. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). 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 i
. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). 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 term
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