Natural auxins 



recovery of biosynthetic ability is manifest up to about 1 kr. Recovery is 

 virtually complete at either one or two weeks with the lowest dosages; 

 partial recovery has been attained in two weeks by the plants receiving 1 kr; 

 an inhibition with no recovery is shown by the seedlings receiving 5 kr. 

 Again the similarity to Figure 2 is clear. The parallel effects of radiation on 

 auxin production (as indicated by free auxin assays and morphological 

 studies) and on tryptophan conversion to lAA offers strong supporting 

 evidence that the two processes are synonymous. 



The effect of ionizing radiation on auxin formation may be tied a little 

 closer, albeit less rigorously, to the accepted pathway of indoleacetate 

 formation. Figure 6 indicates that the presumed immediate precursor of lAA, 



Figure 8. The relative conversion 

 of crude indoleacetaldehyde to auxin by 

 cell-free homogenates of X-irradiated 

 mung bean seedlings. 



5x10^ 

 rontgcns 



indoleacetaldehyde, piles up during the conversion of tryptophan to lAA by 

 homogenates of irradiated tissues. The same phenomenon was observed in 

 the conversion of infiltrated tryptophan by irradiated plants. The evidence 

 that we are really dealing with indoleacetaldehyde in these experiments is 

 not unequivocal. The substance is neutral, possesses no or little activity in 

 the Avena curvature test when tested directly, but is converted to lAA by 

 coleoptile preparations and forms adducts with dimedone and bisulphite. 



We assumed that this substance was indoleacetaldehyde. Its accumulation 

 suggested that the radiation block occurred at the enzyme which oxidizes the 

 aldehyde to the acid, analogous to the biochemical blocks in Neurospera 

 mutants. It appeared desirable, therefore, to examine more directly the effect 

 of irradiation on the activity of the terminal enzyme. Figure 8 shows the 

 relative conversion of crude preparations of indoleacetaldehyde to lAA by 

 homogenates of irradiated tissues. Here again there is apparent the unusual 

 in vivo sensitivity and dose response to ionizing radiation. It may be con- 

 cluded that the radiation damage to lAA biosynthesis from tryptophan 

 occurs at the oxidation of indoleacetaldehyde to lAA. If this is true, these 

 experiments support the view that the aldehyde is an intermediate in auxin 

 biogenesis. 



We might at this point begin to examine more critically certain portions of 

 the pathways of indoleacetate formation represented in Figure 1. We men- 

 tioned that the coleoptile can convert tryptamine to auxin and that leaves 

 and leaf preparations of the pineapple plant form both the aldehyde and lAA 

 from the amine. Even though active amine oxidases which could produce the 



70 



