Energy status and functioning of phosphorus-deficient soybean nodules

Characterization of the effects of long-term P deficiency and of onset and recovery from P deficiency on bacteroid mass and number per unit nodule mass and energy status of soybean (Glycine max L. Merr.) nodules was used to investigate the mechanisms by which P deficiency decreases symbiotic N₂ fixation. The continuous P deficiency treatment (0.05 millimolar P) significantly decreased the whole plant dry mass, P, and N by 62, 90, and 78%, respectively, relative to the P-sufficient control (1.0 millimolar) at 44 days after transplanting. Specific nitrogenase activity was decreased an average of 28% over a 16-day experimental period by P deficiency. Whole nodules of P-deficient controls contained 70 to 75% lower ATP concentrations than nodules of P-sufficient controls. Energy charge and ATP concentrations in the bacteroid fraction of nodules were not significantly affected by P treatment. However, ATP and total adenylate concentrations and energy charge in the plant cell fraction of nodules were significantly decreased 91, 62, and 50%, respectively, by the P deficiency treatment. Specific nitrogenase activity, energy charge, and ATP concentration in the plant cell fraction increased to the levels of nonstressed controls within 2, 2, and 4 days, respectively, after alleviation of external P limitation, whereas bacteroid mass per unit nodule mass and bacteroid N concentration did not increase to the level of nonstressed controls until 7 days after alleviation of external P limitation. All of these parameters except bacteroid mass per unit nodule mass decreased to the levels of the P-deficient controls by 11 days after onset of external P limitation. Concentration of ATP in the bacteroid fraction was not significantly affected by alteration in the external P supply. Energy charge in the bacteroid fraction from plants recovering from P deficiency was decreased to a small (10%) but significant extent (P < 0.05) at two sampling dates relative to P-sufficient controls. These ATP concentration and energy charge measurements indicate that P deficiency impaired oxidative phosphorylation in the plant cell fraction of nodules to a much greater extent than in the bacteroids. The concurrence of significant changes in specific nitrogenase activity (2 days) and in the energy charge (2 days) and ATP concentration (4 days) in the plant cell fraction during recovery from external P limitation is consistent with the conclusion that P deficiency decreases the specific nitrogenase activity by inhibiting an energy-dependent reaction(s) in the plant cell fraction of the nodules.