no 



Fundamentals of Auxin Action 



The physiological processes involved in fruit-set will be discussed 

 in more detail in chapter XI. \n l^riel, it is evident that auxin is a 

 primary control of fruit-set. In normal pollination, an auxin-producing 

 system is activated which results in fruit-set, whereas in parthenocarpy 

 the unfertilized ovary contains sufficient auxin — either due to natural 

 production or artificial application. 



It is difficult to distinguish between the role of auxin in fruit-set 

 and its role in the actual growth of the fruit. Assuming that such a 

 distinction can be made, it can be said that in general there are two 

 phases of auxin production by young fruit. In the earliest stages of 

 growth, auxin production is essentially carried out in the ovary tissue 

 surrounding the young embryo (Gustafson, 1939), whereas in later 



A BCD 



Fig. 50. The role of ovules in growth of strawberry fruits, and their replacement 

 with auxin. (A) normal intact fruit; (B) removal of achenes from the sides of the 

 fruit results in localized giowth; (C) removal of all achenes prevents fruit growth; 

 (D) removal of all achenes and application of auxin (100 mg./l. /3-naphthoxyacetic 

 acid) permits normal growth again (redrawn from Nitsch, 1950). 



Stages the major source of auxin is the young developing embryo. That 

 developing embryos are a particularly rich source of auxin has been 

 dramatically shown in several instances (Wittwer, 1943; Luckwill, 

 1948; Mitchell et al, 1951) and the great role that these auxin sources 

 jilay in development of the fruit has been abinidantly demonstrated by 

 Nitsch (1950) working with strawberry fruits. His experiments have 

 indicated that the greatest part of growth of the fruit is brought about 

 by the auxin produced in the developing achene. It can be seen in 

 figure 50 that removal of some of the ovules or young achenes caused 

 localized fruit growth around the remaining ones {B). Complete re- 

 moval of the ovules deprived the fruit of the hormone needed for 

 growth (C), and supplying auxin replaced the function of the achenes 

 in causing fruit growth (D). 



