DRY-MATTER AND NITROGEN ACCUMULATION IN RICE INOCULATED WITH A NITROGENASE-DEREPRESSED MUTANT OF ANABAENA-VARIABILIS

Cyanobacteria contribute to the N economy of flooded rice (Oryza sativ a L.) fields by reducing N-2 to NH3 The purpose was to determine the e xtent to which a nitrogenase-derepressed mutant strain of the cyanobac terium Anabaena variabilis (strain SA-1), capable of excreting newly f ixed N as NH3, could provide N for rice plants grown outdoors in pots. Growth of plants inoculated with the mutant strain was compared with growth of plants inoculated with the parent strain (A. variabilis, str ain SA-0) or fertilized with inorganic N fertilizer [(NH4)(2)SO4] at r ates of 0 (control), 25, 50, and 100 kg N ha(-1). Rice plants inoculat ed with the mutant cyanobacterium produced more panicles than plants i noculated with the parent strain. Dry matter and total N contents of t he root, shoot, and grain were significantly greater (P < 0.05) in the plants inoculated with the NH3-excreting mutant strain than plants th at were inoculated with the parent strain of the cyanobacterium, Under these conditions, the contribution of the mutant cyanobacterium to gr owth and yield of rice plants was equivalent to the application of 71 to 73 kg N ha(-1) as (NH4)(2)SO4. Estimates of the rate of N-2-fixatio n (acetylene reduction) in pots inoculated,vith the NH3-excreting muta nt cyanobacterium were about twice the amount observed in pots inocula ted with the wild-type strain, even in the presence of (NH4)(2)SO4. Po ts inoculated with the mutant strain also had more NH3 in the flood wa ter than pots that were inoculated with the parent strain. The NH3-exc reting strain of A. variabilis shows potential for development for use as a biofertilizer in paddy rice production in areas where inorganic fertilizer N is unavailable or expensive and in rice production system s that aim to minimize environmental pollution from inorganic N fertil izers.