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). 



