112 



Fundamentals of Auxin Action 



The first of these is the actual auxin content of the organ which may 

 be abscissed, particularly with respect to the gradient of auxin levels 

 between the organ and the stem to which it is attached. The impor- 

 tance of the auxin gradient was established by Shoji et al (1951). They 

 found that abscission of leaves occurred when auxin production be- 

 came low and the gradient of auxin across the abscission layer became 

 small. This gradient is a function of the rate of auxin production in 

 the organ and the auxin level in the stem. Either of these can be 

 altered by the addition of auxins or materials modifying auxin action. 



100 



5 DAYS 



TIME AFTER TREATMENT 



Fig. 51. The dual effects of auxin on abscission of excised bean leaf petioles. The 

 addition of auxin (105 mg./l. of indoleacetic acid) to the distal end of the petiole 

 prevents abscission, whereas addition to the proximal end forces more rapid abscis- 

 sion (Addicott and Lynch, 1951). 



A general demonstration of the role of auxin production in the 

 prevention of leaf abscission was made by Myers (1940). His measure- 

 ments of auxin levels revealed that coleus leaves produced relatively 

 small amounts of auxin in the winter season and much larger amounts 

 in the spring. Applications of auxins to prevent abscission were much 

 more effective in the winter during the period of relatively low auxin 

 production. Many workers have reported that removal of leaf blades 

 caused subsequent petiole abscission. The ability of auxins to substi- 

 tute for the leaf blade in preventing that abscission has clearly dem- 



