Developmental Effects of Auxins 1 1 3 



onstrated the role of auxin (LaRue, 1936). If, instead of removing 

 the leaf, one applies substances which cause the death of the leaf 

 tissue, abscission will also result. Livingston (1950) has shown clearly 

 that commercial products designed to force foliar abscission do not 

 themselves promote abscission, but essentially kill the leaf tissue. The 

 effective auxin level of the leaf can be likewise modified by the addi- 

 tion of anti-auxins or auxin competitors. Hall et al (1953) reported 

 that the anti-auxin trans-cinnamic acid can accelerate leaf abscission, 

 and Weintraub et al (1952) have shown that TIBA (2,3,5-tri-iodo- 

 benzoic acid) is remarkably effective in causing stem abscission. The 

 action of TIBA in this regard may be a result either of its competitive 

 inhibition of auxin action, or more likely of its disruption of the 

 polarity of auxin movement, a property demonstrated by Niedergang 

 and Skoog (1952). 



An interesting possible role of auxin in causing abscission is in 

 the dropping of the corolla and stamens of flowers at the time of 

 fruit-set. The abscission of these flower parts normally occurs promptly 

 after fruit-set, and of course fruit-set is associated with a high auxin 

 content of the ovary. With the advent of high auxin levels, abscission 

 of the floral parts distal to the auxin supply would be predicted on 

 the basis of the data in figiue 51. It may be worthy to note in this 

 connection that the spraying of Horal parts of tomatoes with auxins 

 to force fruit-set sometimes prevents abscission of floral parts aftei 

 fruit-set. In these cases the spray apparently brings about an auxin 

 level sufficiently high in the corolla and stamens to prevent their 

 abscission for a week or so. Subsequently these persistent floral parts 

 are ruptured and finally forced off by the enlarging young fruit. 



The abscission of cotton bolls correlates with the accumulation of 

 some auxin antagonist in the bolls (Carne et al, 1954). The antagonist 

 apparently reduces the effectiveness of the auxin supply in the boll, 

 and abscission results. 



An actual destruction of auxin has been indicated as a cause of 

 abscission in leaves infected with the fungus, Omphalia flavida. This 

 parasite is responsible for large losses in coffee production by causing 

 extensive leaf abscission, and an enzyme produced by the parasite has 

 been found which destroys indoleacetic acid (Sequeira and Steeves, 

 1954). Abscission resulting from this pathological situation is thus 

 apparently a consequence of auxin phenomena. 



The important role that auxins play in the developmental func- 

 tions of fruit-set, the control of abscission and the control of fruit size, 

 has led to many uses of growth regulators in commercial fruit pro- 

 duction. These will be discussed in chapters XI to XVII. 



