Physiological interactions in Azolla-Anabaena system adapting to the salt stress

The effect of the different levels of salinity on some important physiological parameters among six species of Azolla has been investigated. Salinity reduced the growth (dry weight) and nitrogenase activity in Azolla-Anabaena association while heterocyst frequency was sharply increased. Minimum sodium accumulation and electrolyte leakage (EL) was observed up to 20 mM NaCl whereas nitrate was accumulated significantly. Beyond 20 mM NaCl, fronds having higher amount of Na+ and EL inhibited the accumulation and reduction of nitrate suggests that 20 mM NaCl is the critical concentration. Highest superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities and proline content might be the probable reason for the low level of EL, malondialdehyde (MDA) content and Na+ accumulation in Azolla species. Lower Na+ accumulation, solute leakage and lipid peroxidation along with the higher nitrogen demand and the proline accumulation with a high ability of oxygen radical scavenging in A. microphylla might be the reasons to grow at higher NaCl concentrations.