Growth and accumulation of 15N in rice inoculated with the parent and a nitrogenase-derepressed mutant strain of Anabaena variabilis

Rice plants were grown in a temperature-controlled greenhouse in 1.2 L glass fleakers. The rooting media were inoculated with either the parent strain or a nitrogenase-derepressed mutant strain (which excreted NH4 4 + produced by nitrogenase) of the cyanobacterium Anabaena variabilis and exposed to 15 N 2 . Dry matter and total N accumulated in the roots and shoots of plants inoculated with the mutant strain were significantly greater than from plants inoculated with the parent strain. Significantly higher levels of 15 N 2 accumulated in the roots and shoots of plants inoculated with the mutant strain, which indicated that more fixed N was readily available for root uptake and assimilation. Roots and shoots of uninoculated plants exposed to 15 N 2 had a small, consistent but nonsignificant increase in levels of 15 N compared with treatments that were exposed only to the natural atmospheric abundance of the isotope. These results show that the NH 4 + -excreting mutant strain of A. variabilis has the potential to increase N input for plant growth in rice production systems.