Occurrence and Role of Growth Hormones 79 



Furthermore the neutral precursor forms an addition product with 

 dimedon which, like the bisulfite addition product, is specific for 

 aldehydes (Gordon and Nieva, 1949). 



The conversion of indoleacetaldehyde to indoleacetic acid can be 

 carried out by using crude enzyme preparations from various plant 

 materials (Larsen, 1949, 1951; Gordon and Nieva, 1949; Ashby, 1951). 

 As already noted, Larsen has observed that the yield of the free acid 

 is lower than one might expect on a mol for mol basis. Using naphtha- 

 leneacetaldehyde as a substrate, he has demonstrated that the enzyme 

 preparation produces only one-half mol of acid for each mol of alde- 

 hyde consumed, and he proposes that the oxidation must be a dis- 

 mutation which produces one-half mol of the acid and one-half mol 

 of the alcohol, as shown in figure 37. 



It should be pointed out here that the indoleacetaldehyde is ap- 

 parently not an auxin itself, but is probably active in growth only 

 upon conversion to the acid (Larsen, 1949). The two major pieces of 

 evidence for this conclusion are the observation that the aldehyde 

 produces a growth response only after a delay in time (presumably 

 because it must first be converted to the acid), and the observation 

 that the concentration curve obtained by the Avena test is quantitative 

 only over a very narrow range of concentrations. Perhaps the latter 

 occurs because the enzyme system for the conversion of the aldehyde 

 to the acid becomes saturated at higher concentrations of the aldehyde. 



Some interesting data supporting the contention that indoleacetal- 

 dehyde affects growth after conversion into the free acid have been 

 obtained by Bentley and Housley (1952). They grew Avena coleoptile 

 sections in solutions of various strengths of indoleacetaldehyde, obtain- 

 ing the growth data shown in figure 39 A. After the coleoptile sections 

 had been removed from the solutions, the neutral substances remain- 

 ing in the solution were separated from the acidic substances, and a 

 fresh set of coleoptiles was then grown in each of these fractions. The 

 growth responses shown in figure 39 B indicate that sufficient acidic 

 growth substances were present in the solution to account for the 

 entire amount of growth obtained in the first dilution series. It may 

 be deduced, then, that the apparent stimulation of growth by the 

 aldehyde was in fact due to the formation of the free acid in the 

 solution during the test. 



Another pathway by which indoleacetic acid may be formed in 

 plants is through the oxidation of indoleacetonitrile, a compound 

 found to occur naturally in plants by Jones et al (1952). The nitrile 

 shows apparent auxin activity in the Avena straight-growth test (Bent- 

 ley and Housley, 1952), but its lack of activity in the pea straight- 



