Changes in ammonia-assimilation enzymes in response to different nitrate levels in Azolla pinnata and A. microphylla

The effect of varying concentrations of nitrate (2.5-15 mmol/L) has been studied in two species of the water fern Azolla in terms of their relative growth rate, heterocyst frequency of the symbiont Anabaena azollae, free ammonia, and activity of ammonia-assimilating enzymes. Both species showed optimum growth rate with 2.5 mmol/L of nitrate. Suppression of cyanobacterial heterocyst frequency occurred at relatively higher levels (15 mmol/L) of nitrate. Ammonium content released in nitrate treated plants increased sharply during later periods (7-14th day) of incubation. Invariably, all dosages of nitrate activated glutamine synthetase activity at 24 h in A. pinnata, while no significant change in this enzyme activity was observed in A. microphylla during this period. Further prolongation of treatment, up to the 14th day, resulted in suppression of this enzyme in both the species of Azolla with all levels of nitrate. Glutamate dehydrogenase enzyme exhibited more activities with higher concentrations of nitrate (≥ 10 mmol/L) in A. pinnata and lower dosages (≤ 5 mmol/L) in A. microphylla during later periods of incubation.