Plant growth‐regulating and systemic fungicidal activity: The aryloxyalkylcarboxylic acids

Abstract

INVESTIGATIONS on synthetic plant growth-regulating substances closely followed the discovery by Kogl et aZ.lg that indole-3-acetic acid was markedly active in promoting cell-elongation. Other indole derivatives were studied by Kogl and Kostermans,2O and Hitchcock's ; the work of Zimmerman and Wilcoxon5* revealed activity in I-naphthylacetic and other arylacetic acids, and in 1938 Irvine16 demonstrated that 2-naphthoxyacetic acid was an active growth-substance.The aryloxyalkylcarboxylic acids have since become an important group of plant growthregulating substances and certain of these compounds, e.g. 2 : 4-dichlorophenoxy-, a-methyl-qchlorophenoxy-and 2-naphthoxy-acetic acids, are now being widely used in agriculture and horticulture."? % 38 All such compounds possess the structural requirements for activity put forward by Koepfli, Thimann and Went18 in 1938. Thus they possess (I) a ring system containing at least one double bond (2) a side chain carrying a terminal -COOH group with at least one carbon atom between it and the ring system and (3) a particular steric relationship between ring and side chain. The molecular structure of most synthetic growth-substances conforms to these requirements. Veldstra,s6 however, on the basis of results obtained in polarographic studies, has reduced them to the following: (a) a ring system with a high surfaceactivity (b) a carboxyl group (or its dipole) in a definite spatial position with regard to this ring system. The evidence relating to all these factors has already been reviewed 1, s 6 9 38 and will not be considered again here. I t is important to note, however, that certain substituted benzoic acids have been shown to be effective for promoting cell-elongationsl 48 and for inducing morphogenic effects in In these compounds, the polar -COOH group is presumably in the plane of the ring, though SkoogS1 has suggested that the orientation of this group may be affected by the nuclear substituents. The importance of the configuration of the 4 O O H group in relation to the ring is well shown in cis-and trans-cinnamic and I : 2 : 3 : 4-tetrahydronaphthylideneacetic acids. The -COOH group is well removed from the plane of the ring in the cis forms of both of these compounds and only these forms are physiologically active.89 s8 On these and other considerations Veldstrase has suggested that growth substances exert their physiological activity at the protoplasmic membrane where, following adsorption and orientation at the surface boundary, the compound exerts an effect on membrane permeability and thus regulates transport. Promotion of growth then arises from the resulting increased flow of metabolites. Other workers have suggested that the action of growth substances is essentially chemical44 or that they act as coenzymes in some essential metabolite reaction.22. 261 341 46Now although the aryloxyalkylcarboxylic acids fulfil the requirements of Koepfli et al., not all of them are plant growth-regulating substances ; indeed, many are completely inactive. Thus, other factors must...

On the Mechanism of Action of Growth Regulators

“…These are (a) the effect of optical isomerism of the side chain (18,35,36), (b) the effect of geometrical isomerism of the side chain (31,32,33), (c) the ortho effect in benzoic acids (1,16), (d) the ortho effect in phenoxyacetic acids (7,13,17), (e) the inhibiting effects of lactones (6,32) and maleic hydrazide (4,9,14,40), (f) the inhibiting effect of cysteine (2), (g) the inhibiting effects of reagents specific for sulfhydryl groups (28,29), (h) the inactivity of phenoxyacetic acids without two open para positions (13), (i) the effect of electron-releasing and electron-attracting ring substituents (16). It is the purpose of this paper to present data on the relative activity of some growth regulators and to consider recent proposals for the reaction mechanism of the growth regulators in relation to the requirements listed above.…”

The Interaction of Some Auxin Antagonists and 2,4‐D in Root Growth

“…For the naphthalene derivatives this relation is often strongly infhienced by the point of attacbement (1 or 2) of the side chain. 1-NA does thus show a slightly higher activity than 2-NA in (he root test (unpublished results) as well as in the pea test (ti, p. 121, 145), while the reverse is true tor tlie fj-(naphthalene)-propionic acids ((i) and for the a-(riyplit'it^^y)-'*lky'carboxylic acids (6,8). For napbthoxyacetic acid the shift from 2-position to 1-position does not result in a lowered activity only hnt changes the character of the compomid fnndamentally, the 1-derivative being an effective auxin antagonist in tbe root test (unpublished resnits).…”

“…To the ring system a side chain with snch proi)erties or snch a steric relationshi]) to the ring system as make an anxin fnnction of tbe substance im-|)ossible, is then atlached. The presence of a hydrogen atom on the carbon atom alpha to tbe carhoxyl group has been shown to be necessary for auxin activity in a-laryloxy)-alkylcarhoxylic acids (8), and the inactive a-(aryloxy)-/,so-l)utyric acids may perhaps generally he antiauxins. For a-fpcblorophenoxy)-iiO-hutyric acid this has actually been shown (3).…”

Stereoisomerism in Plant Growth Regulators of the Auxin Type

“…Naphthyl-oxy, naphthyl-thio, and naphthyl-seleno compounds.-In 1951 the resolution of a-(2-naphthoxy)-propionic acid (2-NOP) and the growth effects of the antipodes were reported by two groups of workers (1,37,99,114). The auxin activity of the racemic substance was found to reside wholly in the D( + )-form, while the L-form exerted antiauxin effects (1,100).…”