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 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 N(2) for 20 to 22 h. Incorporation of 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) to the nutrient solution increased root-associated nitrogenase activity, producing a strong N label which could be traced into the shoots. Fixed 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 N into the shoots was evident within 4 days after the roots were exposed to 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 N enrichment of plant tissues. Inoculated plants did exhibit greater root dry weight than uninoculated plants, however.