Studies on plant growth-regulating substances: XXXII. Chlorosubstituted α-hydroxy-and α-methoxy-phenylacetic acids

The plant growth-regulating activities of all the mono- and di-chloro-substituted α-hydroxy-phenylacetic (mandelic) acids and their methoxy derivatives have been determined in the wheat coleoptile cylinder, pea segment and pea curvature tests. In general, chloro-substituted α-hydroxy acids were less active in all three tests than the corresponding α-methoxy acids. The α-methoxy compounds and the dichloromandelic acids were more active in pea than in wheat tissues. These results are discussed in relation to the plant growth-regulating activities of the corresponding phenylacetic and phenoxyacetic acids.     

Studies on plant growth-regulating substances XXV. The plant growth-regulating activity of cinnamic acids

Effects of ring substitution on the plant growth-regulating activities of trans- and cis-cinnamic acids have been investigated in the wheat cylinder, pea segment and pea curvature tests. Most of the cis- acids were shown to be active. Substitution of fluorine, chlorine or bromine into the ring of cis-cinnamic acid in most cases increased the activity. The results are discussed in relation to mode of action and chemical structure/biological activity relationships: 4-chlorobenzoic acid is shown to act as a competitive antagonist towards 4-chloro-cis-cinnamic acid.     

Studies on plant growth-regulating substances LIII. The conversion of certain substituted ethylamines to active growth substances in plants

A number of substituted phenylethylamines have been synthesised and their plant growth-regulating activities, in standard wheat and pea tests, have been determined and compared with those of related substituted phenoxyethylamines, phenylacetic acids and phenoxyacetic acids. The effects of applying the most active amines to various plant species are described and the results are considered in relation to the possible ability of the plant to convert the amine into the physiological active acetic acid derivative.     

Studies on plant growth-regulating substances XXVI. Isatic and anthranilic acids

The plant growth-regulating activities of isatic acid and twenty-six of its derivatives, together with the twenty-seven corresponding anthranilic acids, have been assessed in the wheat cylinder, the pea segment and the pea curvature tests. Activity was sustained by substitution in the 4- and 5-positions of isatic acid but decreased by substitution in the 3- and 6-positions. In the anthranilic acid series, the parent acid was inactive but the introduction of a large grouping (bromine or iodine) into the 5-position conferred activity. The 3,6- and 5,6-dichloro and the 3,6-dibromo acids were also active; compounds substituted in the 4-position to the carboxyl group or disubstituted in the 3,5-positions, were, as expected, inactive. In metabolism experiments on wheat and pea tissues with isatic and 5-chloroisatic acids the corresponding anthranilic acid was formed, together with an unidentified non-acidic metabolite in each case. There was no evidence that the growth regulating activity of isatic acids was related to this breakdown and it is concluded that the acids possess activity per se. The results are briefly discussed in terms of recent theories relating chemical structure to plant growth-regulating activity.     

Studies on plant growth-regulating substances: The metabolism of indole-3-acetonitrile and related compounds in wheat and pea tissues

The plant growth-regulating activity of a number of new indole derivatives is reported. It is shown that indole-3-acetonitrile (IAN) is converted to indole-3-carboxylic acid by metabolism within wheat and pea tissues, and the mechanism of this a-oxidation reaction has been studied. The relevant indole compounds were synthesized and their metabolism investigated by T.L.C. techniques. N -Methylindole-3-acetonitrile was also shown to be degraded by a-oxidation in wheat and pea tissues and this was separately investigated. While no definite conclusions can be drawn, the evidence indicates that conversion of indole- and Af-methylindole-3-acetonitriles to the corresponding indole-3-aldehyde-cyanohydrins can occur. These compounds then become metabolized to the aldehydes and then to the respective indole-3-carboxylic acids. Indole- and A⁷-methylindole-3-glyoxylic acids do not appear to be involved in the a-oxidation reaction to any significant extent. Relevant studies on the metabolism of indole-3-acetaldehyde-cyanohydrin are also described.     

Studies on plant growth-regulating substances. XXX. The plant growth-regulating activity of substituted phenols

The growth-regulating activity of a number of substituted monophenols and related compounds has been assessed in the wheat cylinder, pea segment, pea curvature and tomato-leaf epinasty tests and the effect on activity of position, size and electronic nature of the substituents has been studied. The results indicate that, for high auxin activity, electron-attracting substituents, having certain steric properties must be substituted in the 2- and 6-positions. Furthermore, at least one of these ortho-substituents must be capable of intramolecular bonding with the hydrogen of the phenolic hydroxyl group. Any substituent in the para-position to the hydroxyl group leads to complete loss of activity and activity is reduced by meta-substitution. This new group of phenolic plant growth regulators is considered in relation to other synthetic auxins from the viewpoints of structural requirements for activity and current theories on mode of action.     

