Whole Plant Studies using Radioactive 13-Nitrogen

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Abstract
Methods are described for studying the uptake, by hydroponically grown Zea mays seedlings, of ammonium and nitrate ions labelled with radioactive nitrogen-13, which has a half-life often minutes. For nitrate only, some of the activity absorbed by the root exchanges back out again into the root bathing solution. The amount of this activity is about five times too large to be attributable to exchange with ions in the root cortical apoplasm. Much of it must be transferred from the root symplasm with a half-time of exchange of 2–5 min. After exposing the root to the labelled solution, equilibrium rates of transport to, and distribution in the shoot were attained within 2 min, for nitrate, or 5 min, for ammonium. The pools within the root, and the transport pathway through which the label passes must therefore rapidly attain the specific activity of the nutrient solution. Distribution patterns through the plant are reasonably consistent with earlier work on nitrogen assimilation and transport.