Developmental Effects of Auxins 1 1 1 



ABSCISSION 



That auxins can control abscission ol various plant parts was first 

 suggested by the work of Laibach (1933) when he demonstrated that 

 orchid pollinia contained a material which prevented leaf abscission. 

 LaRue (1936) was the first to use a synthetic auxin to control leal 

 abscission, and subsequently auxins have been found to control abscis- 

 sion not only in leaves and petioles but in flowers, fruits, and even 

 stems. 



Morphologically, abscission can take place in two ways. A meri- 

 stematic abscission layer may form across the base of the organ. En- 

 largement of cells just distal to the merislematic zone may then force 

 the organ to abscise. This meristematic type of abscission is characteris- 

 tic of most leaves (coleus, pepper, tomato, citrus) and most flowers and 

 young fruits (apple, tomato). The other general type of abscission in- 

 volves a fracturing of the cell wall independently of meristem 

 activity. This type of abscission is common in some types of leaves 

 (Impatiens) and in the abscission of mature fruits (apple, peach, 

 citrus). In this latter type there appears to be a degeneration of the 

 cellulose materials of the primary cell wall followed in turn by a hy- 

 drolysis of the pectic materials (Sampson, 1918). The middle lamella 

 is apparently not always the locus of the breakdown, but layers of 

 the primary wall immediately adjacent to the middle lamella are 

 places where these changes may also occur and the fracture actually 

 take place. The distinction between these two morphological types of 

 abscission is difficult to draw sometimes. Organs which normally 

 abscise by activity of the meristematic abscission layer can be forced to 

 abscise by means of wall fracture upon exjDOSure to ethylene gas 

 (Gawadi and Avery, 1950). Each of the morphological types of abscis- 

 sion can be prevented by auxin applications. 



The effects of auxin on abscission, like the effects on so many 

 other physiological functions, are of a dual nature. This has been 

 demonstrated in the remarkable experiments of Addicott and Lynch 

 (1951), showing that auxin can either accelerate or retard abscission 

 depending upon the locus of its application. These two opposite effects 

 are demonstrated in figure 51 from which it can be seen that auxin 

 applied to the distal side of an abscission layer retards abscission, 

 whereas auxin applied to the proximal side has the opposite effect. 

 This observation may have a strong bearing on the use of auxins 

 either to thin fruits or to hold fruits on trees, depending upon the 

 time and place of application. 



Several factors are recognized in the auxin control of abscission. 



