The Uptake of Growth Substances: VI. A COMPARATIVE STUDY OF THE FACTORS DETERMINING THE PATTERNS OF UPTAKE OF PHENOXYACETIC ACID AND 2,4,5-TRICHOLOROPHEN-OXYACETIC ACID, WEAK AND STRONG AUXINS, BYGOSSYPIUMTISSUES2

Abstract

Using segments of etiolated hypocotyls of Gossypium, a comparative study has been made of the processes which determine the patterns of uptake of a very weak auxin, phenoxyacetic acid (POA), and a very powerful one, 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). When segments are placed in solutions of POA-1-¹⁴C, a continuous increase in the radioactivity of the tissues is accompanied by the formation of radioactive metabolities which can be separated from POA by techniques of paper chromatography. At the same time there is a progressive increase in the amount of radio-activity which cannot be removed by transferring the tissues to buffer. Uptake is inhibited by low temperature, anaerobiosis, 2,4-dinitrophenol, and iodoacetate. It is concluded that the accumulation of POA involves its metabolic conversion to products which do not readily diffuse out into the external medium. With 2,4,5-T-1-¹⁴C the radioactivity of the segments at first increases rapidly but this is followed after two hours by a phase of rapid decrease. No radioactive metoabolites can be detected by paper chromatography and all of the ¹⁴C taken up can be rapidly removed by transfer to buffer. The magnitude of the decrease in radioctivity during the second phase of uptake is balanced by a release to the medium of a matched amount of radioactive 2,4,5-T. Uptake of 2,4,5-T is somewhat less sensitive to temperature and anaerobiosis than uptake of POA and is by contrast only slightly inhibited by 2,4-dinitrophenol and iodoacetate. Pretreatment of segments in buffer markedly alters the pattern of uptake of 2,4,5-T but not that of POA. It reduces both the amount of 2,4,5-T initially taken up and the amount subsequently released to the medium. In addition, both net loss of radioactivity during the course of uptake of 2,4,5-T and the reduction in the extent of uptake following pretreatment are both arrested by adding streptomycin, but not by the addition of pencillin or chloramphenicol. It is concluded that the uptake of 2,4,5-T involves reversible accumulation by a process whose efficiency decreases with time: the most likely systems are a metabolically linked mechanism for the active transport across a membrane or reversible adsorption on specific binding sites.