282 Auxins in Agriculture 



which apparently translocate insufficient amounts of auxin to kill the 

 roots, whereas annual grasses such as foxtail apparently translocate 

 amounts sufficient for lethal action much more readily. Selectivity may 

 be the expression, finally, of inherent differences in the toxicity of the 

 auxin at the cellular level as is the case of 2,4-D on oats and peas. 

 Thus, excised tissues of oats are known to have much less metabolic 

 susceptibility to a given level of 2,4-D than have comparable tissues 

 of peas (Kelly and Avery, 1949), and the selectivity between these 

 species may be accounted for in that way. 



Selectivity, then, is an expression of differences in susceptibility 

 between plants. It may be accountable as an expression of any of the 

 factors altering susceptibility just enumerated. Thus differences in 

 stage of development, growth rate, morphology or metabolic suscepti- 

 bility of two plants can make possible a selective action of auxin 

 herbicides. 



Woodford (1950) has pointed out that, theoretically, mortality 

 can be related schematically to auxin concentration in the manner 

 shown in figure 115. In his original discussion he was comparing mor- 

 tality curves between two herbicides, but the same plan can be 

 modified to compare mortality curves between two species. Thus one 

 would find that plant A would show a 50 per cent mortality at a 

 lower concentration of a given auxin than plant B. Both will suc- 

 cumb if a sufficiently high concentration is given. By applying the 

 concentration indicated in the figure as concentration X it is pos- 

 sible to kill selectively plant A without destroying any of plant B. 

 This difference in sensitivity, whatever is the cause, makes selective 

 killing possible. 



In general it is not considered to be sufficient to kill 50 per cent 

 of the weeds, but Woodford suggests that the concentration to kill 

 90 per cent of the weeds may be of greater value. As indicated in the 

 curve, the final destruction of 100 per cent of the plants may require 

 a much higher concentration than that required for a 90 per cent kill, 

 hence the latter figure is probably more valuable and more economi- 

 cally utilized. The tenacity of the last 10 per cent of the weed popula- 

 tion is evident in the data of van Overbeek and Velez (1946), which 

 indicate that the amount of 2,4-D required for 100 per cent kill was 

 as much as half again greater than the amount for 90 per cent kill. 



SELECTION OF AUXINS 



The most potent herbicidal materials among auxins appear to 

 be 2,4-D, 2,4,5-T (2,4,5-trichlorophenoxyacetic acid), and MCPA 

 (2-methyl,4-chlorophenoxyacetic acid). 



