Nitrogen fixation (C 2H 2 reduction) associated with roots of intact ZEA MAYS in fritted clay at reduced oxygen tensions

Abstract

The role of oxygen tension pO 2) as a limiting factor for associative nitrogen fixation was studied by measuring acetylene reduction (AR) associated with the roots of intact Zea mays. The plants were grown in a greenhouse in plastic (PVC) cylinders containing fritted clay as the rooting medium. Periodically during a 10–12-week growth period, a silicone rubber seal was applied at the base of the stem and the rooting medium was exposed to a pO 2 in the range from 0 to 14 kPa for 2–3 days (up to 13 days in one experiment). The pO 2 in the fritted clay was controlled by sparging the cylinders with known concentrations of O 2 (air diluted with N 2). Rates of AR were measured daily while the roots were exposed to the reduced pO 2. The highest AR activity occurred when the cylinders were sparged with 1–2 kPa O 2, regardless of the plant's stage of growth. The activity increased with prolonged exposure (up to 48 h) to the low pO 2. Substantial AR activity (500–700 nmol C 2H 4g −1dry wt h −1) occurred when the roots were exposed to 0 kPa O 2, but only after several days of the anoxic treatment. Little or no activity was exhibited by roots exposed to ⩾6 kPa O 2. Plants assayed under 1–2 kPA O 2 supported increasing rates of AR as they grew vegetatively, but the activity declined during flowering. Plants exposed to higher pO 2 supported increasing rates of AR through vegetative growth and flowering, though the activity was always very low. Root growth, but not root respiration, was inhibited by pO 2⩽6 kPa. The results indicate that AR activity associated with roots of intact Zea mays is enhanced by pO 2 of 1–2 kPa and strongly inhibited by pO 2⩾ 6 kPa.