PHOSPHORUS DEFICIENCY INCREASES THE ARGON-INDUCED DECLINE OF NODULE NITROGENASE ACTIVITY IN SOYBEAN AND ALFALFA

Open-flow assays of H-2 evolution in Ar:O-2 (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) n utrition either limiting (low-P) or non-limiting (control) for plant g rowth. Nodule growth was more limited than shoot growth by P deficienc y. Phosphorus concentration was less affected in nodules than in other parts of the low-P plants. During assays, nitrogenase activity declin ed a few minutes after exposure of the nodulated roots to Ar. The magn itude of this argon-induced decline (Ar-ID) was less in alfalfa than i n soybean. In both symbioses the magnitude of the Ar-ID was larger in low-P than control plants. Moreover, the minimum H-2 evolution after t he Ar-ID, was reached earlier in low-P plants. The Ar-ID was partly re versed by raising the external partial pressure of O-2 in the rhizosph ere. The magnitude of the Ar-ID in soybean was correlated negatively t o nodule and shoot mass per plant, individual nodule mass, H-2 evoluti on in air prior to the assay, and nodule N and P concentrations. Possi ble reasons, including nodule size and nodule O-2 permeability, for th e increase in Ar-ID in P-deficient plants are discussed and an interpr etation of the P effect on nodule respiration and energetic metabolism is proposed.