234 Auxins in Agriculture 



are principally effective only until fruit-set has taken place. An advan- 

 tage of auxin as a thinning agent is that there are other physiological 

 means by which auxin may act as well. 



The Abortion of Embryos 



The capacity of auxins to cause the abortion of embryos in young 

 fruits was first demonstrated by Swanson et al (1949) in Tradescatitia 

 fruits. In studies of the action of auxins in thinning apples, Murneek 

 (1952) noted that the abscissed fruits uniformly contained aborted em- 

 bryos, whereas the fruits remaining on the tree contained a large pro- 

 portion of normal embryos. The tissue showing the most conspicucjus 

 deterioration in the abscissed fruits was the endosperm. Miuneek (1952) 

 proposed that the thinning action was a consequence of the ai)orti()n of 

 the endosperm or perhaps of the inhibition of normal embyro growth 

 (Murneek and Teubner, 1953). The endosperm tissue is the principal 

 source of auxin to the young apple fruit (Luckwill, 1948), and deprived 

 of its auxin supply, the fruit would be expected to abscise. Comparison 

 of the frequency of aborted embryos and the shedding of young apple 

 fruits in figure 99 indicates that a large wave of seed abortion occurs 

 three weeks after auxin application at petal-fall, and this event is then 

 followed one week later by a wave of fruit drop. 



Direct Abscission Control 



The two thinning mechanisms described above are each probably 

 effective through interference with the auxin production in the flower 

 or fruit. It is well known that abscission can be obtained either by 

 lowering of the auxin supply distal to the abscission zone or by supple- 

 menting the auxin stq^ply proximal to the abscission zone, as shown in 

 figure 51 (page 112). A considerable body of evidence indicates that a 

 third thinning effect of auxin involves supplementing of the proximal 

 auxin supply. 



In studies of the thinning of olive fruits, Hartmann (1952) has 

 found that the application of auxin to the fruits was ineffective in 

 thinning, whereas the application of auxin to the foliage of the tree 

 resulted in the shedding of the fruits. Application to the foliage alone 

 was actually more effective than an overall spray which covered both 

 foliage and fruits. If the auxins were acting exclusively upon the 

 flower or fruit one would certainly expect that direct application onto 

 that organ would be more effective than applications to more distant 

 parts. It is suggestive, then, that auxin applications may thin olive 

 fruits by so altering the auxin gradient across the abscission zone as 

 to force abscission. 



