222 Auxins in Agriculture 



Time and Placement 



Studies of the relative effectiveness of auxin sprays applied to 

 tomato flowers at different stages of development show that fruit-set 

 can be induced even in small, unopened flower buds (Roberts and 

 Struckmeyer, 1944). The greatest relative effectiveness is obtained, 

 however, when the petals have just begun to open. This is in contrast 

 to responsiveness of the various flower stages to pollination, for of 

 course no response to pollen is obtained until the petals have begun 

 to open. Maximum response to pollen occurs several days later. These 

 differences in responsiveness to parthenocarpic fruit-set and to pol- 

 lination can be seen in figure 94. It is interesting to note that more 

 flowers can be set with auxin than with pollen at every flower stage 

 previous to full-bloom. The ability of tomato flowers to be set by 

 auxin persists up to the time that the flower is abscissed from the 

 plant, and even after abscission (Leopold and Scott, 1952). 



The position of the flower on the flower cluster has been found 

 to have a bearing on responsiveness to auxin treatment. The first 



10 Days 



AGE OF FLOWER 



Fig. 94. The relative effectiveness of pollination and forced parthenocarpy 

 with auxin (jg-naphthoxyacetic acid) at various stages of flower development 

 (data of Roberts and Struckmeyer, 1944), 



