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Dry Matter and Nitrogen Accumulation in Rice Inoculated with a Nitrogenase-Derepressed Mutant of Anabaena variabilis

Dep. of Agronomy, Agronomy Physiology Lab., P.O. Box 110965, Univ. of Florida,, Gainesville, FL 32611-0965
USDA-ARS, Columbia Plateau Conserv. Res. Ctr., P.O. Box 370, Pendleton, OR 97801
USDA-ARS, Crop Genetics and Environmental Res. Unit, Agronomy Physiology Lab., P.O. Box 110965, Univ. of Florida,, Gainesville, FL 32611-0965
Dep. of Microbiology and Cell Science, P.O. Box 110700, Univ. of Florida, Gainesville, FL 32611-0700

^* Corresponding author (lhajr{at}nervm.nerdc.ufl.edu).

Cyanobacteria contribute to the N economy of flooded rice (Oryza sativa L.) fields by reducing N₂ to NH₃. The purpose was to determine the extent to which a nitrogenase-derepressed mutant strain of the cyanobacterium Anabaena variabilis (strain SA-1), capable of excreting newly fixed 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, strain SA-0) or fertilized with inorganic N fertilizer [(NH₄)₂SO₄] at rates of 0 (control), 25, 50, and 100 kg N ha^–1. Rice plants inoculated with the mutant cyanobacterium produced more panicles than plants inoculated with the parent strain. Dry matter and total N contents of the root, shoot, and grain were significantly greater (P < 0.05) in the plants inoculated with the NH₃-excreting mutant strain than plants that were inoculated with the parent strain of the cyanobacterium. Under these conditions, the contribution of the mutant cyanobacterium to growth and yield of rice plants was equivalent to the application of 71 to 73 kg N ha^–1 as (NH₄)₂SO₄. Estimates the rate of N₂-fixation (acetylene reduction) in pots inoculated with the NH₃-excreting mutant cyanobacterium were about twice the amount observed in pots inoculated with the wild-type strain, even in the presence of (NH₄)₂SO₄. Pots inoculated with the mutant strain also had more NH₃ in the flood water than pots that were inoculated with the parent strain. The NH₃-excreting 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 systems that aim to minimize environmental pollution from inorganic N fertilizers.

Received for publication May 28, 1997.