¹⁵N₂ Fixation by Bacteria Associated with Maize Roots at a Low Partial O₂ Pressure

Abstract

Nitrogen fixation by bacteria associated with roots of intact maize plants was measured by exposing the roots to ¹⁵N₂ 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 of ¹⁵N₂ for 20 to 22 h. Incorporation of ¹⁵N into the roots was 200 times greater at 2 kPa of O₂ than at 10 kPa of O₂. Adding l-malate (1 g of C liter⁻¹) to the nutrient solution increased root-associated nitrogenase activity, producing a strong ¹⁵N label which could be traced into the shoots. Fixed ¹⁵N 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 ¹⁵N into the shoots was evident within 4 days after the roots were exposed to ¹⁵N₂ at 2 kPa of O₂. Inoculation with Azospirillum lipoferum yielded no significant differences in shoot dry weight, total nitrogen content, percent nitrogen, or ¹⁵N enrichment of plant tissues. Inoculated plants did exhibit greater root dry weight than uninoculated plants, however.

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