Effect of 2,3,6-Trichlorobenzoic Acid and 2,6-Dichlorobenzoic Acid on the Geotropic and Phototropic Responses of Seedlings of Various Species.

Abstract

Shoots of oats, barley, and cucumber and 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. In this method hulled oat seeds are inserted after germination near the upper edge of a small cube of cellulose sponge which is placed in the test solution contained in a glass humidity chamber. The chamber is maintained under red light at 25[degree] C until the seedlings are 96 hours old. A shadowgraph of the seedlings is made and the angle formed by the seed and shoot is measured. 2,3,6-Trichlorobenzoic acid was shown to be much more active in this test. Of two widely studied growth regulators, 2,4-dichlorophenoxyacetic acid and 2,3,5-triiodobenzoic acid, only the latter was capable of interfering 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.