Vander Beek-geo- and Phototropic Response of Seedlings

Shoots of oats, barley, and cucumber andc various other seedlings, grown in the dark in dilute solutions of 2,3,6-trichlorobenzoic acid or 2,6-dichlorobenzoic acid, failed to assume the normal vertical position. This was presumed to be due to repression of the geotropic response. Other plant species responded similarly to these two acids. A method was developed whereby a quantitative relationship was established between concentration of 2,3,6-trichlorobenzoic acid or 2,6-dichlorobenzoic acid and extent of repression of the geotropic response of shoots of oats. The former acid was shown to be much more active in this test. Of the two widely studied growth regulators, 2,4-dichlorophenoxyacetic acid and 2,3,5-triiodobenzoic acid, only the latter was found to be capable of interference with the geotropic response. The phototropic response of seedlings of a number of species was also similarly impaired when treated with 2,3,6-trichlorobenzoic acid and 2,6-dichlorobenzoic acid. A structural requirement, which appears to be associated with positions 2 and 6 of the benzene ring, for activity in the repression of the geotropic and phototropic responses was demonstrated; among the variously substituted benzoic acid derivatives tested, only 2,3,6-trichlorobenzoic acid and 2,6-dichlorobenzoic acid were active. The author wishes to thank Dr. A. G. Norman for his advice and encouragement throughout these studies.