Nitrogenase Activity and Nodule Gas Permeability Response to Rhizospheric NH3 in Soybean

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Abstract
This study was conducted on soybean (Glycine max L. Merr.) nodules to determine if exogenous NH3 exerts a controlling influence over nitrogenase activity through changes in nodule gas permeability (P), and if decreasing carbohydrate availability, as a result of low-light treatment, increases the sensitivity of root nodules to NH3. Nodulated root systems of intact plants were exposed to one of several NH3 concentrations ranging from 0 to 821 microliters per liter for an 8-hour period. Treatments were conducted under high-light (2300 micromoles per square meter per second) or low-light (800 micromoles per square meter per second) conditions. Increasing the NH3 concentration and length of exposure of NH3 caused a progressive decline in acetylene reduction activity (ARA). There was generally a greater reduction in ARA under the low-light treatment compared to the high-light treatment at a particular NH3 concentration. The NH3 concentration necessary to decrease P was greater than that needed to decrease ARA, and there was no evidence of a causal relationship between P and ARA in response to NH3.