284 Auxins in Agriculture 



of 2,4-D and 2,4,5-T were applied to bean plants, and the resulting 

 inhibition effects were measured. The strongest effects were obtained 

 with the free acids and with the diethanolamine salts. The calcium 

 salt is the least soluble of the ones tested, and produced the least 

 effects on growth. The diethanolamine salt has been recommended as 

 a derivative showing high solubility and high activity (McNew and 

 Hoffman, 1948), and the di- and triethanolamine salts have subse- 

 quently become among the most widely used derivatives for aqueous 

 formulations of 2,4-D. 



Because of their more effective penetration, ester derivatives of 

 auxin show greater contact toxicity to plants, although their trans- 

 location within the plant is considered to be poorer than that of the 

 acids or salts (Robbins et al, 1952, p. 195). The possible difference in 

 translocation between salts and esters was not confirmed by Hauser 

 and Young (1952), undoubtedly because they injected the auxin into 

 the transpiration stream. 



The volatility of different esters is approximately inversely pro- 

 portional to the size of the alcoholic part of the ester as shown in 



9 80 



60 



40 



a: 



LiJ 

 CD 



I- 



UJ 



o 



UJ 20 

 Q_ 



CH3 



R-Na R-OCH3 R-OCHg R-OCH R-OCH2 R-O9H2 



CH3 CH3 CH2 CH2 



CH2 CH2 



CH3 CH2 



N- N- 



^ , , Sodium Methyl Ethyl "so- 



Control salt Ester Ester Propyl Butyl Arnyl 



2,4-D DERIVATIVE 



Fig. 117. Effectiveness of vapors of various esters of 2,4-dichlorophenoxyacetic 

 acid in inhibiting germination of seeds of ten common species (tomato, bean, 

 corn, cucumber, rye, wheat, turnip, cotton, pea, and squash averaged together) 

 showing greatest volatility effect of the smallest ester derivatives (calculated from 

 Mullison and Hummer, 1949). 



