CHAPTER IX 



Methods of Applying Auxins and 

 Their Persistence in Plants and Soils 



In the experimental use of auxins, certain precautions must be fol- 

 lowed in making up solutions or other auxin preparations. A variety 

 of methods may be employed in ajjplying the auxin preparations. The 

 experimenter needs to have a working knowledge of these techniques, 

 as well as an appreciation of the persistence of auxins in plants, in 

 soils, and in the greenhouse after application. 



SOLVENTS AND SOLUTES 



The usual means of applying auxins is in aqueous solution. Some 

 salts of the acids are considerably more soluble in water than the free 

 acids themselves. For this reason, the salts are ordinarily used in ex- 

 periments requiring high concentrations of auxins. 



The acidity of a solution of auxin greatly influences its effective- 

 ness as well as its solubility. This influence of acidity appears to be 

 twofold — affecting the entry of the auxin into plant cells, as well as 

 the action of the auxin inside the cells. The distinction between these 

 is difficult to draw. 



Tests of the relative effectiveness of auxin solutions of various 

 pH values in stimulating the growth of Avena coleoptile cells have 

 shown that greatest influence is obtained under very acid conditions, 

 and the effectiveness drops as the acidity is decreased (Bonner, 1934). 

 Since the drop was most rapid in the range of acidity at which dis- 

 sociation of the auxin takes place, it was believed that the auxin 

 functions in growth as the undissociated molecule. More recently, 

 however, it was found that the effects of auxins upon the growth of 

 wheat roots could be precisely correlated with the calculated amount 

 of auxin anions present at the surface of the cells (Lundegardh, 1949). 

 Thus the question of whether auxin acts in growth as the intact acid 

 or as the anion is not settled, but it is quite clear that applied auxins 

 are generally most efficacious in acid solutions of a pH from 3 to 5.5. 

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