288 Auxins in Agriculture 



level had been reached. Another expression of plant injury is the 

 distortion response of leaves, and in figure 118 it can be seen that 

 these may appear at concentrations a million times more dilute than 

 the lethal concentration in the case of tomato and pigweed. In the 

 velvet leaf, distortion responses did not appear until near-lethal levels 

 were applied. 



Herbicidal responses to increases in concentration of auxins are 

 more precipitous than are many other auxin effects. For this reason 

 most herbicidal work is done over very narrow concentration ranges, 

 such as comparison of i/^, 1/2, and 1 pound per acre of 2,4-D or of 

 other experimental ranges equally narrow. 



At the time of the discovery of the herbicidal action of auxins, 

 the majority of herbicides then available had to be applied at con- 

 centrations many times greater. For example, arsenical herbicides 

 were applied at rates as high as 1500 pounds per acre which repre- 

 sents quite a contrast to 2,4-D which kills most annual broad-leafed 

 plants at rates of less than two pounds per acre. It has been com- 

 monly observed that the effectiveness of herbicidal auxin sprays does 

 not continue to increase as the concentration is raised, for the appli- 

 cation of very high concentrations may lead to the rapid killing of 

 the foliage before the herbicide has been translocated into the re- 

 mainder of the plant. This concentration phenomenon limits the use- 

 fulness of some auxins like 2,4-D in killing deep-rooted or perennial 

 species. The concentration curve of herbicidal effectiveness of auxins 

 for many plants shows a reduction in activity if very high concen- 

 trations are used because of this obstacle to translocation. 



The quantity of auxin solutions applied varies greatly. Almost 

 all early herbicidal applications were made using one hundred gallons 

 or more of solution per acre. This volume represents approximately 

 the amount which can be retained on the foliage without excessive 

 runoff. Low-volume concentrate sprays are being used more and more 

 frequently, in many instances at volumes as low as four to six gallons 

 of solution per acre (Foster, 1952) or even one to two gallons per 

 acre when applied in oil by airplane. There is some evidence that the 

 selectivity of auxin herbicides is somewhat reduced by the use of 

 low-volume sprays (Miller and Dunham, 1950), but the economy of 

 the method often makes it worth while. 



It seems quite clear that in the inajority of cases the concentra- 

 tion of auxin applied means very little per se, but rather the actual 

 amount of auxin which is applied determines the herbicidal effect 

 (Robbins et al, 1952, p. 170). 



A difficulty encountered with Canada thistle is that the leaves 



