showed considerable increase in the activity of catalase (EC 1.11.1.6), while only H. verticillata exhibited significant increase in the activity of ascorbate peroxidase (EC 1.11.1.11) in response to the salt treatment. A higher constitutive level of the antioxidative enzymes in the salt-tolerant N. gramenia than in the salt-sensitive H. verticillata coupled with significant increase in the activity of these enzymes, particularly of catalase and SOD, in the former upon salinity treatment suggested their active involvement in salt tolerance. The oxidative stress induced upon the salt treatment also indicated a possible involvement of the antioxidative enzymes in the process. The ion interaction study revealed significant modifying effect of Ca(2+), and also of Mg(2+) and K(+), on the changes induced by Na(+) in the activity of catalase and SOD suggesting that relative build-up of these ions inside cells could be of much importance in regulation of synthesis of antioxidative enzymes.