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 CO₂production in nodulated roots were performed in situ with soybean{Glycine max (L.) Merr.) cv. Kingsoy grown hydroponically withorthophosphate (Pi) nutrition either limiting (low-P) or non-limiting(control) for plant growth. Nodule growth was more limited thanshoot growth by P deficiency. During ARA assays, nitrogenaseactivity declined a few minutes after exposure of the nodulatedroots to C₂H₂, and this acetyleneinduced decline (C₂H₂-ID) wastwice as intense at low-P. Moreover, the minimum ARA after theC₂H₂-ID was reached about 10 min earlier at low-P. The intensityof the C₂H₂-ID was correlated negatively with nodule mass perplant and positively with the ratio of shoot/nodule mass. Afterinitial exposure to C₂H₂, the nodulated-root CO₂ productionwas transiently stimulated and, moreover, this increase was2-fold higher at low-P. Then, the nodulated-root CO₂ productiondecreased with nodule C₂H₄ production. During the C₂H₂-ID, thenodule nitrogenase-linked respiration, which was computed asthe variable component of the linear regression between CO₂and C₂H₄ production, was 2-fold higher at low-P. Furthermore,the microscopic observation of nodule sections revealed thatstarch deposits were decreased at low-P. However, nitrogenaseactivity, i.e. ARA before the C₂H₂-ID, was not affected by Pdeficiency. It is argued that P deficiency increased the C₂H₂-IDbecause it increased nodule permeability to O₂ diffusion. Key words: Acetylene reduction, nitrogen fixation, phosphorus, respiration, soybean, Glycine max (L.) Merr