Herbicide Selectivity Mechanisms in Plants!
How does a herbicide act in selective manner has been an interesting field of study with the weed scientists ever since the announcement of 2, 4-D and MCPA as very successful herbicides to control broadleaf weeds in winter grains. An examination of the available literature on the subject reveals that herbicidal selectivity can be either- (i) Innate or (ii) Induced.
The innate selectivity results from specific plant morphological, physiological, and biological features in relation to the herbicide structure. The induced selectivity, on the other hand, is brought about either by hindering the herbicide from reaching the site of its action in plants or by bringing about a genetic change in plants to develop specific biochemical tolerance mechanisms.
In this article the possible herbicide selectivity mechanisms in plants are described, as under:
Mechanism # 1. Differences in Morphology of Plants:
Initially, differences in the morphology of plants were advocated as the main factors to explain the selectivity of postemergent 2, 4-D and MCPA between the broadleaf plants and grasses. The broadleaf plants with their wide, horizontal leaves were supposed to retain more herbicide spray than the narrow, erect leaves of grasses. Also, the growing points of broadleafs are well exposed to the herbicide sprays, while those of grasses are well covered within the whorls of the leaves.
In fact, at very young stage, the growing points of grasses are hidden below the soil surface. Although this theory of selectivity of certain herbicides to specific plants still holds good to some extent yet, largely it has been blown-up by the fact that now we have such highly selective herbicides as can destroy graminaceous weeds in graminaceous crops and broadleaf weeds in broadleaf crops, without any injury to the crops.
Mechanism # 2. Herbicide Deactivation Mechanisms in Plants:
Today this is the most accepted theory of herbicide selectivity in the world. It is a now extensively proved that the tolerant plant species, varieties, and chemotypes are able to deactivate specific herbicides by either their rapid metabolism and/or conjugation. The metabolism of the herbicide is breaking open its molecules to non- phytotoxic moieties, leading finally to the release of their elemental constituents like CO2, H2O, CI, NO2 and SO2, which are partly utilised within the plant and partly let outside in the atmosphere.
Metabolism is a chemical reaction, dependent on the availability of specific enzymes at the site of action. Conjugation, as different from metabolism, is the removal of intact herbicide molecules from the main stream of the plant biochemical system. It is achieved by the tolerant plant by either herbicide adsorption on the protein films in the cells or by its combination with glucose and amino acids with the help of certain enzymes.
Sometimes, instead of the herbicide molecules their more phytotoxic initial metabolites may also be subjected to inactivation by conjugation of its molecules. This portion of the herbicide or its conversion products may sometimes survive as residues in the crop up to its harvest time.
The susceptible plants lack the above processes of metabolism and conjugation in them for particular herbicides.
A few examples of the mode of biochemical deactivation of certain herbicides in specific crops and weeds are provided from the available literature in Table-10.1. It may be noted here that different plants may adopt different mechanisms to resist phytotoxicity of the same herbicide.
In certain herbicides like MCPB and 2.4-DB there is an entirely different process of selectivity. These herbicides are non-phytotoxic as such, but the susceptible plants metabolise these to phytotoxic forms viz., MCPA and 2, 4-D, respectively, while the tolerant species fail to do so. This process of selectivity is called reverse metabolism.
The selectivity of a herbicide for a given plant can be vitiated by- (i) abnormal atmospheric conditions, (ii) interaction with other chemicals, and (iii) abnormal plant growth.
Mechanism # 3. Differential Herbicide Intake by Plants:
Plants differ in the rapidity with which they absorb a given herbicide and then translocate it to site of the action or metabolism, as the case may be. Usually, if a herbicide is absorbed and then translocated rapidly to its action points, the plant is prone to its phytotoxicity. A slow rate of absorption, translocation, or both can impart a plant capability to withstand a given herbicide since its deactivation mechanism is then able to keep pace with the incoming herbicide molecules, provided the plant possessed the herbicide metabolism mechanism in the first place.
To cite a few, well studied examples, the tolerance of wild cucumber (Sicyos angulatus) to 2,4-D and susceptibility of cucumber (Cucumis sativus) to it, have been explained on the basis of much slower rates of its absorption by the former than the latter species.
Likewise, metribuzin is known to be absorbed only slowly in tolerant winter wheat, while the susceptible downybroome absorbs this herbicide rapidly. The selectivity of 2, 4-D between sugarcane and beans has been explained on the basis of differences in the translocation of the herbicide in the two species; it being fast in the beans and slow in sugarcane.
It may be pointed out here that, in general, rapid translocation of a herbicide molecule in a plant, combined with absence of a suitable herbicide deactivation mechanism with it, makes the plant doubly susceptible to it. However, this generalisation may not always be true. Sometimes, a slow rate of herbicide translocation may cause better weed kill.
For example- for obtaining more effective control of some perennial weed with a translocated type of herbicide, it is frequently recommended to spray the weed foliage with its split doses so that each time the herbicide gets translocated to the underground parts of the weed easily, without damaging the tissues on the way.
That way more herbicide could be translocated to the deep underground rhizomes and tubers of the target weed than if a heavy dose was applied at one time, which will scorch the conducting tissues and result in reduced downward herbicide translocation.
Further, when a plant has a strong deactivation mechanism for a given herbicide, it can withstand even fast translocation of the herbicide since it is simultaneously deactivated inside it. The tolerance of bindweed (Convolvulus arvensis) to diphenamid, which is translocated fast in the plant, is because of such a reason.
Further, it is an obvious fact that no more herbicide can be translocated than what has been absorbed by the plant. Thus, translocation of the herbicide is dependent upon its rate of absorption by the plant. So to say, absorption, translocation, deactivation, and phytotoxicity of herbicides are all inter-related mechanisms in plants, and the herbicide selectivity is an interplay of these phenomena, as far as the physiological and biochemical aspects of herbicide selectivity in the plants are concerned.
Mechanism # 4. Agronomic Manipulations:
Although the weed scientists classify herbicides as selective and non-selective compounds, yet, many a time it is possible to use a non-selective, or a relatively less selective herbicide, for obtaining selective weed control in agriculture by certain agronomic manipulations.
Conversely, sometimes, a selective herbicide may prove non-selective when it is used at higher than recommended doses or when it is applied by a wrong method and/or at a wrong time. There may be some herbicides which are known to possess variable inherent selectivity to specific crops, like simazine and atrazine in maize and apple, and propanil in rice. But even in such cases, agronomic factors are very important to let the crop plants display their physiological tolerance to such herbicides.
Every herbicide has dose limits within which it proves selective in the recommended crop-weed systems. This limit may be wide for some selective herbicides and narrow to very narrow in others, depending upon the crop in question and other prevailing agronomic factors. Beyond these limits even a selective herbicide can behave like a non-selective one. 2, 4-D is perhaps the best known selective herbicide used round the world for weed control in wheat.
Yet, at a dose of more than 1.0 kg ha-1, it may injure the crop in proportion to the increase in dose over the recommendation. Similarly, a herbicide like oxyfluorfen is selective to certain crops at 0.05-0.5 kg ha-1 but it may cause severe injury to the crops when these doses are exceeded even by a few grams per ha.
Further, the selective dose of a herbicide is not static for all situations. As one would note, in each recommendation a range of doses is provided, giving some lower and higher limits. It is so because the optimum dose, i.e. when maximum weed control with least adverse crop effect is obtained, differs with local agronomic practices and prevailing soil and climatic conditions.
This is why the agronomists conduct intensive research in varied agro-climatic situations and determine optimum doses of herbicides in different crops which will provide selective, effective, and economic weed control.
Directed application, spot application, and early postemergent treatments, are modified methods aimed at achieving selective control of weeds with partially selective or non-selective herbicides.
Treatment of weeds in sugarcane and potato with a non-selective herbicide like paraquat, a couple of days before emergence of the crop, is perhaps the best example of manipulating herbicide selectivity. By this early postemergent treatment the yet-to-emerge sugarcane and potato shoots escape herbicide injury, while the existing flush of young weeds is completely damaged.
Premergence application of a herbicide is adopted largely to achieve positional selectivity to the crop, making use of the fact that most weed seeds germinate from within about 2.5 cm depth of soil where the preemergence herbicides travel easily in moist soils. The crop seeds, on the other hand, are sown usually below this depth in the herbicide-free-zone.
Any abnormal situation vitiating this delicate zonal differentiation of herbicide movement in soil, such as induced by an excessive rain, can vitiate this positional selectivity of the herbicide. Also, on dry soils the preemergence application of herbicides usually provides eratic results.
An interesting observation was made by the author when very efficient selective control weeds in bottlegourd was obtained with the herbicide simazine, which is otherwise phytotoxic to the crop.
The agronomic manipulation involved here was an even spray of the herbicide on the freshly prepared seedbed; making holes in the soil with a stick at the desired spacings; placing one bottlegourd seed in each hole; and filling the hole with untreated, fresh soil brought in a bucket from some nearby field.
It permitted a perfect germination and growth of the crop and a weedfree environment for up to 4 weeks, whereafter the crop shoots creeped over to completely cover the ground. The treatment gave very high vegetable yields.
The stage of plant growth at the time of treatment with a herbicide is also very important in determining the herbicide selectivity; when applied either too early or too late, the selectivity of the herbicide may be at stake. For instance, 2, 4-D is most selective to wheat when it is applied at its fully tillered stage.
At this time, in North India the wheat plants usually possess 5-6 leaves. The selectivity of 2, 4-D to wheat decreases both when it is applied either earlier to this stage (seedling stage) or later at its jointing and boot stages. Similar is the case with other herbicide-crop systems.
In the last decade or so it has been observed that although a crop may be tolerant to a given herbicide, yet some of its cultivars may be susceptible to it because of chance inclusion of some susceptible parent line in the breeding programme of that crop.
In India it was first observed in wheat in the case of 2, 4-D. HD 2009 (Arjun) was perhaps the first wheat cultivar reported 2, 4-D susceptible variety of wheat with us. Such observations have led the wheat breeders to subject all their new wheat material to screening against 2, 4-D before it was released for cultivation. In the USA, maize lines are intensively screened against the triazine herbicides for the same reason.