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

ABSTRACT

Nodulated cowpea (Vigna unguiculata L. Walp. cv Vita 3Bradyrhizobium 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. Rootshoot 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.