252 Auxins in Agriculture 



the most effective preparation, having a longer effect than the simple 

 salts or the esters. The spray does not prevent the fall of immature 

 fruits, but curiously limits its effects to the larger mature fruits. 



As with oranges, an increase in fruit size of lemons can be ob- 

 tained if the spray is applied sufficiently far ahead of harvest time. 

 Stewart and Hield (1950) recommend the application of the spray 

 seven months before the harvest to increase fruit size. 



The lemon tree grows in spurts throughout the year and if the 

 auxin spray is applied during a growth spurt, fairly great dieback o! 

 the new growth will occur. If the spray is applied between the growth 

 spurts, when the amount of immature tissues is at a minimum, sucli 

 injury is essentially prevented. From the studies of the effects of auxins 

 on tissues of different ages discussed in chapter V, this is j:)recisely 

 what one woidd expect. 



Other Fruits 



Fruit drop in the grapefruit can be effectively controlled by 2,4-D 

 (Stewart and Parker, 1948). Fruits sprayed in June are held on the 

 tree until December. 



Pre-harvest drop in the Stewart apricot is a serious problem, ap- 

 parently owing to abortion of the embryo at various stages in the 

 growth of the fruit. The embryo is a major source of auxin in ma- 

 turing fruits and the abortion of the embryo would be expected to 

 result in a strong decrease in auxin production within the fruit. Fruit 

 drop due to this curious circumstance in apricot can be effectively 

 suppressed by spraying with 10 mg./l. naphthaleneacetic acid. Hesse 

 and Davey (1942) reported that when untreated controls dropped 25 

 per cent of the fruit, the spray reduced this drop to 8 per cent. It is 

 interesting to note that after the application of naphthaleneacetic acid 

 to apricots ten days is required for a difference in the rate of fruit 

 drop to appear. 



In apricots, the use of 2,4,5-T for control of fruit drop has also 

 been noted (Crane, 1953). The auxin is effective in preventing abscis- 

 sion for the remarkably long period of 50 to 60 days. 



Control of fruit drop in Bartlett pears with naphthaleneacetic 

 acid is a general practice in California. Good control can also be ob- 

 tained with 2,4-D, but with greater chance of injury effects (Batjer 

 et nl, 1948). , 



No stone fruit other than the apricot has been found to respond f 

 to auxin sprays to prevent pre-harvest drop. Peaches and olives have 

 been treated without success. Attempts to prevent dropping of grapes 

 from picked clusters have met with no success. 



