Effect of pO(2) on Growth and Nodule Functioning of Symbiotic Cowpea (Vigna unguiculata L. Walp.).

Nodulated cowpea (Vigna unguiculata L. Walp. cv Vita 3:Bradyrhizobium CB 756) plants were cultured with their whole root system or crown root nodulation zone maintained for periods from 5 to 69 days after planting in atmospheres containing a range of pO(2) (1-80%, v/v) while the rest of the plant grew in normal air. Growth (dry matter yield) and N(2) fixation were largely unaffected by pO(2) from 10 to 40%. Decrease in fixation at pO(2) below 5% was due to lower nodulation and nodule mass and, at pO(2) above 60%, to a fall in specific N(2)-fixing activity of nodules. Root:shoot ratios were significantly lower at pO(2) below 2.5%. The effect of pO(2) on nitrogenase activity (acetylene reduction), both of whole nodulated root systems and crown root nodulation zones, varied with plant age but was generally lower at supra- and subambient extremes of O(2). H(2) evolution showed a sharp optimum at 20% O(2) but was at most 4% of total nitrogenase activity. The ratio of CO(2) evolved to substrate (C(2)H(2)+H(+)) reduced by crown root nodulation zones was constant (6 moles CO(2) per mole substrate reduced) from 2.5 to 60% O(2) but at levels below 2.5 and above 80% O(2) reached values between 20 and 30 moles CO(2) per mole substrate reduced. Effects of long-term growth with nonambient pO(2) on adaptation and efficiency of functioning of nodules are discussed.