Effect of NaCl on nitrogen fixation of unadapted and NaCl-adapted Azolla pinnata Anabaena azollae

Exposure of Azolla pinnata-Anabaena azollae to NaCl induced a sharp decline in nitrogenase activity. A lethal dose of NaCl (40 mM) for the growth of A . pinnata-A. azollae complex could inhibit the nitrogenase activity by 39%, revealing for higher sensitivity of general growth to NaCl than the nitrog enase activity. Salinity caused a distinct shift in the profile of heterocy st differentiation in cavities of individual leaves with an increase in het erocyst frequency towards the apex, as well as increased the heterocyst fre quency. An observed 16% heterocyst frequency in unadapted complex under non -saline condition rose to 18.7% after exposing it to 60 mM NaCl in comparis on to 17.4% in adapted complex at identical NaCl concentration. The heteroc yst frequency and its pattern in different leaf lobes of unadapted complex exposed to 60 mM NaCl was significantly different, whilst the difference wa s non-significant between unadapted complex under non-saline condition and the adapted complex at 60 mM NaCl. The increase in heterocyst frequency did not match with the nitrogenase activity. Dinitrogen fixation ability was m aximum in mature median leaf cavities and minimum in apex, while NaCl-induc ed reduction in the nitrogenase activity was most marked in the cyanobionts occupying mature median leaf cavities. (C) 2001 Elsevier Science B.V. All rights reserved.