Auxin-type Herbicides

Grossmann K, The mode of action of quinclorac A case study of a new auxin- type herbicide, in Cobb AH, Kirkwood RC (eds.). Herbicides and their Mechanisms of Action, Sheffield Academic Press, UK, pp. 181—214, 2000. 

The revolution in selective weed control came about in the early 1940s with the discovery of the hormone herbicides 2,4-D and MCPA (see Section 3.3 on Auxin-type herbicides). These compounds were truly selective and very active. They controlled broad-leaved weeds in cereal crops when applied post-emergence. 

Pyridines and Pyridazinones. Pyridine herbicides arc auxin-type herbicides generally used fur selective control of hroadleaved weeds in cropland, rangelands, and noncroplands. The pyridazinones arc used primarily for ihe... 

Hall et al. (1998) reported that an ALS-resistant biotype of false cleavers was cross-resistant to a broad range of ALS inhibitors, as well as to an auxin-type herbicide, quinclorac, which had never before been applied to these fields. A similar case of quinclorac multiple resistance in smooth crabgrass has been reported in California when plants were previously treated with ACCase herbicides. Data suggest a target site-based mechanism of resistance involving the accumulation of cyanide derived from stimulated ACC synthesis, which is a precursor of ethylene (Abdallah et al., 2004). 

A series of benzylnitramines were prepared by either nitration of the carbamates or N-alkylation of nitrourethane, followed by ammonolysis. These represent one-phenylnitramines, known broadleaf herbicides. Nastic responses and growth inhibition observed for this class suggested similarity to the auxin type herbicides. To determine if die preference for the (R) -configuration extended to nitramines, l-(2 6 -dichlorophenethyl)nitramine was resolved and the absolute configuration determined by asymmetric synthesis. A comparison of the (+) and (-) isomers in herbicidal and in vitro assays was performed and the results are discussed. 

Dicamba in the form of its dimethylamine salt is a translocatable auxin-type herbicide, which by pre- or postemergence treatment, usually combined with one or more phenoxyalkanoic acids, is used for the control of grasses, broad-leaved weeds and several shrubs. In combinations mainly 2,4-D, MCPA, and mecoprop are used. Its recommended rate is 80-130 g a.e./ha. 

We proposed a metabohc pathway of phosphonates with structure lo in plants when these compounds lo were initially designed. lo might be metabolized to substituted phenoxyacetic acids (lo-a) as auxin-type herbicide and acylphospho-nates (lo-c) as PDHc El inhibitors after the hydrolysis and oxidation by esterases and oxidases took place in plants (Scheme 1.21). 

The herbicidal activity of IC-IF including lA and IB were evaluated at different rates in the greenhouse. Substituted phenoxyacetic acid as an structure unit of auxin-type herbicide was contained in the stracture of lA-IF, therefore an auxin-type herbicide 2,4-D was used as a positive control for the test in the greenhouse. Considering 2,4-D against broadleaved weeds for post-emergence at a recommended rate of 0.2-2.0 kg ai/ha, a preliminary bioassay was first carried out at... 

On the other hand, herbicide activity of IG-21, IID-10 and some analogs were also observed on tested plants with symptoms of smnting, necrosis, and morphological effects. Some symptoms are similar to 2,4-D. Maybe IG-21, IID-10 also has auxin-type herbicidal action as they are also the derivatives of 2,4-D acid. 

The alkylphosphonates, such as IG-21 and IID-10 were also observed on tested plants with some symptoms like auxin-type herbicidal action. The present results turned out that the herbicidal action not come from the possible metabolic product 2,4-D, but come from these phosphonates themselves. 

The development of chemicals to selectively control the growth of weeds has been a fascinating success story which has unfolded largely during the last four decades. The dramatic growth of herbicide research that followed the wartime discoveries of the auxin-type herbicides (phenoxyalkanoic acids) resulted in a whole range of compounds and mixtures which are used to eliminate broad- or narrow-leaved weeds from agricultural, horticultural, or forestry crops. 

Since his undergraduate years, his research career has focused on the structure-activity relationships of bioactive compounds, starting with the auxin-type plant-growth regulators and herbicides. In 1961, he joined Corwin Hansch at Pomona College in Claremont, California, as a Postdoctoral Research Fellow. From 1961 to 1963, he and Hansch developed a novel procedure to quantitatively analyze structure-activity relationships of agrochemicals and medicines (QSAR). QSAR is regarded as a forerunner to the development of a number of recent computer-aided... 

Arvik et al. (1971), working with the herbicide picloram (which produces effects in higher plants similar to that of auxin-type synthetic growth regulators), found that it had no influence on the algae of a clay loam soil. These workers treated plots with this herbicide at 0.28, 0.56, and 1.12 kg/ha. Samples were collected from three soil levels, three, six, and 18 mon after treatment. None of the 16 species observed was qualitatively or quantitatively affected by the herbicide treatments. 

Various chemical classes of auxin herbicides with different weed spectra and types of selectivity have been synthesized and commercially introduced over the years. They include phenoxycarboxylic acids, benzoic acids, pyridine carboxylic acids, aromatic carboxymethyl derivatives and a relatively new category, the quinolinecarboxylic acids (Figure 1). These compounds basically act as synthetic mimics of indole-3-acetic acid (lAA), which is the principal natural auxin in higher plants [2,3,5,7]. 

The latter investigations also demonstrate another type of biochemical mode of action of herbicides with auxin effect. 

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