Cyclic Variations in Nitrogen Uptake Rate of Soybean Plants: Effects of External Nitrate Concentration

Net uptake of $NO3-$ by non-nodulated soybean plants [Glycine max (L.) Merr.cv. Ransom] growing in flowing hydroponic cultures containing 0–5, 1.0 and 10-0 mol m⁻³$NO3-$ was measured daily during a 24-d period of vegetative development to determine if amplitude of maximum and minimum rates of net $NO3-$ uptake are responsive to external concentrations of $NO3-$. Removal of $NO3-$ from the replenished solutions during each 24-h period was determined by ion chromatography. Neither dry matter accumulation nor the periodicity of oscillations in net uptakerate was altered by the external $NO3-$ concentrations. The maxima of the oscillations in net uptake rate, however, increased nearly 3-fold in response to external $NO3-$ concentrations. The maxima and minima, respectively, changed from 4.0 and 0–6 mmol $NO3-$ per gram root dry weight per day at an external solution level of 05 mol m⁻³$NO3-$ to 15–2and -2–7 mmol $NO3-$ per gram root dry weight per day at an external solution level of 10–0 mol m⁻³$NO3-$. The negative values for minimum net uptake rate from10–0 mol m⁻³$NO3-$ solutions show that net efflux was occurring andindicate that the magnitude of the efflux component of net uptake was responsive to external concentration of $NO3-$.