Among the various derivatives of phenoxyacetic acids, 2,4-D (salts and esters) and MCPA (salts) are widely used as herbicides. The chemical properties of these herbicides are due to the aromatic radical (phenyl) and the presence of the carboxyl group COOH. 2,4-D (2,4-dichlorophenoxyacetic acid) is a derivative of 2,4-dichlorophenol into whose hydroxyl group an acetic acid residue has been introduced making it a new compound with herbicidal properties. These compounds combine with inorganic and organic bases forming salts, which are easier to dissolve and store, being sufficiently stable in both solid (powder) and liquid (solution) states. With alcohols, they form esters that are widely used as herbicides, as they evaporate more easily and spread further when sprayed.
The physiological activity of phenoxyacetic acid increases when a halogen such as fluorine or chlorine is introduced into the aromatic radical affecting the effectiveness of these herbicides. The position of these halogens is key in determining how strong the herbicide will be. For example, in the dichlorophenoxyacetic acid series, 2,4-dichlorophenoxyacetic acid has the highest physiological activity positing the strongest version of the chemical, and 2,6-dichlorophenoxy-acetic acid has the lowest activity making the chemical less effective. The effectiveness in question decreases depending on the chlorine atom positions as follows: 2,4-D >> 2,5 >» 3,4 > > 3,5 > 2,6. If a hydrogen atom is substituted by an aliphatic hydrocarbon radical in a molecule of a phenoxyacetic acid, the herbicide’s effectiveness is not affected considerably.
These herbicides penetrate well into plants through their leaves and roots but are more frequently sprayed into vegetating plants using aqueous solutions and emulsions. The spraying periods are established depending on the phase of stability of the cultivated plants, the sensitivity of the weeds, and the weather. The herbicides may penetrate through open stomas, especially well if the leaves are young with thin epidermal tissues, and the weather is favourable. They may also penetrate through either the cuticles or cell walls – since, both hydrophilic and lipophilic compounds can permeate easily through them.
Herbicides undergo chemical transformations within the plants that may either lead to the intensification of their toxicity or to complete inactivation of the same making the herbicide harmless. Derivatives of 2,4-D undergo processes of metabolism in three directions: decarboxylation (destruction of the side chain and formation of C02 at its expense), hydroxylation (the introduction of a hydroxyl group in the ring), and the formation of complex compounds with the metabolites. The formation of complex compounds of 2,4-D with the products of metabolism in plants also leads to the loss of phytotoxicity of this herbicide, meaning that it no longer harms the plant.
Herbicides that are derivatives of 2,4-D do not stay the same once they enter the soil as well, decomposing through various chemical and biological processes. Salts of 2,4-D soluble in water may be washed into the deeper soil layers by rain or irrigation, leading to the contamination of groundwater. On the contrary, esters of 2,4-D evaporate from the surface of the soil.
2,4-D herbicides also undergo photochemical decomposition. This process occurs more rapidly in soils with a high microbiological activity, under the influence of various microorganisms.
Microorganisms capable of influencing the decomposition of 2,4-D herbicides include Mycoplana, Rhizobium, Corynebacterium, Arjhrobacter, Achromobacter, Flavobacterium, and certain actinomyces.
Certain sensitive weeds react to these herbicides derived from phenoxyacetic acid quite rapidly. Within hours, the growth of the plant stops, the petioles and young shoots curl, and the entire plant bends abnormally. Thickenings are formed in the lower parts of the plant from which adventitious roots appear. The upper parts of roots thicken and decay, and young roots die off. The thickenings and fasciation of the shoots, leaves, and roots are attended by an increased turgor. As a result, the shoots and roots burst, and the wounds are infected by bacteria and fungi speeding up the plant’s death. The generative organs are equally affected leading to a range of consequent developmental deformities. These morphological changes are very diverse and depend on the age of the plant, its specific features, and weather conditions.
It has been established at present that 2,4-D discontinues oxidising phosphorylation by inhibiting the esterification of the phosphate, disrupting the process of formation of compounds rich in energy (macroergs) – ADP and ATP. It has also been established that 2,4-D strongly affects the synthesis of RNA (ribonucleic acid). This allows us to presume that the herbicides first affect the nucleic acids, and then, through the latter, the biosynthesis of proteins. Disruption of the biosynthesis of structural and enzyme proteins leads to disruption of the processes of metabolism, the result being complete disarrangement of many aspects of metabolism in a plant’s organism. Studies have further shown that 2,4-D noticeably affects the photosynthesis processes, both in terms of the photolytic activity of chloroplasts and photosynthetic phosphorylation. It is also known that 2,4-D has a sharp negative effect on the synthesis and transformation of growth stimulants in plants.
When considering all the aspects of the negative action of 2,4-D on plants, one cannot fail to note that this herbicide, as well as other ones similar to it, inhibits the enzymes of plants. The addition of a large number of foreign molecules, for example, of 2,4-D to the cytoplasmic system causes disruptions such that not one, but possibly all the enzyme systems lose their equilibrium, and this leads to the disruption of most processes.
This can be a partial explanation of such deep disruptions in plants. However, the causes of such irreversible disturbances of the physiological processes have not yet been revealed adequately to date.
The author is an associate professor (retd.) and former head of the department of botany at Ananda Mohan College.