Flower and Fruit Thinning 

 The Prevention of Pollination 



233 



For some time evidence has indicated that auxins may aUer the 

 germination and growth of pollen tubes. For example, in a synthetic 

 medium Addicott (1943) obtained a small inhibition of pollen tube 

 growth with indoleacetic acid, and a greater inhibition of pollen ger- 

 mination. In studies of the means by which auxins may thin apple 

 flowers, Luckwill (1953) has treated flowers with various concentrations 

 of naphthaleneacetic acid and subsequently introduced fresh pollen 

 onto the stigmas. The inhibition of pollen germination following the 

 auxin treatment is evident in figure 98. This action of auxin in thin- 

 ning will of course apply only to flowers. 



Flower thinning materials such as the cresols and phenols obtain 

 their greatest effects through a toxic action on opened, unset flowers 

 (Shepard, 1939; MacDaniels and Hoffman, 1941). For this reason, they 



100 



80 



2 60 



UJ 



O 



or 



LjJ 40 

 Q. 



20 



I \ 

 I \ 



\ 



Abortive 

 seeds 



Extra 



fruit drop 



,^A:>^,\' 





8 



10 



12 



14 WEEKS 



TIME AFTER PETAL FALL 



Fig. 99. The abortion of seeds in Crawley Beauty apples following a fruit- 

 thinning spray with 40 mg./l. naphthaleneacetic acid at petal-fall, and the sub- 

 sequent abscission of the fruits (Luckwill, 1953). Figures represent percentage 

 differences from unsprayed controls. 



