Influence of Nitrate and Ammonium Nutrition on the Uptake, Assimilation, and Distribution of Nutrients in Ricinus communis

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Ricinus communis L. plants were grown in nutrient solutions in which N was supplied as NO3− or NH4+, the solutions being maintained at pH 5.5. In NO3−-fed plants excess nutrient anion over cation uptake was equivalent to net OH− efflux, and the total charge from NO3− and SO42− reduction equated to the sum of organic anion accumulation plus net OH− efflux. In NH4+-fed plants a large H+ efflux was recorded in close agreement with excess cation over anion uptake. This H+ efflux equated to the sum of net cation (NH4+ minus SO42−) assimilation plus organic anion accumulation. In vivo nitrate reductase assays revealed that the roots may have the capacity to reduce just under half of the total NO3− that is taken up and reduced in NO3−-fed plants. Organic anion concentration in these plants was much higher in the shoots than in the roots. In NH4+-fed plants absorbed NH4+ was almost exclusively assimilated in the roots. These plants were considerably lower in organic anions than NO3−-fed plants, but had equal concentrations in shoots and roots. Xylem and phloem saps were collected from plants exposed to both N sources and analyzed for all major contributing ionic and nitrogenous compounds. The results obtained were used to assist in interpreting the ion uptake, assimilation, and accumulation data in terms of shoot/root pH regulation and cycling of nutrients.

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