Competing interactions with other members of vegetative community. Herbicides

you are: Competing interactions with other members of vegetative community

Herbicides name the chemical substances applied to destruction of vegetation. herbicides of the general action killing all kinds of plants, are used for vegetation destruction on racetracks of stadiums, on railways, under lines of high-voltage electricity transmissions, etc. In agriculture herbicides of selective action find application. Value of this or that connection as herbicide depends on its toxicity for weeds-targets, from degree of selectivity of its action, from absence of undesirable by-effects both in the ecological plan, and from the point of view of public health services, and, at last, if this organic substance, from its ability to be exposed to biological decomposition. Sulfuric acid was one of the first herbicides; It kills plants, burning their leaves, and even possesses to some extent selective action as it destroys only shirokolistvennye weeds, and to leaves of cereals does not harm. However sulfuric acid has an essential lack - its corroding action. In the late thirties researchers were engaged in searches of the connections capable selectively to regulate growth of plants. These works, many of which during the Second World War were spent in the closed laboratories, have led to opening hlorfenoksiuksusnyh acids - 2,4-D and 2М-4Х. It has appeared that these connections for the yet not found out reasons possess desirable property - toxicity for one only shirokolistvennyh plants (fig. 15.13); therefore after war them have started to make and apply as herbicides of selective action. Since then the wide set of connections of this type for agriculture is created. Many of them are calculated on any special application, basically on destruction of a certain kind of those weeds or other food cultures. preparations Available now in our disposal are so various that there was even a possibility to destroy annual and long-term weed cereals in crops grain, belonging besides to family (fig. 15.14). To discuss here all these herbicides it is not obviously possible; we will consider only certain general principles on which application of herbicides is based.

Herbicides should be toxic for plants in low concentration; at such concentration they should be capable to break numerous reactions in vegetative cages. Some herbicides, for example diuron, suppress photosynthetic carrying over elektronov and quickly kill plants because in such conditions on light fotoliz waters leads to accumulation vysokookislennyh connections which influence a cellular membrane and break its integrity. The substances influencing growth of plants, for example 2,4-D, on the structure are close to auksinam and in small concentration operate as they. However in higher concentration these substances cause the unbalanced growth which is quite often coming to an end with  destruction of plants. Though 2,4-D it is known already over 35 years, we so, in effect, and do not know till now, what mechanism of its action. It is possible to assume that this herbicide influences the proton pumps localised in a membrane, and also on synthesis of nucleinic acids and the fiber, stimulated, as it is known, and natural auksinom - indoliluksusnoj acid (IUK). However such influence is completely not equivalent normal regulation; the reason, probably, consists that 2,4-D does not collapse IuK-oksidazoj and not so effectively contacts other connections. Thereof concentration 2,4-D gradually grows and at last reaches level at which it becomes already toxic; the normal course of cellular processes as a result of it is broken, there comes disintegration and eventually -  destruction of a plant. Some other herbicides, for example destroying odnodolnye weeds dalapon, operate, possibly, inducing changes in konformatsii fermental and membrannyh fibers that leads to infringement of metabolic processes and to disintegration of various cellular structures.

Any herbicide is effective only in the event that it is capable to get into a plant; for this purpose to it as well as system fungitsidam, certain degree as zhirorastvorimosti, and water solubilities is required. The basic way of application of herbicides - spraying by water solutions; however for penetration through kutikulu and through a cellular membrane herbicide should be to some extent zhirorastvorim. Sometimes, to facilitate herbicide passage through kutikulu, to its solution add adjuvanty, i.e. The agents lowering a superficial tension of water and by that promoting penetration of herbicide in substance kutikuly, close by the nature to wax. The most effective herbicides extend on all plant, moving on ksileme or floeme. To destroy any kornevishchevyj mnogoletnik, for example vjunok or taking roots sumah, herbicide should get on floeme to reserving bodies and to tops of young developing runaways on the rhizome ends. Such ability new herbicide glifosat possesses, for example.

