Cyclic Variations in Nitrogen Uptake Rate in Soybean Plants

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Abstract
Uptake of NO3- by nonnodulated soybean plants (G. max L. Merr. cv. Ransom) growing in flowing hydrophobic culture at 22 and 14.degree. 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-h period. At both root-zone temperatures, rate of NO3- uptake per plant oscillated with a periodicity of 3-5 days. The rate of NO3- uptake per plant was consistently lower at 14.degree. C than 22.degree. C. The lower rate of NO3- uptake at 14.degree. C during the initial 5-10 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.degree. C. Thereafter, the continued reduction in rate of NO3- uptake per plant at 14.degree. C was attributable to slower root growth.