The Uptake of Growth Substances

Abstract

A comparative study has been made of the patterns of uptake of a series of chlorinated phenoxyacetic acids by segments excised from the etiolated shoots of Pisum sativum, Gossypium hirsutum, Sorghum vulgare, Avena sativa, and Triticum vulgare. The compounds investigated were phenoxyacetic acid (POA) itself and the following substituted compounds: 2,4,5-trichloro- (2,4,5-T), 2,4-dichloro-(2,4-D), 2,6-dichloro- (2,6-D), 4-chloro- (4-CPA) and 2-Chloro- (2-CPA). Each acid contained carbon-14 in the carboxyl group and the course of uptake was followed by measuring the changes in the radioactivity of the tissues. A special technique was developed which permits the accurate determination of carbon-14 without resorting to combustion. With the highly active auxins, 2,4-D and 2,4,5-T, the rates of uptake by segments of Gossypium and Pisum are initially high but decrease progressively with time and may end in a phase of net loss. In marked contrast the very weak auxins, 2-CPA and POA, are accumulated in a more or less linear fashion. The courses of uptake for 4-CPA and 2,6-D are intermediate in pattern. For the three graminaceous species uptake of all compounds is cumulative although the final contents are less for 2,4-D and 2,4,5-T than for 2-CPA and POA. In relation to the observed differences in pattern of uptake it is significant that Gossypium and Pisum are susceptible under field conditions to herbicidal applications of 2,4-D, while Triticum, Avena, and Sorghum can be classed as resistant. It is equally significant that the effects of variations in chemical structure on the pattern of uptake can be related to differences in the physiological activity of the compounds.