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Articles

Cyclic Variations in Nitrogen Uptake Rate in Soybean Plants ¹

Department of Soil Science, North Carolina State University, Box 7619, Raleigh, North Carolina 27695-7619

Uptake of NO₃^– by nonnodulated soybean plants (Glycine max L. Merr. cv Ransom) growing in flowing hydroponic culture at 22 and 14°C root temperatures was measured daily during a 31-day growth period. Ion chromatography was used to determine removal of NO₃^– from solution during each 24-hour period. At both root-zone temperatures, rate of NO₃^– uptake per plant oscillated with a periodicity of 3 to 5 days. The rate of NO₃^– uptake per plant was consistently lower at 14°C than 22°C. The lower rate of NO₃^– uptake at 14°C during the initial 5 to 10 days was caused by reduced uptake rates per gram root dry weight, but with time uptake rates per gram root became equal at 14 and 22°C. Thereafter, the continued reduction in rate of NO₃^– uptake per plant at 14°C was attributable to slower root growth.

¹ Supported in part by National Aeronautics and Space Administration grant NCC 2-101. Paper no. 9577 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695.