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

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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.

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