Whole Plant Studies using Radioactive 13-Nitrogen

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Abstract
Nitrate ion uptake by the roots of hydroponically grown maize seedlings was measured using the short-lived isotope 13N. It is shown to be described by a four compartment model, recognizably nitrogen in the root bathing solution, nitrogen which is readily exchangeable from the root, nitrogen bound in the root, and nitrogen transported from the root. Some of the absorbed activity leaks back into the root bathing solution with the efflux from the root, as a fraction of the influx, increasing with concentration to be greater than 0–8 at external nitrate ion concentrations above about 1.0 mol m−3. The capacity of the exchangeable root pool increases with external nitrate ion concentration, approaching the expected cytoplasmic nitrate ion content at the highest external nitrate ion concentrations studied (70 mol m−3). The investigation has highlighted the problems of interpreting uptake profiles in experiments for which the 10 min half-life of 13N dictates experimental times that are comparable with the times for saturation of root pools.