Nitrogen fixation (C2H2 reduction) associated with roots of intact ZEA MAYS in fritted clay at reduced oxygen tensions

Abstract The role of oxygen tension p O 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 p O 2 in the range from 0 to 14 kPa for 2–3 days (up to 13 days in one experiment). The p O 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 p O 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 p O 2 . Substantial AR activity (500–700 nmol C 2 H 4 g −1 dry 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 p O 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 p O 2 ⩽6 kPa. The results indicate that AR activity associated with roots of intact Zea mays is enhanced by p O 2 of 1–2 kPa and strongly inhibited by p O 2 ⩾ 6 kPa.