236 Auxins in Agriculture 



advantage of apparently causing less flagging of the foliage and less 

 epinastic response. It is most effective when applied before petal fall. 



Another auxin, 2,4,5-T, has been used successfully for thinning 

 Anjou pears (Griggs et nl, 1951), but comparative trials with various 

 auxins on other fruits usually indicate that naphthaleneacetic acid is 

 the best for thinning (Hartmann, 1952). 



Various phenols and cresols are used as floxoer thinning sprays 

 more generally than are the auxins. A compound which is finding 

 considerable use as a fruit thinning agent is chloro-IPC (3-chloro- 

 isopropyl-N-phenylcarbamate), which like auxin is effective in thinning 

 both flowers and fruits. 



The concentration of auxin considerably influences the effective- 

 ness. Very few experiments have been published giving information on 

 comparative effects of extensive ranges of (oncentrations, but experi- 

 ments using small concentration ranges indicate thai the response is 

 usually quantitative. In figure 100 it can be seen that the percentage 

 of fruits dropped from the tree increases with greater auxin concen- 

 trations. Southwick and Weeks (1952) have iound, however, that some 

 varieties of apples do not appear to give .i c|iianiitative response to 

 the auxin. 



The effectiveness of any given concentration is strongly dependent 

 upon the vigor of the tree or branch, as more vigorous trees are less 

 effectively thinned by a given concentration than weaker trees or 



10 20 



CONC. OF AUXIN 



30 MG/L 



Fig. 100. Relative effectiveness of various concentrations of auxin (napfithalene- 

 acetic acid) upon thinning of apples (Murneek, 1950). 