Studies on plant growth-regulating substances. XLIII. 2 -Chloro -3 -phenoxypropionitriles

A number of ring substituted 2-chloro-3-phenoxypropionitriles have been prepared and their plant growth-regulating activity has been assessed in the wheat coleoptile, pea segment and pea curvature tests, and also by spray treatments on tomato and dwarf French bean plants. The activities of these compounds are compared with those of previously studied 2-chloro-3-phenylpropionitriles and the results are discussed in terms of chemical structure/biological activity relationships and mode of action.     

Studies on plant growth-regulating substances XXVIII. Halogen-substituted benzoic acids

The plant growth-regulating activities of chloro-, bromo- and iodo-isomers in 2,5-, 2,3,5- and 2,3,6-substituted benzoic acids were assessed in the wheat cylinder, pea segment and pea curvature tests. Their effects in the tomato-leaf epinasty test were also investigated. Replacing an ortho-chloro atom by bromine had little or no effect on activity. An ortho-iodo substituent, however, reduced activity in the 2,3,6-substituted series and almost completely inhibited it in the 2,5- substituted series. This decreased activity of ortho-iodo-substituted isomers is visualized as being due to a steric inhibition of the attachment of the carboxyl group to its receptor unit prior to the initiation of the growth response.     

Studies on plant growth-regulating substances

Series of I, I-dichloro-2-phenoxyethanes, α,α-dichlorotoluenes and I, I-dihalo-2-phenylethanes have been examined for plant growth-regulating activity in a number of tests. Some compounds were shown to be active and the results, together with those obtained from metabolism studies, are discussed in terms of chemical structure/biological activity relationships and mode of action.     

Studies on plant growth-regulating substances. XL. 2-Chloro-3-phenylpropionitriles

The complete series of chloro-, dichloro-, methyl-, dimethyl- and chloro-methyl-ring substituted 2-chloro-3-phenylpropionitriles and several related compounds have been prepared. Their auxin activities have been assessed in the wheat coleoptile, pea segment and pea curvature tests, and the results, which are discussed in relation to molecular structure, support the suggestion by other workers that the observed activity is due to rapid uptake of the nitriles by the plant tissues, followed by metabolism to the corresponding phenylacetic acid. The high growth promoting activity of 2-chloro-3-(3-chloro-2-methyl-phenyl)propionitrile (orthonil) is confirmed but 2-chloro-3-(2,3-dichloro-phenyl)propionitrile and 2-chloro-3-(2,3-dichlorophenyl)butyronitrile are shown to be the most active members of the series.     

STUDIES ON PLANT GROWTH-REGULATING SUBSTANCES: CHLORO-SUBSTITUTED PHENYLACETIC ACIDS

The plant growth-regulating activity of all the mono- and di- chloro-substituted phenylacetic acids, together with the 2:3:6-trichloro- derivative, has been determined in the wheat cylinder, pea segment and pea curvature tests. Each of these compounds showed activity, the 2:3-dichloro- and the 2:3:6-trichloro- derivatives being extremely active. The 3:5-dichloro- acid was inactive in the wheat test but showed moderate activity in the pea tests.
All these findings are discussed in relation to the plant growth-regulating activity of corresponding benzoic and phenoxyacetic acids.     

Studies on plant growth-regulating substances

Nitrophenols structurally similar to 4-hydroxy-3-nitrobenzoic acid and 4-hydroxy-3-nitrobenzaldehyde have been examined for their ability to inhibit chloroplast development in linseed and oat seedlings, and their activity has been estimated quantitatively in specifically designed clover and Lemna bioassays. Twelve compounds were found to be active and activity is considered in relation to chemical structure. A nitro group in the 3-position and a hydroxyl group or ether linkage in the 4-position were found to be essential for activity whereas the nature of the functional group in the 1-position could vary considerably. Possibilities in which activity might arise from metabolism of the applied compound were investigated using excised wheat and pea tissue, and compounds with various groupings in the 1-position were shown to be converted to the corresponding active benzoic acid derivative.     

Studies on plant growth-regulating substances: Water uptake by discs from inulin-storing roots and tubers in response to growth-substance treatment

Discs of tissue cut from chicory and dandelion roots are shown to take up large amounts of water when treated with solutions of the auxin 2,4-dichlorophenoxyacetic acid. The effects of disc size, concentration of auxin and cold-storage of the roots on the magnitude of the response have been studied. A range of plant growth-regulating chemicals has been examined for the capacity to promote water uptake in chicory root discs. The results are considered both in relation to chemical structure and to the growth responses induced by these compounds in the wheat cylinder and pea segment tests. The evidence indicates that the water uptake response by chicory root tissues provides a useful method for assessing plant growth-promoting activity.     

Studies On Plant Growth-Regulating Substances

Eight series of aryloxyacetic, α-aryloxy-propionic, -butyric and -isobutyric acids have been synthesized, and their growth-regulating activity has been assessed using six different test methods, viz. tomato-leaf epinasty, Avena cylinder, Avena curvature, tomato parthenocarpy, tomato-leaf rooting and the Went pea test. The ability to induce morphogenic effects in tomato plants was also studied. The results are critically discussed and shown to be consistent with the view that in tests involving only cell extension the presence of an α-hydrogen atom is usually necessary for growth-regulating activity to be shown. In tests in which cell division occurs this effect is not so clearly defined, though the presence of an α-hydrogen does, in general, correlate with growth-regulating activity.     

