Phosphorus deficiency increases the acetylene-induced decline in nitrogenase activity in soybean (Glycine max (L.) Merr.)

Open-flow assays of acetylene reduction activity {ARA)and CO2 production in nodulated roots were performed in situ with soybean {Glycine max (L.) Merr.) cv. Kingsoy grown hydroponically 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. During ARA assays, nitrogenase activity declined a few minutes after exposure of the nodulated roots to C2H2, and this acetyleneinduced decline (C2H2-ID) was twice as intense at low-P. Moreover, the minimum ARA after the C2H2-ID was reached about 10 min earlier at low-P. The intensity of the C2H2-ID was correlated negatively with nodule mass per plant and positively with the ratio of shoot/nodule mass. After initial exposure to C2H2, the nodulated-root CO2 production was transiently stimulated and, moreover, this increase was 2-fold higher at low-P. Then, the nodulated-root CO2 production decreased with nodule C2H4 production. During the C2H2-ID, the nodule nitrogenase-linked respiration, which was computed as the variable component of the linear regression between CO2 and C2H4 production, was 2-fold higher at low-P. Furthermore, the microscopic observation of nodule sections revealed that starch deposits were decreased at low-P. However, nitrogenase activity, i.e. ARA before the C2H2-ID, was not affected by P deficiency. It is argued that P deficiency increased the C2H2-ID because it increased nodule permeability to O2 diffusion.