Selectivity of herbicides depends on a number of properties, and, each of them separately, possibly, cannot provide selectivity, i.e. Primary action on any one kind of plants. The list of these necessary properties varies depending on the herbicide nature, and especially from a way of its application, for example from, whether process it soil before occurrence of shoots of a cultural plant and weeds or spray leaves already after occurrence of shoots. If herbicides are brought in soil selectivity of their action is influenced considerably by preparation localisation in soil. For example, if herbicide contains only in the top layer of earth it, obviously, will kill any seeds of weeds sprouting in this layer whereas roots of the cultural plants which seeds are closed up deeply, will not absorb him. Seeds ovsjuga (Avena fatua) sprout more low ' herbicidal ' a layer, and nevertheless the preparation destroys them, whereas on wheat and other cultural cereals it does not operate; it speaks that the extended stalk ovsjuga pushes out meristematicheskuju a zone in that layer of earth which contains herbicide, and at cultural cereals of such lengthening does not occur, and consequently them meristem the ethical zone remains below this layer. Some herbicides of selective action are brought in soil after occurrence of shoots of cultural plants but before weeds will start to grow. For destruction on lawns rosichki (Digitaria) a number of the preparations suppressing germination of seeds of this annual weed, but not operating on the long-term cereals forming a grassy cover of a lawn is used.

Efficiency of the herbicide applied after occurrence of shoots, depends on, whether it will reach in a plant of cages-targets. And it is in turn defined by a number of circumstances. The quantity of herbicide getting on leaves depends, for example, on how leaves are located: vertical leaves of cereals intercept much less herbicide, than wide horizontally settling down leaves dvudolnyh. Herbicide drops can be kept on sheet and can flow down from it. It depends on character of a sheet surface and from a sheet inclination: from the inclined surface of a drop covered with wax touch easily roll down, and on horizontal, it is weak kutinizirovannyh leaves they usually are late. Peas with its thick kutikuloj are often absolutely tolerant to herbicides, but the same herbicides become for it highly toxic if somehow to force them to be kept on leaves. After the preparation has got on leaves and was kept on them, its efficiency is defined by, whether he will manage to get into cages. Here again the wax touch kutikuly serves as a barrier: water-soluble connections get through kutikulu more slowly, than through nekutinizirovannuju a surface.

If herbicide does not extend on a plant on a little considerable distance from a penetration place the plant morphology appears the important factor on which its toxicity depends. At cereals, for example, stalks generally short and the top kidney is protected by a sheet vagina, and at dvudolnyh plants both a top kidney, and a stalk are accessible to spraying. Therefore cereals at processing suffer from ' a burn ' leaves a little and easily recover, whereas heavy damages of a kidney and a stalk at shirokolistvennyh weeds cause  destruction of plants. Ability to moving in a plant also can influence selectivity of action. The herbicide getting from sheet in a kidney, destroys, for example, the given kind of plants whereas other, related kind remains to it tolerant only for the reason that in it this herbicide is not capable to move of a little considerable distance from a penetration place.

When the preparation already has adequately extended on a plant, other factors defining its selectivity come into force. The bright example of such situation is a case when a steady kind of plants it is capable to destroy herbicide to nontoxical connections, and sensitive is not capable. Well-known, for example, that herbicide simazin is neutralised in corn plants. Business when the preparation used for spraying in itself does not possess phytotoxicity exactly the opposite is and turns to phytotoxic substance only in plants then kills them; at such situation any plant, not capable to carry out this transformation, will be steady against the given preparation. Having investigated from this point of view hlorfenoksialkilkarbonovye acids with a lateral chain of various length, it is possible to be convinced that some plants possess ability to carry out so-called - oxidation of these acids at which from a lateral chain otshchepljaetsja in each cycle of oxidation a two-carbon fragment. If the initial chain contains behind a deduction karboksilnoj groups odd number of atoms of carbon the r-oxidation end-product appears always toxic derivative acetic acid; if it contains even number of atoms it is formed rather poorly toxic phenolic derivative (fig. 15.15). However not all plants are capable to carry out - oxidation hlorfenoksialkilkarbonovyh acids; representatives this are absolutely not capable to it, for example. Leguminosae. Therefore if to process, for example, any bean culture and the weeds present at it which are not concerning family bean, by a solution hlorfenoksimasljanoj acids (2,4-DM), having in a lateral chain three intermediate atoms of carbon in weeds this connection will turn to hlorfenoksiuksus th acid (2,4-D) which will kill them; in bean plants it remains invariable and accordingly will not cause them harm. It gives the chance to apply 2,4-DM as herbicide in the mixed crops consisting of cereals and any bean culture, used on a forage to cattle after cleaning of cereals. It is possible to imagine also that selectivity of action of herbicide depends on its influence on any important biochemical process which is available for a sensitive kind of a plant, and at steady either is absent in general, or is not so sensitive to the given herbicide, or, at last, is somehow protected. A concrete example such:

