TAPAN KUMAR MAITRA EXPLAINS THE MANY WAYS IN WHICH PESTICIDES PENETRATE PLANTS

PESTICIDES can readily penetrate a plant through its roots, especially if preplanting treatment of the seeds was carried out, or the toxicants were incorporated into the soil. Pesticides penetrate the roots notwithstanding the poor solubility of individual substances in water. This is associated with their solubility in lipids.
Pesticides are evidently absorbed by the roots in the same way as nutrient substances are, by diffusion, exchange adsorption and active transfer of molecules and ions.
This process may be of a passive nature, when the pesticide ions and molecules adsorbed on the surface of the roots penetrate, in an unchanged form, the free space of a cell and further move with the flow of water along the conducting vessels and cells of the green organ tissues. Pesticides may simultaneously enter a plant by metabolism when, being adsorbed on the outer surface of the cytoplasm of the roots’ cells, they are immediately involved in intensive metabolism. As a result of biochemical reactions, the pesticides may be irreversibly decomposed or may form complexes with the components of a cell. The intensity of entry via the roots grows with an increase in the dose.
The movement of a pesticide from the soil solution into a plant depends on soil properties. Clay and humous soils greatly adsorb pesticides and in this connection the latter become less accessible to plants. The moisture content of the soil is of considerable importance. The intensive absorption by a plant of insecticides from the soil in conditions of adequate moisture is closely related to the vigorous inflow and movement of water and nutrient substances.
In the treatment of growing plants, pesticides penetrate them mainly through the leaves (cuticle and stoma) in the form of a liquid or vapour. Penetration through the cuticle depends in many aspects on the anatomic and morphological features of the integument tissues.
According to modern notions, the cuticle consists of these unevenly distributed components: cutin — polymerised high-molecular acids and alcohols having simultaneously hydrophilic and lipophilic properties; cuticular hydrophobic waxes — low-molecular ethers of fatty acids and monatomic alcohols of the fatty series with a short chain; pectin — a hydrophobic substance of an amorphous structure permeable to water and polar compounds; and cellulose — a hydrophilic substance with a fibrous structure having high tensile strength.
The cuticle covers the entire surface of a leaf and the substomatic chamber (the surface of the mesophyllous and palisade cells inside a leaf open to the access of air) in the form of a solid film and is the main obstacle preventing the penetration of pesticides in the leaf. It is characterised by a negative charge and can adsorb water. The cuticle has a lipoid matrix, and lipophilic substances can penetrate through it. The ectodesmae of the cuticle are one of the possible ways for the penetration of hydrophilic compounds.
Pesticides penetrate a plant through its leaves only if they are in the form of a solution or emulsion. After crystallisation, no penetration of the substances takes place. Hydrophilic pesticides can enter via the aqueous phase of the cuticle, first into the acid components of the cutin and then into the pectin and water-permeable cytoplasmic membrane. In conditions of increased humidity, the micropores of the cuticle and cutin are saturated with water. With insufficient moisture, the micropores of the cuticle are filled with air, and the drops of the pesticide solution, encountering air locks in the pores, cannot come into contact with the water in them. In this case, the aqueous way of penetration is greatly hampered.
When there is insufficient moisture in a plant, pesticides penetrate in the lipoid way. Lipophilic substances penetrate through the fat components of the cell membrane.
The penetration of pesticides through the cuticle is determined by their solubility in water or in separate components of the cuticle and cell membrane and depends on the degree of their polarity. The cuticle is well permeable to oils, therefore many pesticides soluble in oil readily penetrate it. Having passed through the cuticle and cuticular layers, the pesticide molecules encounter an obstacle along their path to the protoplast in the form of the cellulose layers of the cell membrane. Pesticides may also enter a plant directly through the epidermis, hark, and integument tissues of its stem.

THE WRITER IS ASSOCIATE PROFESSOR, HEAD, DEPARTMENT OF BOTANY, ANANDA MOHAN COLLEGE, KOLKATA, AND ALSO FELLOW, BOTANICAL SOCIETY OF BENGAL, AND CAN BE CONTACTED AT tapanmaitra59@yahoo.co.in