52 Antoinette Pirie 



(Weber and Gordon 1952a), extracts of irradiated mung bean 

 plants form less auxin from tryptophan than extracts from 

 normal plants. These changes were noticeable immediately 

 after radiation with low doses such as 10-50 r but synthesis 

 of auxin was not stopped completely even by very high doses. 



The synthesis of auxin from tryptophan goes through a 

 series of steps. Weber and Gordon (1951c) found that im- 

 mediately after irradiation of the plant with 25-50 r there 

 was a rise in concentration of an aldehyde. Similarly, using 

 extracted enzymes from irradiated plants Weber and Gordon 

 (1952a) found increased formation of an aldehyde which is 

 considered to be indoleacetaldehyde. They therefore conclude 

 that it is the enzyme which converts indoleacetaldehyde to 

 indoleacetic acid or auxin that is specifically inhibited by 

 radiation. 



If there is any doubt about this — and until details of the 

 work done between 1949 and 1955 have been published 

 there must be a doubt — I think it lies in the conclusion that 

 irradiation affects the conversion of indoleacetaldehyde to 

 indoleacetic acid. The reasons for uncertainty are these: 

 indoleacetaldehyde has not been proven to be an intermediary 

 in auxin synthesis. It has not been isolated from the plant and 

 has only once been synthesized and obtained pure. Gordon 

 has not, I think, been able to use indoleacetaldehyde in any of 

 his work. Assay of indoleacetaldehyde is achieved by conver- 

 sion to auxin either by infusions of soil or by enzymes from 

 leaves or by the aldehyde oxidase of milk, followed by bio- 

 logical assay of auxin formed. Since conversion is not com- 

 plete, the assay involves complicated calculations. 



Brown, Henbest and Jones (1952) who synthesized indole- 

 acetaldehyde, tested the pure substance as an auxin or auxin 

 precursor in various biological tests. They found it to be not 

 more than 10 per cent as active as indoleacetic acid. Weber 

 and Gordon (1952&) have used neutral ether extracts of 

 cabbage as a crude source of indoleacetaldehyde in testing 

 the conversion of the aldehyde to auxin. But Jones and co- 

 workers (1952) have isolated indoleacetonitrile from neutral 



