CHEMISTRY OF THE GROWTH SUBSTANCES 



763 



The various pieces of inconclusive evidence for tryptophan and tryptamine as 

 precursors of lAA cannot be detailed here; the matter has been reviewed more 

 than once (Larsen, 195 la; Gordon, 1954, 1956). 



A wide variety of synthetic substances have auxin activity. These are constructed 

 on the general molecular plan of lAA, that is, with an aromatic ring and an ali- 

 phatic acid side-chain. Examples include indeneacetic acid, VIII, indolebutyric 

 acid, IX, sometimes used for the rooting of cuttings, and naphthalene acetic acid, 



-CH2CH2 

 NH2 



Tryptamine 

 VII 



-CH2COOH 



Indeneacetic acid 

 VIII 



-CHgCHgCHjCOOH 



Indolebutyric acid 

 IX 



or NAA, X, widely used to inhibit the abscission of fruits in the orchard industry. 

 Other substances will be discussed in various parts of this chapter. 



High auxin concentrations are so toxic that synthetic auxins find their widest 

 application as weed-killers. For this purpose NAA was first used, but now the 

 most widely used substances are 2,4-dichlorophenoxyacetic acid or "2,4-D", 

 XI, the corresponding trichloro-derivative "2,4,5-T", XII, and the 2-methyl-4- 

 chloroderivative "Methoxone", XIII. 



CH2C00H 



Naphthalene 



acetic acid 



X 



2, 4-Dichlorophenoxy- 



acetic acid 



XI 



CI CI 



Trichloro-derivative 2-methyl-4-chloro- 

 of XI derivative 



XII XIII 



A large literature has grown up around the characteristic features of the molecule 

 which are required for auxin activity (see Thimann, 1951a; Veldstra, 1953, 1956; 

 Pybus et al., 1 959) . It was originally thought that an acid group, or a structure readily 

 converted to an acid group, must be present, and that this must be held at a dis- 

 tance of one or more atoms from an unsaturated ring. Esters, amides and nitriles 

 are examples of structures readily converted to carboxylic acids, and those active 

 compounds which belong to these types may all owe their biological activity to 

 conversion to the corresponding free acid. In a few cases, however, they show 

 somewhat higher activity in one or another test than the free acid. For example, 

 ethylindoleacetate is 10-100 times more active than lAA in causing partheno- 

 carpy, and indoleacetonitrile is 10-20 times more active than lAA on the growth 

 of coleoptiles. Such behavior has been ascribed to the greater ease of entry of 

 these neutral derivatives into the cells, but of course it might mean that they are 

 active per se, and this is often difficult to disprove. 



The requirement for distance between ring and acid group is exemplified by 

 the inactivity of indole-carboxylic and benzoic acids, but it does not necessarily 



Literature p. 8i6 



