RESUMEN
Anabaena cylindrica filaments were freed from heterocysts by growing in a standard basic medium supplemented with 20 mM KN0[3]for 10 days. The induction of heterocysts in nitratetdeprived cultures of Anabaena was assessed in presence of various Ca[2] concentrations. Heterocyst frequency increased as Ca[+2] concentration was increased from 10[-5] to 4x10[-3]M; the highest heterocyst frequency was achieved at 4x10[-4M Ca[+2]. Total nitrogenase activity declined as the Ca[+2] concentration increased. The heterocyst frequency was studied at the optimum concentration of calcium in conjunction with various concentration of abscisic acid [from 10[10] to 10[-3]M]. Maximum induction of heterocyst formation was recorded at 10[-S] M ABA, although nitrogenase activity declined as the concentration of ABA increased. The presence of ABA at concentration 10[-5]M increased heterocyst frequency at all the Ca[+2] concentrations used