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Increase in permeability to oxygen and in oxygen uptake of soybean nodules under limiting phosphorus nutrition

The O 2 uptake of soybean roots nodulated with Bradyrhizobium japonicum strain PJ 17 and its response to external oxygen pressures (pO 2 ) were measured in situ, 40-45 days after sowing (DAS), on plants grown hydroponically with either limiting (low-P) or non-limiting (control) orthophosphate (P i ) supplied weekly. At this age, shoot mass, nodule mass and number of nodules of low-P plants were respectively 35, 14 and 26% of those of control plants, whereas root mass was not significantly affected by P nutrition. The mean individual mass and size of nodules were also lower in low-P plants. The rate of O 2 uptake for growth and maintenance of root and nodule tissue of low-P plants was 35% of control values. By contrast, the nodule nitrogenase-linked O 2 uptake and the nodule permeability to O 2 of low-P plants were respectively 210 and 190% of those of control plants at ambient pO 2 . However, P deficiency did not significantly affect the specific acetylene reducing activity of nodule nitrogenase at 45 DAS nor the ratio of whole plant N fixed to nodule mass at 75 DAS. It is argued that P deficiency induces an increase in nodule O 2 uptake per electron transfered by nitrogenase and an alteration in the structural features of nodules that are related to permeability to O 2 .