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

This study was conducted on soybean (Glycine max L. Merr.) nodules to determine if exogenous NH(3) 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 NH(3). Nodulated root systems of intact plants were exposed to one of several NH(3) 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 NH(3) concentration and length of exposure of NH(3) 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 NH(3) concentration. The NH(3) 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 NH(3).