Mechanism of Crop Selectivity

Metabolism studies have been conducted on 7 and the closely related triazolo[l,5-c]pyrimidine analog 24 (Eig. 2.4.4). The metabolites identified in wheat Triticum aestivum) for 7 and 24 are shown in Eig. 2.4.4. These studies showed that O-dealkylation of one heterocycle methoxy groups was occurring with 7 in wheat (Eig. 2.4.4). In comparison, both methoxy groups on the heterocycle of 24 underwent O-dealkylation. Table 2.4.8 compares the metabolism rates and activity on wheat and blackgrass Alopecurus myosuroides) for 5, 7 and 24. The order of rank- 

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Before Radosevich and De Villiers found in 1975 that isolated chloroplasts of resistant common groundsel were insensitive to atrazine and simazine (2), it had been erroneously assumed that all living plants would die if the herbicides could reach their target site intact. We now know that mechanisms of selectivity in crops can be due to differences in metabolism rates, uptake, translocation, site of action or avoidance mechanisms. However, the mechanisms of herbicide resistance that have evolved in weeds are usually different from the mechanisms of herbicide selectivity in most crops. This is certainly true with the most prevalent and thoroughly studied cases of herbicide resistance, including the triazines, dinitroanilines, and AHAS inhibitors. 

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. 

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. 

There are three general approaches to controlling Jerusalem artichokes as a weed chemical, mechanical, and crop rotation. The method selected depends upon a number of factors, such as the necessity for continuous cropping, suitable rotational crops, costs, geographical area, equipment availability, weed density, and other factors. 

Tolerance toward the sulfonylureas is known to occur naturally due either to the presence of a form of ALS that is insensitive to the inhibitors (7) or to a mechanism for detoxification of the inhibitors (8). Another mechanism that could in principle lead to tolerance is the overproduction of the target (ALS) enzyme. We are interested in engineering herbicide tolerance in crop plants in order to increase the margin of safety for the application of existing selective chemicals, to achieve selectivity in crops where selective chemicals do not currently exist, and to reduce damage in rotated crops which is due to the presence of herbicide residues. 

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. 

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