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

Abstract

Nitrogen fixation by bacteria associated with roots of intact maize plants was measured by exposing the roots to 15 N 2 at a partial O 2 pressure (pO 2 ) 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 N 2 at a pO 2 of 2 or 10 kPa. Acetylene reduction was measured after the roots were exposed to the low pO 2 overnight. The air-N 2 atmosphere in the cylinders was then replaced with an O 2 -He atmosphere at the same pO 2 , and the roots were exposed to 20 kPa of 15 N 2 for 20 to 22 h. Incorporation of 15 N into the roots was 200 times greater at 2 kPa of O 2 than at 10 kPa of O 2 . Adding l -malate (1 g of C liter −1 ) to the nutrient solution increased root-associated nitrogenase activity, producing a strong 15 N label which could be traced into the shoots. Fixed 15 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 fixed 15 N into the shoots was evident within 4 days after the roots were exposed to 15 N 2 at 2 kPa of O 2 . Inoculation with Azospirillum lipoferum yielded no significant differences in shoot dry weight, total nitrogen content, percent nitrogen, or 15 N enrichment of plant tissues. Inoculated plants did exhibit greater root dry weight than uninoculated plants, however.