UPWARD AND LATERAL MOVEMENT OF SALT IN CERTAIN PLANTS AS INDICATED BY RADIOACTIVE ISOTOPES OF POTASSIUM, SODIUM, AND PHOSPHORUS ABSORBED BY ROOTS

THE STUDY of the movement of mineral elements in plants is greatly facilitated by the use of indicator elements which are not initially present in the plant. Bromine and rubidium have been used for this purpose by F. C. Steward2 in extensive experiments on transport and accumulation. The former element has also been systematically employed in this laboratory as an indicator element. Radioactive isotopes possess certain obvious advantages in following rapid movements of mineral salts, since, first, it is possible to conduct experiments with ions which are known to be involved in the normal physiology of plants, and second, the readiness with which the radioactive isotopes can be detected in extremely small amounts enables one to make observations on small samples of tissues within relatively short periods of time. With the cooperation of Professor E. 0. Lawrence, Director of the Radiation Laboratory, J. P. Bennett of this Laboratory has made a study of the transport of radioactive potassium in the pear tree, and the present writers, also with the cooperation of Professor Lawrence, have conducted experiments on other plants with radioactive potassium, bromine, phosphorus, and sodium, as a development of an investigation of solute movement from root to shoot, especially as influenced by metabolic activities of the root and by transpiration. The purpose of this paper is to present experimental evidence on the path of transport of mineral elements in plants after their absorption by the roots through processes involving metabolic activities of living cells. While the prevailing view of botanists is that the mineral elements move upward in xylem tissues as the primary path of movement (cf. review by Mason and Phillis, 1937), this view has been questioned by Curtis (1935). More recently Clements and Engard (1938), following earlier work, have presented strong evidence, based on ringing experiments, in favor of the xylem as the chief path of upward movement of salt, but the problem has been raised again by Gustafson and Darken (1937) in their report on experiments with radioactive phosphate. By using radioactive isotopes, it was possible to design experiments which, though relatively simple in their general aspect, have provided direct and definite experimental evidence of the path of movement of certain ions in well-developed, foliated plants during periods when factors of light, temperature, and humidity were favorable for plant growth and for transpiration. In transport experiments with radioactive phosphorus, Gustafson and Darken (1937) observed 'Received for publication February 14, 1939. 2 Private communication. radioactivity in all parts of the plant whether the plant had a ring of bark or a section of xylem removed. In some of their experiments, Gustafson and Darken removed a strip of bark 5 to 10 cm. long from the xylem of a geranium plant, leaving both ends of the bark attached to the wood. They found less radioactive phosphorus in the detached bark than in the bark which remained in contact with the wood and concluded that there was either an active diffusion of radioactive phosphorus from the xylem into the phloem or that injury attendant on separating the bark had interfered with the transport through it. Their general conclusion was that both xylem and phloem function in upward movement of mineral salts. The experiments reported here are all "stripping experiments" (fig. 1 a). The technique of stripping was similar to that used by Gustafson and Darken and by earlier investigators. With the aid of a Geiger counter we were able to investigate the amount of radioactivity found in a strip of bark in considerable detail and within short periods of time following the absorption by the roots of the ion under studv. If salts are shown to have moved upward in the stem of the plant, and if it is further shown that the salts are present in some tissues of the plant and absent in others, one can be fairly certain that movement did not take place through the tissues in which the salt is found to be absent. Therefore, with the stripping method a possibility should exist for obtaining a clear answer to the question of whether the path of movement is through the phloem or the xylem. The stripping operation can be performed with a minimum of injury to either of the tissues involved and should allow the normal functions of the plant to proceed with the least possible interruption. For example, Mason and Maskell (1928) found that such treatment did not hinder downward movement of sugars and that contact between bark and wood was not necessary for this transport. EXPERIMENTAL METHOD.-The experimental plants were cotton, geranium, and willow. They were grown in culture solutions [Ca (NO3 ) 2, .005 M; KNO3, .005 M; KH2 PO4, .001 M; MgSO4, .002 M] from seedlings of cotton and from small cuttings of geranium and willow. Some geranium p'ants were also grown in sand cultures, so that there would be no possibility of mechanical disturbance of the roots. No difference was observed in the behavior of plants grown in water culture and in sand culture, with respect to salt absorption and upward transport. A series of experiments was made on the transport of radioactive potassium, sodium, phosphate,