Aliphatic acid, plant

The herbicide 2,4-D is itself a potent phytotoxin. However, a number of structurally related but inactive compounds may be converted by plants to 2,4-D following the activation process and thus act as herbicides. These phenoxyalkanoic acids are co-(2,4-dichlorophenoxy) alkanoic acids. The transformation may be viewed as shown in Fig. 12 as 6-(2,4-dichlorophenoxy)hexanoic acid as the parent compound. The sequence is called /1-oxidation because the steps in which two carbons are removed initially involve the oxidation of the /1-carbon to the aliphatic acid moiety. 

Two aliphatic acids possess, for grasses, many of the growth-distortion and toxicity effects associated with the synthetic auxins on dicotyledonous plants. Trichloroacetic acid and 2,2-dichloropropionic acid (dalapon), as the sodium salts, have been called grass "hormones or auxins, although Wilkinson184 could find no growth stimulation at low concentrations, and described dalapon as an antiauxin from its interference with indole-3-acetic acid effects. The herbicidal properties of trichloroacetate do not depend on its protein-denaturing ability, and those of 2,2-dichloropropionic acid involve, at least indirectly, the synthesis of pantothenic acid. 

The nomenclature of carboxylic acids and their derivatives was discussed in Section 7-6. Many carboxylic acids have trivial names and often are referred to as fatty acids. This term applies best to the naturally occurring straight-chain saturated and unsaturated aliphatic acids, which, as esters, are constituents of the fats, waxes, and oils of plants and animals. The most abundant of these fatty acids are palmitic, stearic, oleic, and linoleic acids. 

A wide range of simple aliphatic acids are ubiquitous in soils and plants. Amongst these are a number of di- and tri-carboxylic adds, oxalic (1), malonic (2), malic (3), tartaric (4) and citric (5), which are implicated as having metal-binding roles. 

The biochemical mode of action of dalapon has not been unequivocally elucidated. The protein precipitating action of chlorinated aliphatic acids, hence of dalapon, is known (Redemann and Hamaker, 1954), and it has also been proved by the investigations of Kemp et al. (1969) that the acid form of dalapon is able to form a hydrogen bond with the amide group of the protein molecule, so that this mechanism, in blocking enzyme activity, may be the cause of the phytotoxic action. Hilton et al. (1959) proved that dalapon inhibits the pantothenic acid synthesis of plants. 

These requirements are valid for the groups of the compounds investigated, they are not valid, however, in the case of benzoic acid, naphthenic acids and aliphatic acid derivatives (e.g., S-carboxymethyl dithiocarbamate), compounds with plant growth-regulating action discovered later (Veldstra, 1952 Van der Kerk et al.,... 

Under anaerobic soil conditions, organic substrates are often not decomposed completely to carbon dioxide. Incompletely oxidized intermediates and end products toxic to plants often accumulate in waterlogged soils. These intermediates and end products include lactic acid, ethanol, acetaldehyde, and aliphatic acids such as formic, acetic, or butyric acid. Ethylene is also sometimes present in abnormally high concentrations in waterlogged or anaerobic soil. 

Toxins. Metabolic products from microorganisms, plants, or animals with a poisonous effect on mammals, especially, humans. T. are mostly immunogenic, i.e., they can induce the formation of specific antibodies (antitoxins) as a result of their antigen character. They belong to widely differing classes of compounds such as proteins, lipopolysaccharides, alkaloids, terpenoids, steroids, aliphatic acids, biogenic amines, and guanine derivatives. 

The fatty acids of lichens show some resemblance to those in non-lichen-forming fungi, but none are identical. Aliphatic acids, much rarer in lichens than in other plant groups, appear to be formed in a very different manner through the tricarboxylic acid cycle. Known higher aliphatic acids and related substances are acaranoic acid, acarenoic acid, (+)-aspicilin, (—)-caperatic acid, (—)-lichesterinic acid, linoleic acid, (—)-nephromopsinic acid, (+)-nephrosteranic acid, (+)-nephrosterinic... 

This section is mainly concerned with the non-volatile, non-nitrogen-containing organic acids. It is restricted to the aliphatic acids and alicyclic acids (except for fatty acids, which are covered in Sect. 6.4.2) the aromatic acids, except for shikimic and quinic acid, are covered in Chap. 7. Since only a limited number of acids have been reported to occur in woody tissues such as bark and heartwood, the acids present in other parts, such as leaves and fruit, are also included. Those found only in herbs are excluded. No attempt has been made to search all the woody plants for the occurrence of these compounds. In the following, a brief introductory explanation about the bioformation and function in plants is described for the respective organic acids (10, 33, 43, 61, 120, 121), and this is followed by tables listing the occurrence of the acid in woody plants, if any. 

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