GROWTH AND ACCUMULATION OF N-15 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 par ent strain or a nitrogenase-derepressed mutant strain (which excreted NH4+ produced by nitrogenase) of the cyanobacterium Anabaena variabili s and exposed to N-15(2). Dry matter and total N accumulated in the ro ots and shoots of plants inoculated with the mutant strain were signif icantly greater than from plants inoculated with the parent strain. Si gnificantly higher levels of N-15 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 N-15(2) had a small, con sistent but nonsignificant increase in levels of N-15 compared with tr eatments that were exposed only to the natural atmospheric abundance o f the isotope. These results show that the NH4+-excreting mutant strai n of A. variabilis has the potential to increase N input for plant gro wth in rice production systems. (C) 1997 Elsevier Science B.V.