The Uptake of Growth Substances: VII. THE ACCUMULATION OF CHLORINATED BENZOIC ACIDS BY STEM TISSUES2

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Abstract
The courses of uptake of benzoic acid (BA) and its 2-chloro- (2-CBA), 2,4-dichloro- (2,4-DCBA), 2,5-dichloro- (2,5-DCBA), and 2,3,6-trichloro- (2,3,6-TCBA) derivatives, all containing 14C in the carboxyl group, have been investigated, employing stem segments of Pisum sativum, Gossypium hirsutum, and Avena sativa. From comparisons of the rates of accumulation by segmetns of different length it is conclueded that for each compound uptake proceeds largely or wholly via the cut surfaces. The initial uptake of BA and 2-CBA by segments of Pisum is depressed as the pH of the solution is raised from 4 to 6.5, but the fall is less rapid than the decrease in the proportion of undissociated molecules. For all three species, BA and 2-CBA, which induced no extension growth, were accumulated at a more or less constant rate. By contrast, the course of uptake of 2,3,6-TCBA, a powerful auxin, exhibited marked deviations from a linear pattern, especially in Avena where uptake became negative between four and six hours. This loss of radioactivity from the tissues was due to the net egress of 2,3,6-TCBA itself into the external solution. In Avena the two dichloro-benzoic acids (2,4-DCBA and 2,5-DCBA) have intermediate trens: net accumulation declined almost to zero but subsequently recovered and proceeded at a rapid rate. These findings are discussed in relation to prior studies of the uptake of substituted phenoxyacetic acids and the concepts of Type 1 and Type 2 accumulation. It is proposed that accumulation of BA and 2-CBA is largely governed by a stable Type 2 process while the initial uptake of the powerful auxins, 2,3,6-TCBA and 2,5-DCBA proceeds via an unstable system, similar or identical to Type 1 accumulation.