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 japonicu m strain PJ 17 and its response to external oxygen pressures (pO(2)) w ere measured in situ, 40-45 days after sowing (DAS), on plants grown h ydroponically with either limiting (low-P) or non-limiting (control) o rthophosphate (P-i) supplied weekly. At this age, shoot mass, nodule m ass and number of nodules of low-P plants were respectively 35, 14 and 26% of those of control plants, whereas root mass was not significant ly affected by P nutrition. The mean individual mass and size of nodul es 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 c ontrol values. By contrast, the nodule nitrogenase-linked O-2 uptake a nd the nodule permeability to O-2 of low-P plants were respectively 21 0 and 190% of those of control plants at ambient pO(2). However, P def iciency did not significantly affect the specific acetylene reducing a ctivity 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 induce s an increase in nodule O-2 uptake per electron transfered by nitrogen ase and an alteration in the structural features of nodules that are r elated to permeability to O-2.