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 st udied in two species of the water fern Azolla in terms of their relative gr owth rate, heterocyst frequency of the symbiont Anabaena azollae, free ammo nia, and activity of ammonia-assimilating enzymes. Both species showed opti mum growth rate with 2.5 mmol/L of nitrate. Suppression of cyanobacterial h eterocyst frequency occurred at relatively higher levels (15 mmol/L) of nit rate. 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, whi le no significant change in this enzyme activity was observed in A. microph ylla during this period. Further prolongation of treatment, up to the 14th day, resulted in suppression of this enzyme in both the species of Azolla w ith all levels of nitrate. Glutamate dehydrogenase enzyme exhibited more ac tivities with higher concentrations of nitrate (greater than or equal to 10 mmol/L) in A. pinnata and lower dosages (less than or equal to 5 mmol/L) i n A. microphylla during later periods of incubation.