. Auxins and plant growth. Auxin; Plant growth promoting substances. 158 Fundamentals of Auxin Action methyl group for a hydrogen on the alpha carbon (XII) or a methyl- ene group for both hydrogens (XIII) does not take away activity, but the substitution of two methyl groups, as in the case of the wo-butyric acid derivatives (XIV) removes activity entirely (Koepfli et al, 1938). Such iso-butyric acids are anti-auxins (Burstrom, 1950). CHCOOH. XII XIII This information suggests then that steric hindrance on the acid side- chain can effectively prevent auxin activity. The presence of hydro- gens
. Auxins and plant growth. Auxin; Plant growth promoting substances. 158 Fundamentals of Auxin Action methyl group for a hydrogen on the alpha carbon (XII) or a methyl- ene group for both hydrogens (XIII) does not take away activity, but the substitution of two methyl groups, as in the case of the wo-butyric acid derivatives (XIV) removes activity entirely (Koepfli et al, 1938). Such iso-butyric acids are anti-auxins (Burstrom, 1950). CHCOOH. XII XIII This information suggests then that steric hindrance on the acid side- chain can effectively prevent auxin activity. The presence of hydro- gens on the alpha carbon is apparently not essential for activity. The substitution of a hydroxyl group on the side-chain has a re- markable effect (Thimann, 1951). In two such auxin derivatives (XV and XVI) auxin activity is eliminated by the hydroxyl substitution. Other acids with hydroxyl groups on the side-chain have been found to lack auxin activity. These include synthetic analogues of auxins a and b (Kogl and de Bruin, 1950). This finding has cast considerable doubt upon the existence of auxins a and b as growth hormones. OH CHzOH X'NCHCOOH f\ CHCOOH The degree of acidity of the acid side-chain plays a part in auxin activity. Salts of the acid may be less effective in growth than the free acid, and substitution of weak acid groups for the carboxyl reduces auxin activity. For example, the substitution of —SO3H (XVII) or —NO2 (XVIII) for the terminal carboxyl will reduce the activity, but such compounds sometimes still show distinct auxin activity or some- times none (Wain, 1949). Ester and amide derivatives of the carboxyl appear to be less effective than the free acid, although they may have distinct auxin activity (Kogl and Kostermans, 1935; Thimann, 1951). Some evidence has suggested that indoleacetonitrile (XIX) may possess auxin activity (Bentley and Housley, 1952), but such activity may be owing to its conversion to the acid plus synergistic properties of the nitril
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