Atrazine tolerance

Bahler, C.C., K.P. Vogel, and L.E. Moser. 1984. Atrazine tolerance in warm-season grass seedlings. Agron. Jour. 76 891-895. 

The development of herbicide-resistant weeds has also been an influence on the selection of herbicides used on field corn or soybean. Weed resistance now affects nearly every decision a farmer makes about herbicide selection either a farmer is trying to control resistant weeds or is selecting herbicides that may reduce the possibility of weed populations becoming resistant. The adoption of the imidazolinone- and sulfonylurea-tolerant com hybrids mentioned above was in part a response to the presence of atrazine-tolerant pigweeds or kochia in many fields. However, a recent decrease in die use of imidazolinone and sulfonylurea herbicides can also be attributed to the development of populations of weeds that have become resistant to these herbicides. 

Weimer, M.R., B.A. Swisher, and K.P. Vogel (1988). Metabolism as a basis for differential atrazine tolerance in warm-season forage grasses. Weed Sci., 36 436 -40. 

Andersen, R.N. and J.W. Gronwald (1987). Noncytoplasmic inheritance of atrazine tolerance in velvetleaf (Abutilon theophrasti). Weed Sci., 35 496 -98. 

Nystrom B, Paulsson M, Almgren K, Blank H. 2000. Evaluation of the capacity for development of atrazine tolerance in periphyton from a Swedish freshwater site as determined by inhibition of photosynthesis and sulfolipid synthesis. Environ Toxicol Chem 19 1324-1331. 

Permissible tolerances for atrazine range from 0.02 mg/kg in meat, milk, and eggs, to 15 mg/kg in orchard grass forage, fodder, and hay (Reed 1982 USEPA 1983). However, the 15 mg/kg tolerance in forage is considered high, and a new upper limit of 4 mg/kg is proposed. This limit would be expressed in terms of atrazine and three major metabolites (Reed 1982 USEPA 1983) ... 

Plant. In tolerant plants, atrazine is readily transformed to hydroxyatrazine which may degrade via dealkylation of the side chains and subsequent hydrolysis of the amino groups with some evolution of carbon dioxide (Castelfranco et al, 1961 Roth and Knuesli, 1961 Humburg et al, 1989). In corn juice, atrazine was converted to hydroxyatrazine (Montgomery and Freed, 1964). In both roots and shoots of young bean plants, atrazine underwent monodealkylation forming 2-chloro-4-amino-6-isopropylamino-s-triazine. This metabolite is less phytotoxic than atrazine (Shimabukuro, 1967). 

USEPA (2006a). Memorandum. Atrazine Finalization of Interim Reregistration Eligibility Decision and Completion of Tolerance Reassessment and Reregistration Eligibility Process. 

To facilitate premixes in sorghum, Ciba-Geigy developed a seed safener to ensure greater tolerance to the chloro-acetanilide mixing partners. The development of the safener had a significant impact on use of atrazine and propazine... 

Shimabukuro et al. (1966) identified 2-chloro-4-amino-6-isopropylamino-i-triazine (G-30033) as a major metabolite in shoots of mature pea plants. These results indicated that a second mechanism for tolerance to atrazine existed in some moderately susceptible plants. Later, Shimabukuro (1967a) was able to demonstrate that atrazine could be metabolized independently in both roots and shoots of young pea plants to 2-chloro-4-amino-6-isopropylamino-.t-triazine. This metabolite was much less phytotoxic than the parent compound. The metabolism of atrazine in resistant com and sorghum, in intermediately susceptible pea, and in highly susceptible wheat was reported by Shimabukuro (1967b). This study revealed two possible pathways for metabolism of atrazine in higher plants. All species studied were able to metabolize atrazine by TV-deal kyI ation of either of the two alkyl groups present. Com and wheat that contain the cyclic hydroxyamate (2,4-dihydroxy-7-methoxy-l,4-benzoxazine-3-one) also metabolized atrazine by conversion to hydroxy-atrazine (G-34048). Subsequent metabolism was postulated to be by conversion to more polar compounds. 

In a study designed to determine the mode of action of atrazine in higher plants, Shimabukuro and Swanson (1969) concluded that atrazine inhibits the Hill reaction and its noncyclic phosphorylation, while being ineffective against cyclic photophosphorylation. Atrazine readily penetrated the chloroplast of resistant as well as susceptible plants. In tolerant plants such as sorghum, the metabolism of atrazine was postulated to occur outside the chloroplasts to form water-soluble and insoluble residues that reduced the concentration of photosynthetic inhibitors in the chloroplasts. 

The metabolism of atrazine and a series of 2-chloro-.v-lriaz.ines were reported by Lamoureux et al. (1972) in excised leaf or shoot tissue of barley, corn, sorghum, and sugarcane. The authors found that the primary route of metabolism was the displacement of the 2-chloro group with glutathione or 7-glutamylcysteine. The overall rate of metabolism in susceptible barley was much slower than in tolerant crops. 

Removal of one alkyl group leaves a partially phytotoxic compound, which is then further metabolized to confer total tolerance to the herbicide. For example, Shimabukuro et al. (1973) showed that sorghum dealkylated atrazine to 2-chloro-4,6-diamino-,v-triazinc (GS-28273), which was nonphytotoxic (Figure 9.1). The biological activities of the mono-dealkylated products were intermediate between those of atrazine and the diamino derivative (Shimabukuro and Swanson, 1969 Shimabukuro et al., 1973). These differences are considered to be the reason for intermediate tolerance in some species. 

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). 

---