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

Abstract

Ricinus communis L. plants were grown in nutrient solutions in which N was supplied as NO(3) (-) or NH(4) (+), the solutions being maintained at pH 5.5. In NO(3) (-)-fed plants excess nutrient anion over cation uptake was equivalent to net OH(-) efflux, and the total charge from NO(3) (-) and SO(4) (2-) reduction equated to the sum of organic anion accumulation plus net OH(-) efflux. In NH(4) (+)-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 (NH(4) (+) minus SO(4) (2-)) 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 NO(3) (-) that is taken up and reduced in NO(3) (-)-fed plants. Organic anion concentration in these plants was much higher in the shoots than in the roots. In NH(4) (+)-fed plants absorbed NH(4) (+) was almost exclusively assimilated in the roots. These plants were considerably lower in organic anions than NO(3) (-)-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.