Pre-Harvest Fruit Drop 243 



It is possible that ethylene production by the maturing fruits may 

 accentuate the tendency to abscise. Ethylene is produced by ripening 

 apples (Gane, 1935) and also brings about abscission in general 

 (Goodspeed et al, 1918). 



The abscission of mature apple fruits takes place principally as a 

 result of a weakening and ultimate fracturing of the middle lamella 

 of cells in or near the abscission zone (McCown, 1943). Auxins ap- 

 plied to the fruit prevent the development of these changes in the 

 middle lamella, possibly through inhibition of enzymes which render 

 pectins soluble (van Overbeek, 1951). Inhibition of pectin methyles- 

 terase with auxin has been observed in vitro, but not in vivo (Bryan 

 and Newcomb, 1954). 



When auxin sprays are used to control pre-harvest drop in apples, 

 auxins applied to the stem end of the fruit are most effective (Gardner 

 et al, 1940). Application to this part would most logically produce the 

 greatest effect in increasing the auxin gradient across the abscission 

 layer, hence preventing abscission in the manner discussed in chapter 

 IV. 



Studies of abscission have shown that for the effective control of 



Q. 

 O 

 CC 

 Q 



y- 

 z 



LlI 



o 

 or 



LU 

 Q. 



Auxin supplied by- 

 Ovory Endosperm 



Embryo 



June drop 



Prehorvest 

 drop 



6 8 



TIME 



10 



12 



14 



16 WEEKS 



Fig. 103. Schematic diagram of the relationship between spurts of auxin pro- 

 duction (above) and fruit drop (below) in apples (adapted from Luck will, 1953). 



