A study of the mechanism of ion absorption by plant roots using radioactive elements.

IF CERTAIN compounds or reactive groups exist in plant cells which are capable of fixing inorganic ions from the culture medium in exchange for equivalent ions released by the plant (Brooks, 1937), the assumed compounds must have the following properties: (l). The ion fixing compounds must be related to the oxidative metabolism of the plant since there is a parallelism between ion absorption and oxygen tension. Under conditions of arrested metabolic activity such as at low temperature or in solutions in equilibrium with low oxygen tensions, very little or no accumulation takes place in root systems (Hoagland and Broyer, 1936, 1942) or in a variety of other cells (Steward, 1937). (2). Tbe ion fixing compounds in the protoplasm must form compounds with ions such as K+, Rb+, Ca++, Br-, NO3-, S04=, HP04=, and others, in which the ions are held by relatively strong bonds since plants are able to accumulate these mineral nutrients offered at extremely low levels of concentration. Furthermore, plant roots can compete with soil colloids which bind nutrient cations very firmly. 1(3). The ion fixing compounds should account not only for the absorption of anions and cations, but also for the very wide range in the absorption of different ions of the same sign. This would presumably require two or more classes of compounds or groups. (4). Although the ions may be bound quite strongly to the ion fixing complex, the combinations so formed must nevertheless possess a high degree of instability since the evidence is that the ions pass into free solution in the vacuoles. Moreover, it is a familiar fact that in response to injury and death, ions absorbed in cells freely diffuse into the surrounding medium. For example, radioactive K could be completely leached from ether killed barley roots (Broyer and Overstreet, 1940). The obvious weakness of the above working hypothesis, and at the same time its chief appeal for the experimentalist, lies in the fact that it depends on the existence of compoiulds -th-at bave not as yet been demonstrated in plants and which may never be susceptible to direct isolation. The-object of the present research has been to gain additional information concerning the fixation of ions in plant cells and indirectly to study the nature of the ion binding. The work consisted of a study of the behavior of radioactive ions in apical segments of the roots of barley using radioactive isotopes of Sr and of I. I Received for publication March 18, 1947. 2Division of Plant Nutrition and Division of Soils. The authors are indebted to the staff of the 60-inch-cyclotron, Crocker Radiation Laboratory, University of California, for the production of the radioelements. Previous experiments on similar lines (Overstreet and Jacobson, 1946) had been made using isotopes of Rb and of P. Thus the new experiments together with the old furnish a comparison between a monovalent and a divalent cation and between a monovalent and a divalent anion. At the outset, it was again considered desirable to determine which segments of the excised root were most active in the accumulation of the ions Sr++-and I-. In order to avoid complications due to longitudinal translocation, these determinations were made at 00 C. Actual experiments indicated that translocation did not occur at this temperature. The general experience is that the accumulation of ions in quantity in the vacuole is at a minimum at such low temperatures. However, the use of the radioactive tracers enables experiments to be made for short periods during which the absorption of much smaller amounts of an ion can be detected even at the low temperatures, although it is perfectly possible that all of the absorbed ion is in the wall and protoplasm rather than in the vacuole. In other words, these experiments concern themselves with what may be the initial binding of ions in the protoplasm rather than the subsequent secretion into the vacuole for which higher temperatures may be necessary. As in previous experiments (Overstreet and Jacobson, 1946), it was not only advantageous but also necessary to use radioactive ions prepared without carrier. To elucidate the nature of ion fixation within the plant, the character of the exchanges between absorbed radioactive isotopes and inactive isotopes from the culture medium has been investigated. Here also radioelements prepared without carrier were used, e.g., Rb+, Sr++ and I-. EXPERIMENTAL.-Radioactive isotopes of Rb, Sr and I were prepared without inactive carriers in solutions of the order of 10-9 mol per liter. The isotopes were produced by means of the 60-inch cyclotron in the University of California Radiation Laboratory according to the following nuclear reactions: