Fixation of dinitrogen-15 associated with rice plants

Rice plants (IR26 and Latisail) obtained at near heading stage from a wetland field were transferred to water culture and exposed to N(2) in a gas-tight growth chamber for 7 days to measure N(2)-fixing activities associated with the rice. The activities measured varied from 6.5 to 11.6 mumol of N(2) fixed per hill per day. The outer leaf sheath had about 2.5 times higher N(2)-fixing activities per unit weight than the root. Slight activities were also found in the basal node and inner leaf sheath. Wrapping basal parts of the stem with aluminum foil did not decrease the activities of N(2) fixation in these parts. Thus, the outer leaf sheath as well as the root are N(2)-fixing sites in rice plants. N(2) fixation found in above-ground parts is not due to photoautotrophic organisms. Less than 10% of the fixed nitrogen was translocated from the fixing sites to the leaf blades and the young panicles.