220 Auxins in Agriculture 



effect of auxin on the initial growth rate of tomato ovaries has been 

 used as an assay for auxins as described in chapter II. 



The selection of the auxin concentration to be used in practice 

 must take into consideration the injurious effects that the spray may 

 have on the plant and undesirable effects which the auxin may have 

 on the fruit. Injurious effects to the tomato plant begin at 50 mg./l. of 

 PC A if the spray is restricted to the flower cluster, and at much lower 

 concentrations if an over-all plant spray is used (see figure 119). Also 

 in many cases the fruit size and quality may be reduced by concentra- 

 tions of PCA or BNOA of 50 mg./l. or above (cf. figure 96). For 

 these reasons higher concentrations of auxin are avoided in practice 

 and 25 to 50 mg./l. are generally accepted as the concentrations 

 giving the best fruit-set with the fewest undesirable side effects. 



The capacity of auxins to set parthenocarpic fruit in the fig has 

 permitted another commercial use (Crane and Blondeau, 1949). In 

 the Calimyrna fig the auxin application replaces the requirement for 



I- 

 Z 

 < 

 _l 

 Q- 



tr 



UJ 



a. 



\- 



UJ 



en 

 or 



100 MG/L 



CONC. OF AUXIN 



Fig. 93. Quantitative differences in effectiveness for forcing fruit-set of field 

 tomatoes witlr various concentrations of auxin, p-chlorophenoxyacetic acid (data 

 of Mann and Minges, 1949). 



