Crop selectivity mechanisms

The remainder of this chapter will describe the mode of action, crop selectivity mechanisms and environmental properties of the sulfonylurea herbicides. Other issues including resistant weeds and recropping intervals will also be discussed. 

A number of possible crop selectivity mechanisms have been investigated (see Brown, H. M. Pestic. Sci. 1990 (in press)). Differential uptake and/or translocation of the selective herbicide between the tolerant crop and sensitive weeds has been ruled out as the basis for crop selectivities in several specific cases. For example, Sweetser et al. ( found no ccnrelation between chlorsulfuron uptake or translocation and sensitivity to this herbicide in a study of 7 plant species, and similar conclusions were drawn in studies of thifensulfuron methyl tolerance in soybeans (28). Lichtner (29) has shown that sulfonylurea herbicide uptake and translocation in plants is not carrier-mediated, but instead depends on the physical properties of the herbicide (pKa, log P) and proceeds through an acid-trapping mechanism common to higher plants. Given this information, we conclude that differential uptake and/or translocation is unlikely to account for any of the sulfonylurea crop selectivities discovered to date. 

Nonchemical or traditional practices, such as weed seed removal, optimal crop seeding rates, crop selection, enhanced crop competitiveness, crop rotation, and mechanical weed control are all important components of an effective weed management program (458,459). In the context of modern intensive chemical herbicide appHcation, nonchemical practices may even represent an innovative approach to weed management and should receive careful consideration. 

Herbicide safeners (also referred to as herbicide antidotes or protectants) fulfill an important role in crop protection. Safeners are chemicals that protect crop plants from unacceptable injury caused by herbicides. Either by placement on the crop seed or by way of a physiological selectivity mechanism, safeners in commercial use do not negatively impact the weed control of the herbicide. Although many herbicides have been developed for use without a safener, some of the strongest and most broad-spectrum herbicides tend towards border-line crop selectivity, which may completely preclude use in a particular crop or at least limit maximum use rates or the crop varieties that can be safely treated. It is for such situations that safeners have been developed. Several books and reviews of safeners have been written over the past 20 years [1-3]. It is not the intention of this chapter to cover in detail older safeners, but rather to focus on more recently developed commercial safeners as well as some of the older compounds still in wide commercial usage. 

Differential herbicide sensitivity at the site of action is a second possible selectivity mechanism, with good precedence in the aryloxyphenoxy and cyclo-hexanedione grass herbicides (2Q, 51). However, several studies now clearly indicate that the inherent sulfonylurea crop selectivities listed in Table 1 are not based on differential sensitivity at the site of action. ALS preparations isolated fix>m crops and weeds are equally sensitive to several selective sulfonylurea herbicides (28.32.33). Although data have not appeared for all of the compounds shown in Table 1, we conclude that differential active site sensitivity is not a general sulfonylurea selectivity mechanism. The clear exceptions to this generalization are the cases of genetically-altered plants (crops and we s) which have acquired through mutation or deliberate transformation a resistant form of the ALS enzyme (see below). 

In addition, naturally growing plants resist plant pathogen and Insect attack because resistance develops over time via natural selection (35). Also, most natural and crop plants have, as a part of their basic physical and chemical makeup, a wide array of mechanisms that help them resist pest attack. These Include chemical toxicants, repellents, altered plant nutrients, hairiness, thorns, and diverse combinations of these (35). 

Selective toxicity is also important in relation to the development of resistance or tolerance to pollutants from two distinct points of view. On the one hand, there is interest among scientists concerned with crop protection and disease control in mechanisms by which crop pests, vectors of disease, plant pathogens, and weeds develop resistance to pesticides. Understanding the mechanism should point to ways of overcoming resistance, for example, other compounds not affected by resistance mechanisms or synergists to inhibit enzymes that provide a resistance mechanism. On the other hand, the development of resistance can be a useful indication of the environmental impact of pollutants. 

S. Murata, A. Yuda, A. Nakano, Y. Kimura, K. Motoba, T. Mabuchi, Y. Miura, H. Nishizawa, and S. Funayama, Mechanisms of selective action of the peroxidizing herbicide ET-751 on wheat and Galium aparine, in Proceedings of the 1995 Brighton Crop Protection Conference-Weeds, vol. 1, pp. 243-248 (1995). 

The second example, the selective effects of BT toxin on lepidopteran larvae and its use as a commercial product, is well known (9). However, the endogenous incorporation of the genes for expressing this toxin (10) into a crop plant would obviate the wasteful topical application of the commercial product. This bioengineering feat of transferring the allelopathic defense mechanism of one type of organism (bacteria) to another (plants) would be a great scientific achievement and commercial success. 

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