Crop cultivars, herbicide

Chemical herbicides have been available for more than a century but major impacts on crop production awaited the development of "truely" selective herbicides or innovations that would permit use of non-selective herbicides in crop situations. We now have some form of selective chemical weed control for most of our major crops. However, continuing problems with herbicide injury to crops as well as poor control of weeds that are botanically similar to crops remind us that further improvements in herbicide selectivity are still needed. Introductions of new selective herbicides will continue but the high costs of these new chemicals are stimulating efforts to make wider use of existing herbicide chemistry. One successful approach has been to genetically improve the tolerance of new crop cultivars to major herbicides... 

However, ideal levels of crop selectivity are difficult to achieve. Several approaches to supplement natural crop selectivity have been used, including development of chemical safeners that improve herbicide tolerance of corn and sorghum (2), restriction of herbicide use to tolerant cultivars (3), transfer of tolerance to crop cultivars by breeding methods (4), and optimization of the site, timing, and method of herbicide application. 

Classical Approaches to the Development of Herbicide Tolerance in Crop Cultivars... 

After use, herbicides decompose slowly, and so affect cultivated plants for many years. In 1990, investigations in many regions of the USSR detected herbicides phytotoxic effects, especially among the si/m-triazine class, on different cultivars in many varied situations [13]. These sym-triazine herbicides, such as protrazin, simazin, atrazine, metazin, and prometrin, were used in different oblasts of the Ukraine, Kirgizia, Kazakhstan, Russia and Moldavia in previous years, especially on corn. Residual herbicide aftereffects led to the suppression and death of crops such as winter wheat, oats, barley, rye, potatoes, beets and sugar beets, linen, onions, watermelons and other melons, and sunflowers. 

Macias LA, Varela RM, Torres A, Mohnillo JMG (1993) Potential allelopathic guaianolides from cultivar sunflower leaves, var. SH-222. Phytochemistry 34 669-674 Macias LA, Molinillo JMG, Galindo JCG, Varela RM, Simonet AM, Castellano D (2001) The use of allelopathic studies in search for natural herbicides. J Crop Prod 4 237-256 Macias AL, Marin D, Oliveros-Bastidas A, Varela RM, Simonet AM, Carrera C, Molinillo JM (2003) Allelopathy as a new strategy for sustainable ecosystems development. Biol Sci Space 17 18-23... 

Herbicide resistance is at present one of the most commonly used traits in commercial cultivars of GM crop plants [1], Since this resistance operates via the production of an enzyme to break down the herbicide and bees lack such substrates, they are extremely unlikely to be harmed by these plants. Bioassays with the purified products of such genes have not been carried out, but some experiments with GM plants have (see Chapter 15). 

The rates of biosynthesis and catabolism are reflected in the pool size of the saponins, which is considerable in the whole plant (i.e., nearly 2 g/plant) however, the bulk of the metabolic activity was found in the leaves, particularly the young ones. We believe that the rates of biosynthesis and catabolism vary with respect to physiological states of development, diurnal variation, time of year that the crop is grown, types of soil, fertilizers, herbicides, insecticides and cultivars, as well as the functionally different parts of the plant. Different patterns of saponins may be formed. Saponins undergo biotransformation within the plant as is shown in the section on bioassay in this paper. Biotransformation also occurs in the soil surrounding the plant often these reactions occur at appreciable rates however, little is known about their effect either in the plant or in the soil. 

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