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Environmental and Stress Physiology

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

U.S. Department of Agriculture, Agricultural Research Service, University of Florida, Gainesville, Florida 32611, Agronomy Physiology Laboratory, University of Florida, Gainesville, Florida 32611

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.

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