Growth responses of paddy rice to an ammonia-excreting mutant cyanobacterium at elevated CO 2 concentration

Abstract

An ammonia-excreting mutant strain of Anabaena variabilis (strain SA-1) was studied as a N supplier for the growth of rice ( Oryza sativa L., cultivar ‘IR-30’) plants at near ambient (330 μmol mol −1) and elevated (660 μmol mol −1 carbon dioxide (CO 2) concentrations. Rice performance with N supplied by the mutant cyanobacterium was compared with that of three other N treatments: rice plants inoculated with the parent strain ( A. variabilis, strain SA-0), plants fertilized with 75 Kg N ha −1 as urea, or control plants (uninoculated, with no nitrogen fertilization). Plants at elevated CO 2 concentration had a higher dry matter content and accumulated more nitrogen than at near ambient CO 2. At either CO 2 concentration, the mutant strain enhanced the shoot and grain dry matter accumulation more than did the parent strain. Total N concentration of the shoot and grain fractions was significantly greater ( P < 0.05) in plants inoculated with the mutant than with the shoot and grain the cyanobacterium. The benefit of inoculation with the ammonia-excreting mutant strain was equivalent to or greater than the response observed in plants receiving 75 kg N ha −1. A combined response of both the rice plants and the cyanobacterium to high CO 2 levels probably accounted for the higher dry matter and N at the elevated than at the ambient CO 2 concentration. The potential of the ammonia-excreting mutant cyanobacterium as a N source for sustainable, low-input rice production in the predicted high CO 2 atmosphere of the next century is promising.