DOWNWARD TRANSLOCATION OF PHOSPHORUS IN SEPARATED MAIZE ROOTS

Abstract

THE DOWNWARD translocation in the root system of phosphorus, which is normally absorbed by the roots and translocated to the shoot, can best be investigated either by shoot-injection of the tracer phosphorus or by the method of root separation. A successful leaf-injection method was developed by Biddulph (1941) for his work on the downward translocation of phosphorus in stems. However, the root-separation method permits an uptake of phosphorus under natural conditions. The roots are divided between two independent culture solutions so that the tracer phosphorus enters the plant in a natural manner through a part of the root system but can be withheld from the remainder. Merrill (1915) reported that the root-separation method was employed by J. Macaire in 1832. Lead acetate was supplied to half the roots of a plant, and was eventually detected in the separated roots. Macaire also observed an efflux of lead acetate from the separated roots into their independent culture solution. Others were unable to repeat this work, and attributed Macaire's findings to the capillary movement of the solute over the adjacent lips of the two containers. Gile and Carrero (1917, 1921) summarized the many applications of the root-separation method and reported their own study of the phosphorus, nitrogen, and potassium absorption by corn and pea seedlings grown with various fractions of their roots deprived of one, two, or all of these elements. They concluded that plants cannot attain their maximum growth unless the roots have access to all three substances. Among the more recent applications of the root-separation method is the work of Maze (1936) on the inorganic requirements of corn. The early work on the efflux of inorganic substances from plant roots was reviewed by Merrill (1915). Jenny and Overstreet (1938), using excised barley roots and radioactive tracers, initiated a series of studies which have shown that the efflux of cations always involves an exchange of cations between the root and its environment, and that there is normally no simple excretion of inorganic salts. Overstreet and Jacobson (1946) extended this study to the rubidium cation and the phosphate anion. They found a considerable exchange of the rubidium ion, but found the phosphate ion to be strongly bound in the roots. The present study employs the root-separation