194 Auxins in Agriculture 



tomato are very much weaker in growth activity than indoleacetic 

 acid, but one may assume that their persistence in the plant is con- 

 siderably longer. 



Auxins which have been applied to foliage generally remain active 

 in the plant for only a limited time. Experiments in which applied 

 auxins were recovered from plants by extraction suggest a rapid loss 

 of the auxins in plants. For example naphthaleneacetic acid has been 

 applied to cocklebur plants through the nutrient solution for differ- 

 ent intervals of time, and the leaves extracted with ether and assayed. 

 It was found that small increases in the amount of free auxin in the 

 leaves were detectable for the first ten days, but that after that no 

 increase was evident (Bonner and Thurlow, 1949). In an effort to 

 determine how long 2,4-D persisted in plants in an active form, 

 Dhillon and Lucas (1950) made water extracts of leaves at intervals of 

 time after localized treatment of leaves. They found that the auxin 

 could not be detected by epinasty tests even one day after application 

 to bean, corn and oat plants, though it was detectable in tomato plants 

 for five days. When large amounts of 2,4-D were placed in a reservoir 

 in a tomato stem, the auxin could be detected for as long as 26 days 

 in the tissues. Hay and Thimann (1954) also studied the persistence 

 of 2,4-D in beans and they were able to recover 90% of the applied 

 auxin 6 hours after application and only 44% after 48 hours. 



Morphological evidence indicates that the growth effects of 2,4-D 

 seem to be established in a relatively short period of time in most 

 species. There has been considerable discussion in the literature about 

 the length of time for which 2,4-D may exert morphological effects in 

 plants. The bulk of evidence indicates that morphological irregulari- 

 ties or distortions in response to the treatment are usually all estab- 

 lished within a few days after the application of the spray. This has 

 been explained in some detail by Eames (1951). Some evidence for 

 such a generalization can be seen in the description of morphological 

 responses to 2,4-D described in chapter V. Morphological irregularities 

 which are brought about in buds may not become evident for as long 

 as a year after the 2,4-D application (Tullis and Davis, 1950). Morpho- 

 logical irregularities in young developing seeds likewise may not appear 

 until the next growing season when the seeds germinate (Pridham, 

 1947); nevertheless, the evidence at hand indicates that the irregulari- 

 ties induced by 2,4-D have been essentially established even in these 

 tissues within a very short period of time after the application (Eames, 

 1950). 



Exceptionally long persistence of 2,4-D in cotton plants has been 

 reported by Mcllrath and Ergle (1953). They found that leaf abnor- 



