Logo

Logo

Weed terminators

Herbicides are chemicals employed to kill weeds. The major part of herbicides relate to organic compounds characterised by a high…

Weed terminators
Herbicides are chemicals employed to kill weeds. The major part of herbicides relate to organic compounds characterised by a high physiological activity and effectiveness at relatively low rates of use. Some inorganic compounds are also used as herbicides. 
Depending on their properties, selective and non-selective herbicides are distinguished. Non-selective herbicides are used for killing all plants on areas where no plant growth is desired — on the shoulders of highways and railways and on railway tracks, on drainage and irrigation canals, along power transmission lines, on sports grounds, et al. Selective herbicides kill or inhibit weed growth without harming crop plants and can be used on plantings of almost all crops. Many weeds can be killed without harming the crop plants upon the proper choice of a type of herbicide, its rate of use, the time and way of treatment. 
Selectivity depends on the anatomical, morphological, and physiological features of the plants, and also on the chemical composition and physio-chemical properties of the herbicide and its physiological activity. Many selective herbicides kill a considerable number of weed species. For instance, the herbicides 2.4-D and MCPA eradicate numerous dicotyledonous weeds in grain fields. Derivatives of 1, 3, 5-triazine – atrazine and simazine – kill many di-and monocotyledo-nous weeds on corn fields. Conversely, some herbicides kill a very limited number of weed species or even one weed. For example, barban used for treating wheat, barley, pea, and corn fields against wild oats acts on this weed a very short time (only in the stage of one or two leaves). Propanil used to kill Japanese barnyard millet on rice fields has a very weak effect on other weeds. Some other herbicides such as dalapon, TCA, and chlorpropham are also characterised by a narrow selectivity.
Topographical selectivity is due to the differences in the anatomical and morphological structure of plants. For example, plants with dense ground tissues, cuticle, a wax coating, and also with dense nap are more resistant to herbicides because their dense ground tissues prevent the penetration of herbicides. Plants with leaves directed vertically upward are also more resistant to herbicides because a considerable part of the herbicide solution drips down from these leaves.
Plants with a deep root system exhibit resistance to soil herbicides. For example, Canadian (creeping) thistle, field sow thistle, Russian knapweed, field bindweed, and common horsetail are resistant to atrazine and simazine because these herbicides remain in the top 10-centimetre layer of soil and do not reach the zone of the active roots. These features explain the use of the above-mentioned herbicides in fruit growing and forest nurseries. They kill many weeds whose roots are concentrated in the top layer of the soil, but do not harm the fruit plants and tree seedlings whose root systems are at a considerable depth.
The resistance and sensitivity of plants to herbicides are associated with biochemical selectivity. The herbicides penetrating into plants undergo various transformations. In some cases this leads to their decomposition and inactivation, in others, to a greater phytocidal activity. Quite often a considerable part of herbicides penetrating into the leaves are excreted through the root system without harming the plant.
Selective herbicides enter plants in different ways — some via the leaves and move along the vessels of the phloem, while others enter via the roots from the soil solution. This explains why they are applied in different ways — herbicides entering via the leaves are used to spray the green organs of plants, while those entering via the roots are incorporated into the soil. It must be noted that such a division is arbitrary because many herbicides (like 2,4-D, MCPA, dicamba, picloram, dalapon, etc) may penetrate into plants via both the leaves and the roots.
During their translocation within a plant, herbicides are partly inactivated because of reaction with the contents of the cells — absorption by the cells, decomposition by enzymes, and the formation of complexes. They move along the phloem into the root system, the generative organs, accumulate in the zones of active growth and in the meristematic tissues, which is just where they induce deep disruptions of the physiological processes leading to perishing of the plants.
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

Advertisement