The development of chemicals to protect agricultural crops is an important activity within the chemical industry. Without them, many crops would suffer dramatic losses.
Some of these chemicals, the insecticides, are also very important in combating human and animal diseases. The environmental and toxicological properties of these chemicals have improved considerably over the last six decades. Research aims to produce chemicals that are not just potent but are specific for the required purpose, whilst not affecting the environment in any other way. Because pests may develop resistance to crop protection chemicals there is a continual need for new products to be developed.
Three groups of chemicals dominate this part of the chemical industry (Figure 1). They are:
Herbicides: substances that kill or inhibit growth of unwanted plants (weeds)
Insecticides: substances that kill arthropod pests, i.e. insects and mites
Fungicides: substances that destroy or prevent the growth of pathogenic fungi
All three are pesticides.
related efficacies, they include a set of groups as a common minimum basic feature responsible for the best fit for the specific molecules in the biochemical target molecule (e.g. a protein) of the pest. This set of groups is called the toxophore. Where possible, the toxophores described in this unit are indicated by shading.
Development of new chemicals
It is estimated that it costs about œ150-200 million to discover a new product, test it thoroughly for its action and its safety for the environment, and develop manufacturing techniques for its synthesis. It takes an average of 10 to 15 years to do this so it is small wonder that, worldwide, only about 12 chemicals are introduced each year. However, these chemicals are key to the efficient production of food.
Herbicides
Herbicides are used to control the growth of unwanted plants (weeds). Modern herbicides generally act by restricting growth. They inhibit the action of one or more of the many receptors that catalyze reactions which are essential to the growth of the plant. There is one group however, the auxins, that kill by overstimulating growth. With selective herbicides, either the target in the weed is affected more than that of the crop, the herbicide is degraded more quickly within the crop, or the uptake or translocation of the active ingredient differs from that of weeds. Non-selective herbicides kill crops as well as weeds.
Herbicides can act in several ways:
direct contact with plant tissues, for example, leaves; paraquat is a typical contact herbicide
- by translocation (systemic herbicides),whereby the compound has the ability to be absorbed by aerial plant parts and is transported to roots (basipetal translocation); glyphosate and growth hormones belong to this group
- by root uptake and transportation, to the upper parts of the plant (acropetal translocation)
- through a combination of both methods; triketones are an example of this group as they are transported downwards and upwards
(a) Bipyridyliums
These are non-selective herbicides that act by interfering with photosynthesis. Paraquat is one of the most used:
(b) Auxins
Auxins are plant hormones. The most important member of the family is indole-3-ethanoic acid (indole-3-yl)acetic
acid, IAA).
(c) Glycines
Glyphosate ((N-phosphonomethyl)glycine) is a non-selective systemic compound useful in the control of broad-leaved weeds and grasses. It is by far the most widely used herbicide with annual sales well in excess of £3 billion.
(d) Sulfonylureas
Characteristic features of these chemicals are their potency on a wide range of weeds and their safety to mammals and the environment. They act in plants to block the biosynthesis of essential amino acids (valine and isoleucine), and thus prevent cell division and cell growth.
(e) Triketones
The triketones act by inhibiting an enzyme that would have led to the formation of compounds needed in the biosynthesis of carotenoids. These, in turn, play a key role in absorbing light energy for photosynthesis and protecting chlorophyll from photochemical damage. Triketones act through application on the foliage or via the roots. In either case there is translocation through the plant. They are particularly effective in controlling a wide range of weeds. Degradation in soils is fairly rapid.
(f) Inhibitors of acetyl-CoA carboxylase
Some herbicides act by inhibiting the action of the enzyme, acetyl-CoA carboxylase, essential for the biosynthesis of carboxylic (fatty) acids, needed by the plant for the production of cell membranes. An example is pinoxaden which is particularly useful in dealing with grass in fields of barley and wheat.