Isotopes as tracers in plants

Abstract

Isotopes afford an extremely useful means for following the absorption and transport of materials in plants and for elucidating the metabolic reactions of plants. The potentialities of the tracer method were early recognized by Hevesy who with Paneth in 1913 (72) used radiolead* as an indicator to determine the solubilities of lead chromate and lead sulfide, and in 1923 (63) followed the absorption and translocation of radiolead in plants. He applied the tracer technique in the same manner that it is used today (63), described its advantages and suggested how it could be employed. The early limitation of the tracer method to naturally occurring radioisotopes was removed by the discovery of artificially induced radioactivity by Joliot and Curie (83), and the use of stable isotopes as tracers was made possible by Urey's methods (146, 150) of separating D, N15, C13, 018 and S34 in usable quantities. Supplementary developments that have been particularly helpful are the production of uranium piles (nuclear chain reactors), cyclotrons and other particle accelerators, simplified mass spectrometers for measurement of stable isotopes, and commercial GeigerMuller counters and electrometers for measuring radioactivity. Since the release of radioisotopes by the Atomic Energy Commission (see Science 103: 697-705. 1946; and the catalog and price list of radioisotopes distributed by the Isotopes Branch, U. S. Atomic Energy Commission, Oak Ridge, Tenn., for lists of radioisotopes and the form in which they are supplied), it has become unnecessary to maintain elaborate high voltage particle accelerators for most tracer work, as neutron bombardment in the uranium pile produces satisfactory radioisotopes of almost all elements of biological interest. Deuterium and 018 are also available through the facilities of the Atomic Energy Commission. N15 and C13 may be purchased from Eastman Kodak Company; individual institutions no longer need maintain thermal diffusion columns for the concentration of C13.