TRANS‐CINNAMIC ACID AS AN ANTI‐AUXIN

Abstract

IT IS WELL RECOGNIZED at present that the vast majority of compounds which display auxin activity have a distinct molecular configuration (recent summaries by Thimann, 1951; van Overbeek, 1950). Among these requirements is a rather specific steric relationship regarding the position of the side chain relative to the ring. This was realized as early as 1935 when Haagen-Smit and Went found that cis-cinnamic acid has auxin activity, while the trans-form is totally inactive. It has also become increasingly apparent in recent years that a close resemblance in molecular structure often exists between a physiologically active compound and its antagonists (summaries by Roblin, 1949; Sexton, 1950). A single deviation from the structure necessary for activity is often most effective in converting a metabolite into its anti-metabolite. In line with this reasoning it was, therefore, suspected that trans-cinnamic acid would display an action antagonistic to the growth promoting activity of the cis-isomer. And, because cis-cinnamic acid has the general qualifications of an auxin, the possibility was considered that trans-einnamic acid might be a general anti-auxin. These possibilities were indeed verified by experiment, and are reported below. In this paper the criterion by which anti-auxin activity is determined, is the capacity of transcinnamic acid to diminish the growth-promoting effect of added auxin. It is realized that reactions which would inactivate the auxin molecule per se, such as oxidations, would also diminish the auxin effect. However, it seems very likely that transcinnamic acid actually does compete with auxin, rather than inactivate it in a chemical sense. LITERATURE ON TRANS-CINNAMIC ACID AND ANTIAUXINS.-Trans-cinnamic acid has not been reported heretofore as an anti-auxin. It is more closely related to the auxin structure, without being an auxin, than any of the many other compounds reported to be antagonistic to auxins. Coumarin, closely related to cinnamic acid, was reported by Veldstra and Havinga (1943) to be antagonistic to naphthaleneacetic acid, together with several other lactones. Further unsaturated lactones having, antiauxin activity have been reported by Larsen (1947), and Thimann and Bonner (1949). Triiodobenzoic acid was shown to have anti-auxin activity by Galston (1946), de Waard and Florschutz (1948), and Thimann and Bonner (1948). Bonner (1949) reported still another type of compound having antiauxin activity: 2,4-dichloranisole. In