2.4.6- Trichlorophenoxyacetic acid, auxin

Inhibition of Limonoid Biosynthesis by Auxins. Auxins are potent inhibitors of nomilin biosynthesis in citrus seedlings (31). For instance, up to 91% inhibition was observed when 10 ppm of indoleacetic acid was fed to the stem of a lemeon seedling two days prior to and two days following feeding of 25 pCi of 14-C acetate (Table 1). Other auxins tested include 1-naphthaleneacetic acid (NAA), indolepropionic acid, indolebutyric acid, 3-indole acetonitrile, ethyl indole-3-acetate, 3-indoleacrylic acid, 3-(2-hydroxyethyl)indole, indole-2-carboxylic acid and 2,3,4-trichlorophenoxyacetic acid. They were all very effective. 

T 2,4,5-trichlorophenoxyacetic acid (2,4,5-trichlorophenoxyethanoic acid) a synthetic auxin formeriy wideiy used as a herbicide and defoUant. it is now banned in many countries as it tends to become contaminated with the toxic chemicai dioxin. 

Herbicides may be selective, as for broad-leaved weeds in cereal crops, or unselective, essentially for land clearance. 2,4-Dichlorophenoxyacetic acid (2,4-D), 24.6, was commercialized in the 1940s it is inexpensive, is easy to make, and kills broad-leaved weeds in cereal crops. Worldwide, it is the most widely used herbicide. It is a synthetic auxin, or plant hormone, acting only on dicots and not monocots. There is a whole family of related compounds, prepared from chloroacetic acid (or other halo acids) and various chlorinated phenols. Agent Orange, used as a defoliant in Vietnam in the 1970s, was a 1 1 mixture of 2,4-D and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). The main concerns about its use relate to the possibility that dioxins are formed as contaminants in its manufacture. A few countries ban its use for control of weeds in domestic lawns. 

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