Seedlings of Ricinus communis L. cultivated in quartz sand were supplied with a nutrient solution containing either 1 mol m−3 NO3 or 1 mol m−3 NH+4 as the nitrogen source. During the period between 41 and 51 d after sowing, the flows of N, C and inorganic ions between root and shoot were modelled and expressed on a fresh weight basis. Plant growth was clearly inhibited in the presence of NH+4. In the xylem sap the major nitrogenous solutes were nitrate (74%) or glutamine (78%) in nitrate or ammonium-fed plants, respectively. The pattern of amino acids was not markedly influenced by nitrogen nutrition; glutamine was the dominant compound in both cases. NH+4 was not transported in significant amounts in both treatments. In the phloem, nitrogen was transported almost exclusively in organic form, glutamine being the dominant nitrogenous solute, but the N-source affected the amino acids transported. Uptake of nitrogen and carbon per unit fresh weight was only slightly decreased by ammonium. The partitioning of nitrogen was independent of the form of N-nutrition, although the flow of nitrogen and carbon in the phloem was enhanced in ammonium-fed plants. Cation uptake rates were halved in the presence of ammonium and lower quantities of K+, Na+ and Ca2+ but not of Mg2+ were transported to the shoot.

As NH+4 was balanced by a 30-fold increase in chloride in the solution, chloride uptake was increased 6-fold under ammonium nutrition.

We concluded that ammonium was predominantly assimilated in the root. Nitrate reduction and assimilation occurred in both shoot and root. The assimilation of ammonium in roots of ammonium-fed plants was associated with a higher respiration rate.

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