Effect of NaC1 on growth, nitrate uptake and reduction and nitrogenase activity of Azolla pinnata-Anabaena azollae

Azolla pinnata -Anabaena azollae, fertilised with 5 mM nitrate in presence or absence of NaCl could utilise both forms of nitrogen, nitrate as well as symbiotically fixed nitrogen; nitrate in the nutrient solution did not reduce the nitrogenase activity albeit protected it against NaCl-toxicity. Dry weight (DW) and chlorophyll content of the nitrate fertilised association increased by 1.4-fold to that of association relying solely on the symbiotically fixed nitrogen. Addition of 20 mM NaCl in the medium reduced the DW and chlorophyll content of the association but enhanced the protein level. NaCl-induced reduction of nitrogenase activity was lessened in the association fertilised with nitrate. A. pinnata could take up NO3- immediately after incubation in the nitrate medium, whereas the freshly isolated cyanobiont required a lag of 60 min. NO3- uptake by A. pinnata involved the carrier mediated biphasic system, the one exhibited high affinity for nitrate (K-s = 94 muM) and the other with low affinity (K-s = 517 muM). In NaCl-stressed association high- and low-affinity systems showed 3.6 and 2.5 times increased affinity for nitrate, respectively. Salinity also increased the affinity of nitrate reductase for nitrate. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.