| Abstract: | Soybean Glycine max (L.) Merrill] plants that had been subjectedto 15 d of nitrogen deprivation were resupplied for 10 d with1.0 mol m–3 nitrogen provided as NO3–, NH4+, orNH4++NO3– in flowing hydroponic culture. Plants in a fourthhydroponic system received 1.0 mol m–3 NO3– duringboth stress and resupply periods. Concentrations of solublecarbohydrates and organic acids in roots increased 210 and 370%,respectively, during stress. For the first day of resupply,however, specific uptake rates of nitrogen, determined by ionchromatography as depletion from solution, were lower for stressedthan for non-stressed plants by 43% for NO3- resupply, by 32%for NH4+ + NO3– resupply, and 86% for NH4+ resupply. Whenspecific uptake of nitrogen for stressed plants recovered torates for non-stressed plants at 6 to 8 d after nitrogen resupply,carbohydrates and organic acids in their roots had declinedto concentrations lower than those of non-stressed plants. Recoveryof nitrogen uptake capacity of roots thus does not appear tobe regulated simply by the content of soluble carbon compoundswithin roots. Solution concentrations of NH4+ and NO3– were monitoredat 62.5 min intervals during the first 3 d of resupply. Intermittent‘hourly’ intervals of net influx and net effluxoccurred. Rates of uptake during influx intervals were greaterfor the NH4+ -resupplied than for the NO3– -resuppliedplants. For NH4+ -resupplied plants, however, the hourly intervalsof efflux were more numerous than for NO3– -resuppliedplants. It thus is possible that, instead of repressing NH4+influx, increased accumulation of amino acids and NH4+ in NH4+-resupplled plants inhibited net uptake by stimulation of effluxof NH4+ absorbed in excess of availability of carbon skeletonsfor assimilation. Entry of NH4+ into root cytoplasm appearedto be less restricted than translocation of amino acids fromthe cytoplasm into the xylem. Key words: Ammonium, nitrate, nitrogen-nutrition, nitrogen-stress, soybean |
