Common groundsel

Radosevich, S.R. and O.T. DeVilliers (1976). Studies on the mechanism of s-triazine resistance in common groundsel. Weed Sci. 24 229-232. 

Triazines were one of the first family of herbicides where weed resistance was widely recognized and documented in the literature. A simazine-resistant biotype of common groundsel was identified in Washington, United States, in 1968. Since then biotypes of at least 66 triazine-resistant weed species have been reported, mostly in the United States, Canada, and Europe (Heap, 2006). 

Weed resistance to the triazine herbicides was first identified in the late 1960s, with a biotype of common groundsel that was resistant to simazine (Ryan, 1970). Since then, resistance to triazine herbicides has been reported in many weed species (Holt and LeBaron, 1990 LeBaron and McFarland, 1990 Gronwald, 1994). Most cases of triazine resistance have been reported in the US, Canada, and Europe, where triazine herbicides have been used extensively in corn monocultures (LeBaron and McFarland, 1990 Stephenson et al., 1990 LeBaron, 1991). Most of the. v-triazinc-resistant weed species have been selected against atrazine and usually show a high level of cross-resistance to other. v-triazine herbicides. In most cases, these weeds also show a low level of resistance to as-triazinones (e.g., metribuzin). Triazine-resistant weeds are often less vigorous than nonresistant weeds, which facilitates their management. 

In contrast, Ernst et al, (1996) have shown that a substitution at Ser264 does not necessarily lead to herbicide resistance. They found both Ser and Gly at position 264 in various sensitive and resistant biotypes of common groundsel. However, all resistant biotypes of black nightshade had Gly at position 264, but some of the sensitive biotypes also had Gly at this position. They suggested that the effect of this mutation in sensitive biotypes was overcome by two additional mutations in these biotypes alanine at position 251 (Ala25i) to arginine (Arg) and valine at position 280 (Val280) to leucine (Leu). In sensitive common lambsquarters, only Ser was present at position 264 either Ser or Gly were detected at position 264 in different atrazine-resistant plants (Ernst et al, 1996). 

Soon after the discovery of triazine-resistant common groundsel, another equally important discovery was made. Radosevich and DeVilliers (1976) found that the mechanism of resistance in this weed was due to insensitive chloro-plasts that were capable of photosynthesis, even in the presence of simazine or atrazine. This was surprising because earlier research had confirmed that there were no differences in plant selectivity or susceptibility due to the origin of chloroplasts. Moreland (1969) had reported that isolated chloroplasts were equally inhibited to simazine whether they came from tolerant com or susceptible spinach. Radosevich and Appleby (1973) had confirmed there were no differences between the susceptible and resistant biotypes of common groundsel due to herbicide uptake, distribution, or metabolism, whereas it is known that com metabolizes triazine herbicides (Shimabukuro, 1985). 

Weed species resistant to the triazines that have been found in one or more fields in 10 or more states, provinces, or countries include smooth pigweed, Powell amaranth, redroot pigweed, common lamb s-quarters, kochia, common groundsel, black nightshade, and annual bluegrass (Tables 10.1(a) and 10.1(b)). 

Within North America and a few other countries, most triazine-resistant weed biotypes have been reported after repeated use of atrazine in com and sorghum. In some areas of Western Europe and other countries, triazine-resistant weeds have been reported after repeated use of simazine in orchards and along roadsides. A few triazine-resistant weeds (e.g., kochia, cheatgrass, and common groundsel) have biotypes with triazine resistance in nurseries and perennial tree crops, as well as along railways and roadsides. 

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. 

Excellent progress has been made in the understanding of the cause, nature, genetics, mechanism and solutions of herbicide-resistant weeds since the first triazine-resistant common groundsel was reported more than 35 years ago. Resistance management programs have been extremely successful in controlling most weeds that have developed resistance to the triazine herbicides. However, research is critical to better understand the rapid increase and spread of many new weed biotypes resistant to several classes of herbicides. 

In some orchards where repeated applications of triazine herbicides have been used, there are isolated instances of triazine-resistant weeds. These include common groundsel in the United Kingdom (Holliday and Putwain, 1977) and common lambsquarters and pigweeds in Czechoslovakia, Poland, and Bavaria. Some studies indicated an increased prevalence of some tolerant weeds in orchards when triazine herbicides were used in certain crop weed systems of common vetch (Heeney et al., 1981a), field bindweed (Meith and Connell, 1985), and quackgrass (Hertz and Wildung, 1978). 

Intraspecific triazine resistance was first discovered in common groundsel (Senecio vulgaris L.) in western Washington in the late 1960s. The subsequent widespread and frequent occurrence of other triazine resistant weeds over the past 20 years, has made triazine resistance the best known and most studied example of herbicide resistance (1). Triazine resistance has also been of great interest because of the importance and extensive use of this group of herbicides. 

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. 

Recent investigations have shown that the prolonged use of triazines increased the tolerance of some weed species in certain places, and even resistance developed. Ryan (1970) found a simazine- and atrazine-resistant population of common groundsel (Senecio vulgaris) in a forest nursery, where these herbicides had been... 

We compared the membrane lipid composition of chloroplasts isolated from species of common groundsel (Senecio vulgaris L.), lambsquarters (Chenopodium album L.), and pigweed (Amaranthus hybridus L.) (Table VIII) (W. 

Differences were also noted in the fatty acid distribution in chloroplast lipids from atrazine-resistant and atrazine-susceptible biotypes (Table IX). Resistant chloroplast membranes contained lipids comparatively richer in unsaturated fatty acids with the exceptions of DGDG from all three biotypes and PE from common groundsel. 

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