Growth Substances in Fruit Setting 



S. H. WITTWER 



POLLINATION and subsequent fertilization are prerequisites for fruit 

 set on most plants. In their absence the ovary will usually fail to 

 enlarge, and abscission occurs shortly. Pollination, pollen germination,, 

 pollen tube growth, gametic fusion (embryo), and triple fusion (endo- 

 sperm) are, with few exceptions, all essential for seed and fruit produc- 

 tion. Closely associated and directly coupled with some (perhaps all) 

 of these processes are complicated hormonal mechanisms which largely 

 control fruit setting and bring about its eventual maturation. 



Certain plant reproductive structures are loci of production and ac- 

 cumulation of natural growth substances. This is especially true of pollen 

 grains and the young fruit or fertilized ovary. Literature concerned with 

 the proposition that the included growth substances in pollen may be 

 causal in the induction of natural fruit setting has been reviewed by 

 Skoog (41) and van Overbeek (47). The small actual amounts of auxin 

 in the usually few pollen grains which function in fruit formation do not 

 indicate that pollen is the source of growth substances necessary for fruit 

 setting. Nevertheless, the facts remain that fruit setting has been induced 

 by pollen extracts and the exact chemical nature of growth sub- 

 stances in pollen continues to attract considerable interest (39,59). Per- 

 haps of greater importance is the rapid accumulation of growth sub- 

 stances in the ovary following fertilization. These hormonal relationships 

 as they exist in the corn plant and their close association with the cyto- 

 genetically important processes of synapsis and syngamy (53) are illus- 

 trated in Figure i. Similar hormonal relationships during the ontogeny 

 of reproductive structures in corn (3,24) and in other plants (16,23,26) 

 have been reported. It is highly probable that normal growth and 

 development in the fruit of most plants is initiated and continued by a 



