282 PLANT BIOCHEMISTRY 



lip is removed and inverted, the growth hormone is transported irom 

 the apical portion to the original basal portion but not in the other 

 direction. 



In addition to indoleacetic acid several other compounds have been 

 isolated from plants which seem to have growth-promoting properties. 

 Indolepyruvic acid has been identified in plants by chromatographic 

 techniques. Although this compound is known to have giowth-pro- 

 moting activity, the tact that it is readily broken down to indoleacetic 

 acid makes it difficult to assay its role as a growth hormone. 



Three other compounds possessing growth-stimulating jjroperties 

 have been identified in plants, although their role as growth hormones 

 is obscme. These substances are r/,?-cinnamic acid, phenylacetic acid, 

 and indoleacetonitrile. In all probability there are still other growth 

 hormones not yet identified, although it seems quite clear that indole- 

 acetic acid is by far the most common. 



A large number of synthetic compounds with growth-stimulating 

 properties similar to indoleacetic acid have been produced. These 

 synthetic compounds, together with the natinally occurring growth 

 substances, have been given the general name auxin. Certain naphtha- 

 lene derivatives were among the first synthetic compounds to be exam- 

 ined for auxin activity. 1-Naphthaleneacetic acid is widely used as an 

 overall growth-stimidating substance. It is particularly useful in the 

 rooting of cuttings and in the treatment of seeds before germination. 

 2-Naphthoxyacetic acid is particularly useful in setting of seedless 

 fruit (parthenocarpy) by the application of this substance to the pistils 

 of various flowers. 



CH2COOH 



^ ^ -OCH2COOH 



\^ 



l-naphthaleneacetic acid 2-naphthoxyacetic acid 



(NAA) (NOA) 



In addition to those organic compounds containing naphthalene or 

 indole rings, certain synthetic benzene derivatives produce growth 

 responses in plants. Both phenylacetic and phenoxyacetic acids have 

 some activity, the latter only very weakly. Whereas phenylacetic acid 

 is useful in setting of certain fruits, phenoxyacetic acid finds its great- 

 est importance as the parent compound of a number of important 

 derivatives, including 2,4-dichlorophenoxyacetic acid (2,4-D) and 

 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). These substances are 

 among those widely used growth regulators classed as herbicides. 



