Parthenocarpy 223 



flowers on a cluster to open respond best to auxin sprays, presumably 

 because of the lack of competition for nutrients for growth. As more 

 liuits are set and flowers further out on the duster begin to open, 

 ilie sensitivity to the auxin sprays is noticeably hnvered (R<)l)erts and 

 .Siiu(kineyer, 1944). 



It has already Ijeen mentioned that auxins applied even at (on- 

 siderable distances from the flowers can influence fruit-set. It appears, 

 however, that auxin sprays (PCA) applied to the face or front of the 

 flower result in fewer fruit abnormalities than sprays applied at more 

 distant points (Leopold and Guernsey, 1953). 



Most workers who have studied the use of auxin sprays on toma- 

 toes have found that although forcing fruit-set on the first 3 or 4 

 clusters is beneficial, auxin applications to flowers on higher clusters 

 have very little effect. This is apparently attributable to differences in 

 competition between fruits. When the plant is small and has few fruits, 

 the application of auxins can act to overcome the limitation of fruit- 

 set. On the other hand when the plants have many fruits already draw- 

 ing on the food reserves of the plant, the application of auxin is usually 

 of little value in forcing fruit-set. 



FACTORS IN SUCCESS 



The use of auxin in setting tomato fruits in commercial practice 

 may be of benefit for the following reasons: (a) it can overcome certain 

 limitations of fruit-set due to failure of pollination, (b) it can delay the 

 loss of the flower through abscission, (c) it brings about an increase in 

 the rate of maturation of the fruit, making possible an earlier crop, 

 and (d) it can bring about an increase in fruit size. 



Overcoming Limitations of Fruit-set 



Certain physiological factors can limit the capacity of tomato 

 flowers to set fruit. Since pollen is essential for natural fruit-set, those 

 factors which limit the effectiveness of pollen — either in reaching the 

 stigma or in affecting fertilization when it has reached the stigma — 

 limit natural fruit-set. 



Tomato plants grown in greenhouses are often so protected from 

 wind and insects that normal pollen dispersion does not take place. 

 Under circumstances in which self-pollination does not readily occur, 

 forcing fruit-set with auxin can be beneficial. 



Certain morphological limitations to pollination develop under 

 conditions of weak light intensities, high nitrogen levels or high tem- 

 perature (Howlett, 1939). Under these several circumstances the style 

 of the tomato flower is prone to become excessively long. Such elonga- 



