INFLUENCE OF THE FORM OF NITROGEN SUPPLY ON ROOT UPTAKE AND TRANSLOCATION OF CATIONS IN THE XYLEM EXUDATE OF MAIZE (ZEA-MAYS L)

Concentrations of inorganic cations are often lower in plants supplied with NH4+ as compared with NO3-. To examine whether this is attributable to impaired root uptake of cations or lower internal demand, the rates of uptake and translocation of K, Mg, and Ca were compared in maize plants (Zea mays L.) with different growth-related nutrient demands. Plants were grown in nutrient solution with either 1.0 mol m-3 NO3- or NH4+ and the shoot growth rate per unit weight of roots was modified by varying the temperature of the shoot base (SBT) including the apical shoot meristem. The shoot growth rate per unit weight of roots, which was taken as the parameter for the nutrient demand imposed on the root system, was markedly lower at 12-degrees-C than at 24-degrees-C SBT. As a consequence of the lower nutrient demand at 12-degrees-C SBT, uptake rates of NO3- and NH4+ declined by more than 50%. Compared with NO3- supply, NH4+-nutrition depressed the concentrations of K and particularly of Ca in the shoot, both in plants with high and with low nutrient demand. This indicates a control of cation concentration by internal demand rather than by uptake capacity of the roots. Translocation rates of K, Mg and Ca in the xylem exudate were lower in NH4+- than in NO3--fed plants. Net accumulation rates of Ca in the shoot were also decreased, whereas net accumulation rates of K in the shoot were even higher in NH4+-fed plants. It is concluded that reduced cation concentrations in the xylem sap of plants supplied with NH4+ are due to the lower demand of cations for charge balance. The lower K translocation to the shoot is compensated by reduced retranslocation to the roots. For Ca, in contrast, decreased translocation rates in NH4+-fed plants result in lower shoot concentrations.