240 Auxins in Agriculture 



creased fruit size. This is probably largely a result of fewer fruits 

 remaining on the trees. Luckwill (1953) has studied the relationship 

 between the thinning action and fruit size achieved as a function of 

 the time of application of the spray. His data, given in figure 102, 

 indicate that for the apple variety used, most effective thinning and 

 greatest fruit size were obtained when the spray was applied at petal 

 fall. Later applications appeared to cause rapid decline of the fruit 

 size, until actual decreases in fruit size were obtained. The reduction 

 in effectiveness in the thinning of the fruits was less rapid. 



The thinning of fruits gives increased cold-hardiness of the trees 

 in the succeeding winter season. This benefit from thinning is also ob- 

 tained from auxin thinning (Edgerton and Hoffman, 1952). 



Environmental Factors 



Temperatures exert a strong influence on the shedding of flowers 

 and fruits, both naturally and in response to auxin sprays. Many 

 species of plants shed flowers at high temperatures. Apple trees have 

 been observed to shed more at 70-75° F than at 55-60° F (Lu and 

 Roberts, 1952). Temperatures near freezing also cause excessive drop 

 of young apple fruits. The temperature regime will not only alter the 

 natural drop, and hence the need for auxin sprays, but it will also 

 alter the effectiveness of spray treatment. Burkholder (unpublished) 

 has observed that auxin thinning treatments were much less effective 

 when applied in cold weather. 



EFFECTIVENESS ON OTHER SPECIES 



Besides the work with apples and peaches, some success in fruit 

 thinning has been obtained with pears and olives. Anjou, Hardy and 

 Winter Nelis pears have been successfully thinned with 100 mg./l. 

 solutions of 2,4,5-trichlorophenoxypropionic acid (De Tar et al, 1950; 

 Batjer and Uota, 1951). Some very interesting results have been ob- 

 tained in thinning olives (Hartmann, 1952). If naphthaleneacetic acid 

 is applied to the flowers, too much thinning results, but resistance to 

 the auxin builds up as the young fruits begin growth. Use of 125 

 mg./l. naphthaleneacetic acid with a light oil in the spray gave good 

 thinning after the fruits had grown to 3 or 5 mm. in diameter. 



Some preliminary work has been done on the possible uses of 

 auxins in removal of all fruits from ornamental trees which have ob- 

 noxious fruits (Chadwick et al, 1951). 



CONCLUSION 



Auxin sprays can be effectively used to remove excessive sets of 

 apple and peach fruits after fidl bloom. Fruit trees which do not 



