280 Auxins in Agriculture 



plant with a solution of 2,4-D and failed to get any more severe 

 response than if he simply sprayed the auxin on the foliage. 



The wettability of the leaf is another morphological factor which 

 can alter susceptibility. Fogg (1947) has observed that diurnal changes 

 occur in the wettability of the leaves of Sinapsis arvensis and has 

 associated these changes with the water content of the leaves. As leaves 

 dry out they are less readily wet by a spray. He also pointed out that 

 the mechanical nature of the leaf surface has a strong influence on 

 the resulting droplet size, a factor which has considerable influence on 

 toxicity of 2,4-D sprays. Large droplets are more effective than small 

 ones (Loomis, 1949). Pubescence of leaves in some cases can increase 

 spray retention from 10 to 100 per cent, and even the angle of orien- 

 tation of the leaves can alter spray retention considerably (Ennis et al, 

 1952). 



Genetic Factors 



There are genetic differences in susceptibility characteristics of 

 various species and varieties. Willard (1947) has reported that an 

 inbred line of corn has been found which is much more sensitive to 

 2,4-D than other corn varieties. Such an inherent difference in sensi- 

 tivity even in closely related species and varieties has been reported 

 for many crop plants, especially among the grains. These differences 

 in response, presumably at the cellular level, remind one of the 

 differences between more and less susceptible species with respect 

 to respiratory responses reported by Kelly and Avery (1949), the 

 differences in potassium metabolism reported by Rhodes et al (1950), 

 and the differences in susceptibility of enzyme preparations to 2,4-D 

 inhibition as reported by Kvamme et al (1949). 



FACTORS IN SELECTIVITY 



Some suggestions have been made concerning the physiological 

 mechanism of selective herbicidal action. Several of these have been 

 mentioned above in the discussion of the possible mechanisms of toxic 

 action. An interesting specific suggestion has been made by Brian and 

 Rideal (1952), who studied the adsorption of MCPA to a variety of 

 materials spread as monolayers on a Langmuir trough. They found 

 that the auxin anion is readily adsorbed onto materials rich in amines 

 and ketones, and that such adsorptive materials are relatively abun- 

 dant in wheat which is a resistant species as compared with tomato and 

 cress which are highly susceptible species. The inference is drawn that 

 the selective action of auxin herbicides may be related to the amount 

 of exogenous auxin which is adsorbed on such proteins or lipoproteins 