Studies on plant growth-regulating substances

The metabolism of certain 2,6-disubstituted phenols that possess high auxin activity in the pea segment, pea curvature and tomato-leaf epinasty tests, but are much less active in the wheat cylinder test, has been investigated in wheat, pea and tomato tissue. Metabolites were identified by thin-layer chromatography and a semi-quantitative assay method was developed. The low activity of 2,6-dihalogenophenols and inactivity of 2-halogeno-6-nitro-phenols and 3-halogeno-2-hydroxybenzonitriles in the wheat cylinder test was caused by rapid metabolic conversion of the compounds in this tissue to inactive compounds by a process involving hydroxylation of the aromatic ring in the para- position. No such inactivation occurred in pea and tomato tissues. Evidence for a novel detoxification of nitrophenols within both pea and wheat tissue was obtained; 2-bromo–6-nitrophenol was converted via 2-bromo-6-aminophenol to N-acetyl-2-bromo-6-aminophenol. Certain 3-halogeno-2-hydroxybenzaldehydes and corresponding aceto-phenones, although fulfilling the necessary structural and electronic criteria for auxin activity, are inactive. Metabolic studies indicate that this is because they are metabolized in wheat, pea and tomato tissues to compounds not possessing the structural requirements for auxin activity.     

Studies on plant growth-regulating substances XXIX. The plant growth-retarding properties of certain ammonium, phosphonium and sulphonium halides

Wheat, pea and dwarf bean seedlings grown under controlled environmental conditions were used to assess the growth-retardant activity of members of series of chloro-substituted benzyl-, trimethyl- and tri-n-butyl- ammonium bromides. The influence on activity of trialkyl groupings other than tri-methyl and tri-n-butyl was also studied using compounds with the 4-chloro-benzyl ring structure. 3-Chloro- and 4-chloro-benzyltributylammonium bromides were the most effective compounds. The activity was similar to that of 2,4-dichlorobenzyltributylphosphonium bromide (Phosphon-D) and they showed little phytotoxicity. A series of chlorophenoxymethyltributylammo-nium and phosphonium salts were found to have lower activity than the corresponding chlorobenzyl derivatives. Allyldimethylsulphonium bromide retarded the growth of wheat seedlings but, like the aliphatic trimethylam-monium bromides tested, it was only slightly active in the pea-seedling test. The results are considered in relation to the chemical structure of the compounds studied. In particular, the influence of chlorine substitution in the ring of benzyltributylammonium salts on their growth-retardant activity is compared with the effect of similar substitution on the auxin activity of phenoxyacetic acids.     

STUDIES ON PLANT GROWTH-REGULATING SUBSTANCES

Fifteen series of aryloxyacetic, α-aryloxy-propionic, -butyric and -isobutyric acids and the corresponding arylthio-analogues have been synthesized and their growth-regulating activity has been assessed in the tomato-leaf epinasty, wheat cylinder and the pea curvature tests. It is shown that in general arylthio-derivatives are less active than their aryloxy-analogues. Although for a cell elongation response the presence in the molecule of an a-hydrogen atom is usually necessary, three exceptions to this generalization have now been found. A lower activity in the 3:5-dichloro- compared with the 2:6-dichloro-compounds has been demonstrated. Evidence is presented to show that an ortho hydrogen atom in the nucleus is not an essential requirement for cell elongating activity as is postulated by other workers.     

Studies on plant growth-regulating substances. XLIX. Phenoxyalkenes

A series of phenoxyalkenes has been examined in a number of tests for plant growth-regulating and selective herbicidal properties. Some of these compounds were found to possess high activity. The results are discussed in terms of chemical structure/biological activity relationships and mode of action.     

Studies on plant growth-regulating substances XLV. Phenoxyacetaldehyde derivatives

Plant growth-regulating activity in a series of phenoxyacetaldehyde derivatives has been assessed in a number of tests. Metabolic experiments using labelled ¹⁴C compounds are discussed.     

STUDIES ON PLANT GROWTH-REGULATING SUBSTANCES XI. AUXIN ANTAGONISM IN RELATION TO A THEORY ON MODE OF ACTION OF ARYL- AND ARYLOXY-ALKANECARBOXYLIC ACIDS

A study has been made of the antagonistic effects of a range of phenoxy and other acids on the growth-promoting activity of a number of auxins as assessed by the Avena cylinder and pea curvature tests. The compounds examined as inhibitors were selected on a logical basis, these molecules not possessing the structural requirements considered necessary for growth-regulating activity, yet possessing groupings which might allow them to accumulate at a site of action from which the growth response by auxins might be initiated. Our previous demonstrations that inactive stereoisomers of x-aryloxy propionic acids can competitively inhibit the activity of their active enantiomorph has been confirmed and a similar antagonism has been found to operate with other inactive aryloxy acids. Such competitive antagonism is discussed in relation to a 'three-point' contact theory on mode of action.