Effect of pO2 on Growth and Nodule Functioning of Symbiotic Cowpea (Vigna unguiculata L. Walp.)

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Abstract
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 pO2 (1-80%, v/v) while the rest of the plant grew in normal air. Growth (dry matter yield) and N2 fixation were largely unaffected by pO2 from 10 to 40%. Decrease in fixation at pO2 below 5% was due to lower nodulation and nodule mass and, at pO2 above 60%, to a fall in specific N2-fixing activity of nodules. Root:shoot ratios were significantly lower at pO2 below 2.5%. The effect of pO2 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 O2. H2 evolution showed a sharp optimum at 20% O2 but was at most 4% of total nitrogenase activity. The ratio of CO2 evolved to substrate (C2H2+H+) reduced by crown root nodulation zones was constant (6 moles CO2 per mole substrate reduced) from 2.5 to 60% O2 but at levels below 2.5 and above 80% O2 reached values betwen 20 and 30 moles CO2 per mole substrate reduced. Effects of long-term growth with nonambient pO2 on adaptation and efficiency of functioning of nodules are discussed.