We while cannot name, but there are bases to think that something similar underlies stability of certain kinds of plants to some herbicides (for example, at the heart of stability of cereals to 2,4-D). In general to judge the specific reasons of selective phytotoxicity of such herbicide as 2,4-D, it will be possible only after the mechanism of its action becomes quite clear to us.

The Majority of herbicides are adequately safe for the person if not to deviate the ordered doses and to observe all rules of their application. However, with introduction new, more not enough the tested preparations arise sometimes and new problems. Rather serious problem has arisen, for example, in connection with application of herbicide 2,4,5 (fig. 15.16).

It has been intended for destruction of wood vegetation and widely applied in quality defolianta in woods and mangrovyh bogs of Vietnam during recent war. In the beginning it was considered harmless to the person and animals, however later by laboratory researches has been proved that preparations of this herbicide available on sale cause numerous anomalies of development in embryos of mice and rats. It was possible to show that this teratogennaja ability (ability to cause ugliness) is connected with presence at selling preparations 2,4,5 impurity 2,3,7,8-tetrahlordi-benzodioksina (THDD; fig. 15.17), formed at synthesis 2,4,5 as an undesirable by-product. Old preparations 2,4,5 contained THDD in quantity nearby 25 ch. On million, in new samples this impurity makes already less than 0,1 ch. On million; therefore, recognising that such low level it is safe, 2,4,5 began to apply for prorezhivanija afforestations even near to habitation. Meanwhile recently the new disturbing circumstances forcing seriously to think by expediency of the further application 2,4,5 were found out. First, in laboratory experiments on rodents and primacies it was found out, what even five parts THDD on billion () in a diet of these animals it is enough considerably to raise at them frequency of any occurrence of malignant tumours. Secondly, at plants under the influence of minute quantities THDD infringements

A mitosis, and it specifies that the given connection can possess not only cancerogenic or teratogennymi, but also mutagen properties. At last, it was found out that in those areas where 2,4,5 it was applied for prorezhivanija afforestations or to processing of pasturable grounds, significant amounts THDD are found out not only in the beef fat, but also in female milk. Danger which represents THDD for a human population, has not received still a due estimation because it is not known yet, whether this connection in a food chain collects and whether it can collapse and be deduced from an organism.

All this data has compelled environment Protection department to forbid application 2,4,5. Those who insists on its further use, specify that the dangers connected with it it is much less, than benefits which it gives. Supporters of its interdiction object, what even the one and only case of a birth of the dead child, occurrence of a cancer or congenital ugliness - the expensive price that as a matter of fact represents simply means for simplification of our work. prorezhivanie afforestations, they believe, it is possible to conduct and manually; on the thinnest end it is possible to use during any time 2,4,5, but only until any other herbicides deprived of such harmful collateral action will be found.

Recently it was found out also that herbicide atrazin, more often applied to processing of crops of corn, forms the derivatives possessing mutagen action in plants;

To show this mutagen action it was possible on plants and microorganisms. Thus, though itself atrazin in due time has been tested and recognised by not mutagen, products of its metabolic transformations in a vegetative organism have appeared harmful. To refuse now application of this valuable herbicide difficultly because it undoubtedly would entail behind itself corn curtailment of production. And still we, maybe, will be compelled to take this step - for the sake of health of people. Herbicides - the huge blessing for modern agriculture, but their application demands constant vigilance; only provided that it can be safe.