248 Auxins in Agriculture 



are unable to harvest their fruit at the ideal stage of ripeness, and 

 where delays are necessary an auxin spray finds its usefulness to 

 hold the fruit on the tree. In some crops it is desirable to delay 

 harvest in order to await a more profitable market. This is true in the 

 case of lemons which are in greatest demand in the summer months. 

 The fruits ripening in spring can be held on the tree with auxins 

 until the more favorable marketing season has arrived. 



Effectiveness of the Auxin Application 



Fortunately, one of the commonest environmental conditions lead- 

 ing to severe fruit drop is apparently also a factor in increasing the 

 effectiveness of pre-harvest drop sprays. That is high temperature, 

 which not only increases fruit drop, but likewise may increase the 

 effectiveness of a spray (Batjer, 1942). The experiments on this point 

 are not entirely conclusive, however, for sprays applied at different 

 times of day were used for the various temperature conditions. How- 

 ever, beneficial effects of high temperature have been confirmed in 

 several other instances (Davidson, 1940; Overholser et al, 1943; Bar- 

 low, 1952). In autumn seasons with excessively high temperatures as- 

 sociated with drought conditions, auxins may be entirely ineffective in 

 preventing abscission (Klackle, unpublished). The occurrence of low 

 temperatures after application of auxin sprays may increase the dura- 

 tion of the effect in controlling pre-harvest drop. 



Several physiological factors will likewise help to determine the 

 effectiveness of a spray. It has already been mentioned that the extent 

 of development of abscission processes at the time of spray application 

 can make a late spray almost ineffective. The several factors listed as 

 encouraging the natural drop of apples are all factors in the usefulness 

 of a given spray. Each of them can be interpreted as either altering 

 the carbohydrate or the auxin levels in the pedicel consequently 

 affecting fruit drop. If the auxin and/or the carbohydrate level of the 

 fruit pedicels drops too low to prevent abscission naturally, auxin 

 sprays will be effective. However, in instances where the carbohydrate 

 level drops excessively low as in the case of weak or dying trees, the 

 auxin sprays will not prevent abscission of the fruits at all (Thomp- 

 son, 1951; Griggs et al, 1951). It appears also that wormy or otherwise 

 moribund fruits will not be retained on the tree by auxins (Erickson, 

 1951). 



Varietal differences have been reported in several instances. It ap- 

 pears that fruit drop is more readily controlled in early than in late 

 apple varieties (Vyvyan, 1946). 



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