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 product ion in nodulated roots were performed in situ with soybean (Glycine ma x (L.) Merr.) cv. Kingsoy grown hydroponically with orthophosphate (Pi ) nutrition either limiting (low-P) or non-limiting (control) for plan t growth. Nodule growth was more limited than shoot growth by P defici ency. During ARA assays, nitrogenase activity declined a few minutes a fter exposure of the nodulated roots to C2H2, and this acetylene-induc ed decline (C2H2-ID) was twice as intense at low-P. Moreover, the mini mum ARA after the C2H2-ID was reached about 10 min earlier at low-P. T he intensity of the C2H2-ID was correlated negatively with nodule mass per plant and positively with the ratio of shoot/nodule mass. After i nitial exposure to C2H2, the nodulated-root CO2 production was transie ntly stimulated and, moreover, this increase was 2-fold higher at low- P. Then, the nodulated-root CO2 production decreased with nodule C2H2 production. During the C2H2-ID, the nodule nitrogenase-linked respirat ion, which was computed as the variable component of the linear regres sion between CO2 and C2H2 production, was 2-fold higher at low-P. Furt hermore, 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 n odule permeability to O-2 diffusion.