Protection against salt toxicity in Azolla pinnata-Anabaena azollae symbiotic association by using combined-N sources

Mishra, A. K. and S. Singh, Satya (2006) Protection against salt toxicity in Azolla pinnata-Anabaena azollae symbiotic association by using combined-N sources. Acta Biologica Hungarica, 57 (3). pp. 355-365. ISSN 0236-5383

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Abstract

Protection from salt stress was observed in the terms of yield (fresh and dry weight, chlorophyll and pro- tein) and nitrogenase activity. Azolla pinnata appeared highly sensitive to 40 mM external NaCl stress. Fronds of Azolla unable to grow beyond a concentration of 30 mM NaCl and accordingly death was recorded at 40 mM NaCl on the 6th day of incubation. Yield was inhibited by various levels of NaCl (0, 10, 20 and 30 mM). Addition of combined-N to the growth medium protected the association partially from salt toxicity. Among the N-sources (NO 3 – , NH 4+ and urea) tried, urea mitigated the salt-induced tox- icity most efficiently. Reduction in nitrogenase activity was observed when intact Azolla was grown in nutrient medium either supplemented with different levels of NaCl or combined nitrogen. Only NO 3 – (5 mM) protected the enzymatic activity from salt toxicity while other concentrations of ammonium, nitrate and urea slowed down the salt-induced inhibition of enzyme activity in Azolla-Anabaena associ- ation. These results suggested that an optimum protection from salt stress could be obtained by using a combination of combined nitrogen sources. The reason for this protection might be due to the availabil- ity of combined nitrogen to the association, nitrogen is only available through the biological nitrogen fix- ation which is the most sensitive to salt stress.

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