Radioisotope techniques, to the naive, may represent a miracle in scientific research the answer to all investigational difficulties. On the other hand, to the cynic they may be considered as a fad which creates more problems than it solves. The truth seems to be somewhere between the two extremes. The real importance of these techniques are recognized by workers who are well grounded in their fields and who have been involved in the application of these techniques. They are able to recognize the important problems in their own fields, are familiar with the experimental material, and should be able to interpret results of their experiments. Nevertheless, it is only fair to recognize the many difficulties in handling, obtaining and storing radioisotopes. This paper is a brief discussion of some of the fundamentals of this research tool which has developed in recent years and some of its applications in plant research. It may give some ideas as to the possibilities of this technique in attacking some of the problems in agriculture in general and in plant research in particular. A brief review of some chemical and physical concepts familiar to all may be of some interest. All matter is composed of discrete particles, atoms, of the elements, or of larger particles, molecules, made up of combinations of these elementary atoms. Further, that "isotopes" are atoms of an element having the same chemical properties as all other atoms of the same element but differing slightly in weight. In nature a few of these isotopes are radioactive that is they are unstable and spontaneously charge into atoms of another (or the same) element with radiation of energy in the form-gamma rays, or alpha or beta particles. With the advent of controlled nuclear reactions, it has become possible to produce radioactive isotopes which do not occur in nature, by the exposure of selected elements to neutrons. Limited quantities of such "synthetic" elements had been produced earlier by the bombardment of specific target materials by other particles such as protons or alpha particles but the yields were small and the operation expensive. These atoms have some readily recognized characteristics: The radiator given off by, and the time required for disintegration of, the unstable atoms is characteristic of the particular isotope. For example, P32, the important radioisotope of phosphorus, emits only beta particles with a maximum energy of 1.69 Mev and decays at a rate such that onehalf of the initial quantity remains after 14.3 days and each ucceeding 14,3 days interval reduces the amount remaining by 50% (a "halfIife" of 14.3 days) while the isotope of, S35 gives off betas of only 1/10 the energy and has a half life of 87.1 days. C14, particularly useful in the study of biological systems, has a half life of 5568 years; emitting beta particlesof 0.155 Mev. The alpha particles, beta particles, and gamma rays emitted by the disintegrating atoms are "ionizing radiations", that is they have the common ability to produce electrons and positive ions in matter through which they pass. These pairs of oppositely charged ions are formed at the expence of energy loss from incident radiation. Instruments for the detection and measurement of ionization are extremely sensitive, the entrance of a single alpha particle into the "sensitive volume" of a modern counter can be readily detected. Since the radiation arising from the decay of radioactive atoms permits the detection of individual decaying atoms of individual decaying atoms of the element, it is possible to make quantitative measurements of extremely small quantities. For example, with good counting methods one can conveniently measure 2 X 10 - 11 brams of radioactive carbon. This extreme sensitivity of detection combined with the circumstance that the radioactive elements behave in chemical or biological systems as do their inactive counterparts, provide a technique for obtaining data hitherto an attainble and of unusual scope, the "Tracer" Technique". We can now get answers to such questions as: What proportion of a nutrient added to the soil is utilized by the crop? Is a hormone or growth regulator localized or widely translocated? And, more important perhaps, we have a means of making essentially direct observation on the fundamental or basic reactions of biological systems. 84 The "Tracer" method is not new. As early as 1913 Hevecy in Germany used a natural radioisotope of lead to study the solubility dead isotope to study the uptake and distribution of this element by plants. With the invention of the cyclotron the first man-made isotopes became available, but work was markedly limited by availability and cost. Since 1946, however, the development h as been accelerating. Through the U.S. Atomic Energy Commission usable quantities of some 20 isotopes of particular interest in plant research have become available at quite reasonable cost. These include an isotope of Carbon, C14, widely used in research on the fundamental photosynthetic process and as a label or tag in many organic compounds used as herbicides pesticides and metabolic precursors or intermediates; p32, radiophosphorus, a major plant nutrient and label for the new organophosphate insecticides; and usable isotopes of Sulphur Calcium and Iron and the important trape or micro-nutrients Cobalt Copper, Zinc, and Molybdenum. As one might expect, much of the early work, 1947 - 1952, was directed to those problems of rather "practical" nature where the results would be of general interest and would serve, too, to establish the value of the method. The U.S. Department of Agriculture during this period and in collaboration with many State Experimental Stations, carried out extensive studies on the behaviour of phosphatic fertilizers and the effect of soil type, fertilizer, management practices and crop on the uptake of phosphorus by crop plants. Evaluation of a fertilizer material or of an application method has relied on yield response - a time consuming procedure often set at naught by circumstances over which the experimenter had no control. A failure to obtain response to applied nutrient might be due to too little or too much rainfall at a critical stage of growth. Now we have a means of determining quite directly whether or not two sources of phosphorus, say, are equal in availability to a particular, crop on a selected soil or of selecting that method of application which places the fertilizer where it will do the most good since we can now differentiate between the fertilizer phosphorus and that originally present in the soil. Hence we can determine, even though circumstances may conceal or prevent yield response, what proportion of the total plant phosphorus came from that added. As an illustration of the method, suppose we add 100 black marbles identical except for color to a collection of white marbles, which are being mixed and sampled. If, we take samples of 100 marbles and each is found to include 10 black ones, then our sampling procedure is getting 10% of the added marbles. In using radioactive phosphorus we have "tagged" the fertilizer component so it may subsequently be recognized and evaluated although not in quite a black and white fashion as the illustration.Prof. Dr. Aly M. Lasheen Guru besar Plant Physiology Fakultas Pertanian Universitas Indonesia Bogor.