. Auxins and plant growth. Auxin; Plant growth promoting substances. Herbicides 281 in the plants. Thus the relative resistance of wheat may be owing to the strongly adsorptive action of such materials toward auxin. Auxin molecules which are adsorbed in this manner would be unavailable for participation in metabolic events. Gallup and Gustafson (1952) have observed that the absorption and translocation of a radioactive analogue of 2,4-D by grass species tended to be less than that by the broadleafed species tested. It was suggested that differences in absorption and translocation could well co
. Auxins and plant growth. Auxin; Plant growth promoting substances. Herbicides 281 in the plants. Thus the relative resistance of wheat may be owing to the strongly adsorptive action of such materials toward auxin. Auxin molecules which are adsorbed in this manner would be unavailable for participation in metabolic events. Gallup and Gustafson (1952) have observed that the absorption and translocation of a radioactive analogue of 2,4-D by grass species tended to be less than that by the broadleafed species tested. It was suggested that differences in absorption and translocation could well contribute to selectivity of auxin herbicides. In approaching the question of selectivity one must recognize that all plants have a potential susceptibility to 2,4-D. That is, if a suf- ficient amount of the auxin is introduced into the plant death will follow. Selectivity is an expression of differences between plants in the capacity of an auxin preparation to enter, be translocated, or finally to express toxicity at the cellular level. Thus selectivity may be owing to differences in the ability of an herbicide to enter as in the case of peas compared with mustard. A salt of MCPA can evidently enter the mustard foliage more readily than the more waxy foliage of the pea. The ester derivatives of the same herbicide apparently enter the foliage of either plant equally well (Buchholtz, 1952), and conse- quently the salt of MCPA is selective between the plants and the ester is not. In other cases selectivity may be due to differences in translocation, as in the case of perennial species like Johnson grass 100. LU O cr LU CL CONC. OF AUXIN Fig. 115. Theoretical mortality curves for two different plant species sprayed with different concentrations of an herbicidal auxin. Arrows indicate relative con- centrations required to kill 50 per cent of the plants of each species (modified from Woodford, 1950).. Please note that these images are extracted from scanned page images that may have b
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