Phosphorus deficiency increases the argon-induced decline of nodule nitrogenase activity in soybean and alfalfa

Open-flow assays of H2 evolution in Ar:O2 (80:20, v/v) by nodulated roots were performed in situ with soybean [Glycine max (L.) Merr.] and alfalfa [Medicago sativa L.) grown in sand with orthophosphate (Pi) nutrition either limiting (low-P) or non-limiting (control) for plant growth. Nodule growth was more limited than shoot growth by P deficiency. Phosphorus concentration was less affected in nodules than in other parts of the low-P plants. During assays, nitrogenase activity declined a few minutes after exposure of the nodulated roots to Ar. The magnitude of this argon-induced decline (Ar-ID) was less in alfalfa than in soybean. In both symbioses the magnitude of the Ar-ID was larger in low-P than control plants. Moreover, the minimum H2 evolution after the Ar-ID, was reached earlier in low-P plants. The Ar-ID was partly reversed by raising the external partial pressure of O2 in the rhizosphere. The magnitude of the Ar-ID in soybean was correlated negatively to nodule and shoot mass per plant, individual nodule mass, H2 evolution in air prior to the assay, and nodule N and P concentrations. Possible reasons, including nodule size and nodule O2 permeability, for the increase in Ar-ID in P-deficient plants are discussed and an interpretation of the P effect on nodule respiration and energetic metabolism is proposed.