Ammonium content, nitrogenase activity and heterocyst frequency within the leaf cavities of Azolla filiculoides Lam

Abstract Ion selective new microelectrodes have been used to measure the ammonium concentrations within the various leaf cavities from the apical to the basal ones, of Azolla filiculoides Lam. Ammonium is present in solution within all leaf cavities, and its concentration varies considerably from the apex to the base. Median leaf cavities, which have the highest rate of nitrogenase activity, contain 0.6–0.8 mM of ammonium and exhibit numerous cyanophycin granules accumulated within the Anabaena vegetative cells. Basal cavities contain 6 mM ammonium, the lowest nitrogenase activity and lowe cyanophycin granules in Anabaena . The mechaniems involved in ammonium accumulation in the basal leaf cavities are discussed.