Triazine-resistant biotypes

The high efficacy of triazine herbicides and their repetitive use in crops and noncrop situations has resulted in the selection of weeds that are resistant to these herbicides or are not well controlled at the lower rates now being used. In most instances, triazine resistance is due to an alteration in the herbicide-binding site in PS II. Despite the widespread occurrence of triazine resistance, these herbicides are still widely used, even in fields in which triazine-resistant biotypes are known to occur. The rate of increase in the selection for triazine-resistant weed species depends in part on the integration of alternative weed control strategies, in addition to the use of triazine herbicides, for control of these weed species. Due to their resistance mechanism, many triazine-resistant weeds are less competitive than their susceptible counterparts. 

Burke, J.J., R.F. Wilson, and J.R. Swafford (1982). Characterization of chloroplasts isolated from triazine-susceptible and triazine-resistant biotypes of Brassica campestris L. Plant Physiol., 70 24—29. 

Fuerst, E.P., C.J. Arntzen, K. Pfister, and D. Penner (1986). Herbicide cross-resistance in triazine-resistant biotypes of four species. Weed Sci., 34 344-353. 

The levels of infestations or seriousness of triazine resistance within each species varies gready. The author is aware of 19 cases where the resistant weed is no longer present or cannot be identified as resistant to triazine herbicides. In other cases, the current status is unknown. Several triazine-resistant biotypes are likely to be of little or no agronomic importance within a geographical area. 

Triazine-resistant weeds have an impaired electron transport system. Reduced electron transport, in turn, reduces photosynthetic activity and fitness. Under controlled conditions, most triazine-resistant biotypes exhibit impaired photosynthesis. Significant fitness costs from resistance (10-50%) have been reported in most studies (Warwick, 1991). This substantial fitness cost or handicap has been important in the management of triazine-resistant weeds (Radosevich et al., 1991 Bergelson and Purrington, 1996). Anderson et al. (1996b) found that the competitive advantage of triazine-susceptible waterhemp over triazine-resistant waterhemp isolated from one field in Nebraska was equal to or less than that for other species or isolates. This indicates that additional factors contributed to the slow and limited distribution of resistance for this waterhemp biotype. 

Clay et al. (1991) found triazine-resistant biotypes from two different weed species (i.e., American willowherb and common groundsel) were also resistant to two powdery mildews. They proposed that the relationship may be due to the gene responsible for triazine resistance being closely linked to the inability of the mildews to infect those weeds. 

There is generally a lack of fitness or ability in the triazine-resistant biotypes to compete with the crop or with other nontriazine-resistant weeds as a result of the altered triazine binding site at the D1 protein in PS II. 

Until the mid-1990s, multiple-resistance (i.e., resistance to more than one herbicide mode of action within the same biotype) had not been reported within North America. However, Foes et al. (1996) found a kochia biotype from western Illinois resistant to atrazine and several ALS-inhibiting herbicides. Lopez-Martinez et al. (1996) reported that a triazine-resistant Echinochloa species found in atrazine-treated com also showed cross-resistance to quinclorac. Clay and Underwood (1989) and Clay (1989) reported that one triazine-resistant biotype of American willowherb was also resistant to paraquat from a hop garden in the United Kingdom treated annually for 25 years with simazine and paraquat. 

Gasquez, J. and H. Darmency (1983). Variation for chloroplast properties between two triazine resistant biotypes of Poa annua L. Plant Sci. Letter, 30 99-106. 

Gressel, J., Y. Regev, S. Malkin, and Y. Kleifeld (1983). Characterization of an. v-triazine-resistant biotype of Brachypodium distachyon. WeedSci., 31 450. 

Data in our tables are not entirely in agreement with those reported by Heap (2006) for triazine-resistant and ALS-resistant weeds due to variations in data collection techniques. Also, some triazine-resistant biotypes, mostly within... 

Past experience has shown that weeds resistant to triazines can be managed or confined within a reasonable limit. In the U.S., the total area of land or crops infested with triazine resistant weeds is still relatively small and does not seem to be expanding rapidly. In most areas of the U.S. where triazine resistant weed populations have evolved, it has not been necessary or desirable to discontinue the use of the triazine herbicide of choice, due to the many triazine susceptible weeds that are usually prevalent. In a few cases, the resistant biotypes have even disappeared (e.g., some triazine resistant biotypes of Setaria sp. in Nebraska). However, in other countries, especially when resistance strategies were not followed, resistant biotypes quickly became serious problems. 

While the triazine resistant biotypes of virtually all weed species studied differ substantially from their susceptible counterparts in photosynthetic efficiency at the level of the light reactions, data describing whole plant performance is actually quite variable. For example, in one report, relative differences in C02 assimilation rate between susceptible and resistant biotypes varied among six species examined 136). In other reports, resistant biotypes were actually more productive and competitive than susceptible under some conditions (49-51). It is difficult to interpret such data since most research to date has been conducted with weed populations of uncertain genetic backgrounds. 

Resistant biotypes sometimes grow better in the presence of the herbicide than without the herbicide. For instance, Lipecki (pers. comm.) found that a triazine-resistant biotype of Amaranthus hybridus had double the dry weight per plant at 5 kg/ha simazine than without the herbicide. It would be useful to have more such quantitative experiments. This lower resistant biotype productivity when the herbicide is not present results in a stronger lack of competitive fitness in the off years. 

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