Studies on nitrogen metabolism in tomato with use of isotopically labeled ammonium sulfate.

The stable isotope of nitrogen, N 15, has been employed extensively as a tracer in studies of nitrogen transformations in animals and bacteria, and its use has yielded much information difficult or impossible to obtain otherwise. The extension of this method to studies involving the higher plants, however, has been very limited. Hevesy and coworkers (1) studied the absorption of ammonium ions labeled with Nib in the sunflower and showed that there was a definite replacement of the nitrogen present in the mature leaf. Vickery and others (2) employed tobacco and buckwheat and found at the end of 72 and 47 hours, respectively, incorporation of N15 from N15H&1 into the principal nitrogenous fractions of the plants. The absorption, translocation, and utilization of inorganic nitrogen by plants has been extensively investigated by means of traditional methods. The demonstration in animals of a rapid and continuous interchange between the various forms of nitrogen (3), however, raises serious doubt concerning the interpretation of some of these earlier findings. Studies of the metabolic pathways for the incorporation of inorganic nitrogen into tissue protein and other nitrogenous constituents of higher plants have therefore been begun with the stable isotope of nitrogen as a tracer. This communication present,s certain limited observations of nitrogen metabolism in tomato plants.