Chemical Nature of Growth Regulators 173 



to have some effects on flowering (Galston, 1947; Bonner, 1949) which 

 may be interpreted as possible reductions in auxin effectiveness. 



Coiunarin (XXIX) and other unsaturated lactones have long 

 been known to inhibit growth and germination. The synergistic effects 

 with auxin were discussed in tlie previous section. The inhibition of 

 growth is thought to take place through interference with some step 

 closely allied with auxin action (Thimann and Bonner 1949), and 

 the inhibition was effectively vitiated by a sulfhydryl protecting agent, 

 as mentioned above. The concept of coumarin interfering with auxin 

 action through a sulfhydryl effect has been criticized by Mayer and 

 Evanari (1951; 1952) who found that coumarin increased the inhibi- 

 tion of germination by auxins. 



XXIX XXX XXXI 



Other lactones may have very diverse effects on auxin action. 

 Scopoletin (XXX) inhibits the destruction of auxin by indoleacetic 

 acid oxidase (Andreae, 1952). Kinetic analysis of the interaction by 

 the Lineweaver and Burk (I9'^4) method indicates that the lactone 

 may be a competitive inhibitor against the auxin, an observation 

 which suggests that if the auxin and the lactone have so much in com- 

 mon in the one enzymatic reaction one might almost expect a similar 

 interaction with the auxin receptor itself. It is curious that competi- 

 tive inhibition has not been found in the growth interactions. An- 

 other lactone, umbelliferone (XXXI), has been observed to do quite 

 the opposite of scopoletin, and actually increases the enzymatic de- 

 struction of indoleacetic acid (Andreae and Andreae, 1953). 



Besides the lactones, a wide variety of other naturally occurring 

 auxin inhibitors has been found, including those compounds dis- 

 cussed above as auxin synergists plus a host of unidentified inhibitors. 

 These compounds may play key roles in such phenomena as dormancy, 

 seed germination and even the ecological relationships controlling 

 plant distribution (see review of Bonner, 1950). 



The general subject of anti-auxins is a very interesting one and 

 one which can be expected to find many applications in the near fu- 

 ture. The possibility of applying an anti-auxin to lower the effective 

 auxin level in a plant is now a reality. Anti-auxins may find important 

 uses in the modification of any of the many auxin functions in plants 



