. 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). The biogenesis of natural auxins place; either the amine or keto-acid could act as potential precursors of the aldehyde and auxin. Many experimental results indirectly support the concept of tryptophan as the primary precursor. There is the almost ubiquitous occurrence of the tryptophan-IAA enzyme system in metabolically active tissues. There is also a parallelism in the relative enzyme activity and substrate c
. 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). The biogenesis of natural auxins place; either the amine or keto-acid could act as potential precursors of the aldehyde and auxin. Many experimental results indirectly support the concept of tryptophan as the primary precursor. There is the almost ubiquitous occurrence of the tryptophan-IAA enzyme system in metabolically active tissues. There is also a parallelism in the relative enzyme activity and substrate concentration with known auxin production loci, and a parallelism between endogenous auxin level and tryptophan availability. Further support for the thesis that native auxin or lAA is formed from tryptophan can be adduced from experiments dealing with the responses of Figure 2. Relative free auxin levels in kidney bean plants follow- ing X-irradiation. The values shown were derived from the ^ig free auxin found per plant [bio-assay).. plants to ionizing radiation (Gordon, 1955). Low doses of ionizing radiation cause an immediate depression of free auxin levels in various plant species. For example, Figure 2 indicates the relative auxin levels in kidney bean plants at various intervals after exposure to X-i-adiation. It may be observed that all the dosages employed, beginning at 25 r, caused an immediate depression of free auxin level. With the lower radiation dosages apparent recovery to control level was attained within one or two weeks. Inhibition and no recovery resulted after irradiations of 5 and 10 kr. The assumption that the lowering of auxin levels was caused bv reduced rates of auxin production was substantiated by a number of studies on the effect of radia- tion on morphological phenomena known to depend on continued auxin production. This sensitivity to irradiation and the pattern of temporary inhibition and recovery is closely paralleled by the effect of radiation o
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