Ammonium and nitrate uptake by soybean during recovery from nitrogen deprivation.
Soybean [Glycine max (L.) Merrill] plants that had been subjected to 15 d of nitrogen deprivation were resupplied for 10 d with 1.0 mol m-3 nitrogen provided as NO3-, NH4+, or  NH4(+) + NO3- in flowing hydroponic culture.  Plants in a fourth hydroponic system received 1.0 mol m-3 NO3- during both stress and resupply periods.  Concentrations of soluble carbohydrates 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 ion chromatography as depletion from solution, were lower for stressed than for non-stressed plants by 43% for NO3- resupply, by 32% for NH4(+) + NO3- resupply, and 86% for NH4+ resupply.  When specific uptake of nitrogen for stressed plants recovered to rates for non-stressed plants at 6 to 8 d after nitrogen resupply, carbohydrates and organic acids in their roots had declined to concentrations lower than those of non-stressed plants.  Recovery of nitrogen uptake capacity of roots thus does not appear to be regulated simply by the content of soluble carbon compounds within roots.  Solution concentrations of NH4+ and NO3- were monitored at 62.5 min intervals during the first 3 d of resupply.  Intermittent 'hourly' intervals of net influx and net efflux occurred.  Rates of uptake during influx intervals were greater for the NH4(+)-resupplied than for the NO3(-)-resupplied plants.  For NH4(+)-resupplied plants, however, the hourly intervals of efflux were more numerous than for NO3(-)-resupplied plants.  It thus is possible that, instead of repressing NH4+ influx, increased accumulation of amino acids and NH4+ in NH4(+)-resupplied plants inhibited net uptake by stimulation of efflux on NH4+ absorbed in excess of availability of carbon skeletons for assimilation.  Entry of NH4+ into root cytoplasm appeared to be less restricted than translocation of amino acids from the cytoplasm into the xylem.
Rideout
J W
JW
Department of Agronomy and Soils, Clemson University, Florence, SC 29501-9603, USA.
Chaillou
S
S
Raper
C D
CD
Morot-Gaudry J-F
D000596:Amino Acids / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D009566:Nitrates / Q000493:pharmacokinetics*; D009584:Nitrogen / Q000378:metabolism*; D018517:Plant Roots / Q000378:metabolism / Q000502:physiology; D000644:Quaternary Ammonium Compounds / Q000493:pharmacokinetics*; D013025:Glycine max / Q000378:metabolism* / Q000502:physiology; D013997:Time Factors
D000596:Amino Acids; D009566:Nitrates; D000644:Quaternary Ammonium Compounds; D009584:Nitrogen