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Environmental and Stress Physiology

Cyclic Variations in Nitrogen Uptake Rate of Soybean Plants ¹

Ammonium as a Nitrogen Source

Department of Forestry, North Carolina State University, Raleigh, North Carolina 27695, Department of Soil Science, North Carolina State University, Raleigh, North Carolina 27695

When NO₃^– is the sole nitrogen source in flowing solution culture, the net rate of nitrogen uptake by nonnodulated soybean (Glycine max L. Merr. cv Ransom) plants cycles between maxima and minima with a periodicity of oscillation that corresponds with the interval of leaf emergence. Since soybean plants accumulate similar quantities of nitrogen when either NH₄⁺ or NO₃^– is the sole source in solution culture controlled at pH 6.0, an experiment was conducted to determine if the oscillations in net rate of nitrogen uptake also occur when NH₄⁺ is the nitrogen source. During a 21-day period of vegetative development, net uptake of NH₄⁺ was measured daily by ion chromatography as depletion of NH₄⁺ from a replenished nutrient solution containing 1.0 millimolar NH₄⁺. The net rate of NH₄⁺ uptake oscillated with a periodicity that was similar to the interval of leaf emergence. Instances of negative net rates of uptake indicate that the transition between maxima and minima involved changes in influx and efflux components of net NH₄⁺ uptake.