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Nitrogen fixation by bacteria associated with roots of intact maize plants was measured by exposing the roots to15 N2 at a partial O2 pressure (pO2 ) of 2 or 10 kPa. The plants were grown in a mixture of Weswood soil and sand and then transferred to plastic cylinders containing an N-free plant nutrient solution. The solution was sparged continuously with a mixture of air and N2 at a pO2 of 2 or 10 kPa. Acetylene reduction was measured after the roots were exposed to the low pO2 overnight. The air-N2 atmosphere in the cylinders was then replaced with an O2 -He atmosphere at the same pO2 , and the roots were exposed to 20 kPa of15 N2 for 20 to 22 h. Incorporation of15 N into the roots was 200 times greater at 2 kPa of O2 than at 10 kPa of O2 . Addingl -malate (1 g of C liter−1 ) to the nutrient solution increased root-associated nitrogenase activity, producing a strong15 N label which could be traced into the shoots. Fixed15 N was detected in the shoots within 5 days after the plants were returned to unfertilized soil. In a similar experiment with undisturbed plants grown in fritted clay, movement of fixed15 N into the shoots was evident within 4 days after the roots were exposed to15 N2 at 2 kPa of O2 . Inoculation withAzospirillum lipoferum yielded no significant differences in shoot dry weight, total nitrogen content, percent nitrogen, or15 N enrichment of plant tissues. Inoculated plants did exhibit greater root dry weight than uninoculated plants, however.

15 N 2 Fixation by Bacteria Associated with Maize Roots at a Low Partial O 2 Pressure