Effect of pO(2) on the Formation and Status of Leghemoglobin in Nodules of Cowpea and Soybean

Nodulated cowpea (Vigna unguiculata [L.] Walp. cv Vita 3: Bradyrhizobium strain CB756) and soybean (Glycine max [L.] Merr. cv White Eye: Bradyrhizobium strain CB1809) were grown with their root systems maintained in a flowing gas stream containing a range of pO₂ (1-80%, v/v) in N₂ for up to 28 days after planting. At the extremes of sub- and supra-ambient pO₂, the levels of leghemoglobin (Lb) in nodules were reduced. However, neither the proportional composition of Lb component proteins (eight in soybean, three in cowpea) nor their oxidation state was affected by pO₂. Short-term changes in pO₂ (transferring plants grown with sub- or supra-ambient pO₂ in the rhizosphere to air or vice versa) caused a significant decline in Lb content and, in cowpea but not soybean, where pO₂ was increased, a higher percentage of oxidation of Lb. Combining data on changes in Lb level of cowpea nodules grown in sub-ambient pO₂ with those for their structural adaptation to an under supply of O₂ indicated that, despite the nodules having a lower level of Lb, the amount per infected cell was increased by up to twofold and per bacteroid up to fivefold (in those from 1% O₂) compared to those grown in air. Progressive decline in pO₂ resulted in a progressive increase on this basis, indicating a close relationship between Lb content and the adaptation of nodule functioning to external O₂ level.