Cyclic Variations in Nitrogen Uptake Rate in Soybean Plants

Uptake of NO(3) (-) by nonnodulated soybean plants (Glycine max L. Merr. cv Ransom) growing in flowing hydroponic culture at 22 and 14 degrees C root temperatures was measured daily during a 31-day growth period. Ion chromatography was used to determine removal of NO(3) (-) from solution during each 24-hour period. At both root-zone temperatures, rate of NO(3) (-) uptake per plant oscillated with a periodicity of 3 to 5 days. The rate of NO(3) (-) uptake per plant was consistently lower at 14 degrees C than 22 degrees C. The lower rate of NO(3) (-) uptake at 14 degrees 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 degrees C. Thereafter, the continued reduction in rate of NO(3) (-) uptake per plant at 14 degrees C was attributable to slower root growth.