106 • Chemical Control of Flowering 



AUXINS. AUXIN ANTAGONISTS, AND OTHER 

 GROWTH REGULATORS 



Since auxins were widely known long before the gibberellins, 

 there has been more work on their effects on flowering. In addition 

 to auxins, one must consider also the effects of auxin antagonists. 

 This broad term is used here to cover any substances believed to 

 act in a manner opposed to that of auxin. Such action may be 

 exerted through a molecular structure sufficiently similar to that 

 of an auxin to interact with the same biochemical site, yet not 

 sufficiently similar to participate further in whatever system auxin 

 normally acts. Such an auxin antagonist, competitive with auxin 

 molecules, would be a true "antiauxin." Other auxin antagonists 

 may act by interfering with native auxin synthesis, by blocking the 

 transport of auxin from the site of action, or by interfering with 

 the effectiveness of auxin in some other way. Finally, many other 

 organic compounds effective as growth regulators— capable of modi- 

 fying development in various ways— have also been tested on 

 flowering. All of these topics will be considered briefly. None of 

 the results so far has provided much clear information on flower- 

 ing, since most of the evidence suggests that the effects obtained 

 are extremely indirect. 



As noted in the preceding chapter, studies on the changes in 

 native auxin levels associated with flower induction are incon- 

 clusive. In considering the effects of applied auxins, one should 

 bear in mind that these frequently cause all kinds of abnormalities 

 in growth, depending upon the concentrations (see, for example, 

 Audus, 1959; Leopold, 1955). With respect to auxin effects on 

 flowering, comparison of earlier reviews (for example, Lang, 1952; 

 Bonner and Liverman, 1953) with more recent ones such as 

 Leopold's (1958) or the excellent critical article by Lang (1959) 

 indicates a marked decline in the certainty with which any general 

 statement can be made. 



There have been indications that auxin treatment promotes 

 flowering in LDP and inhibits in SDP. The results of some of the 

 papers on this question should illustrate the general uncertainty. 



In experiments by Liverman and Lang (1956) flower initiation 

 in annual Hyoscyamns and Silene was promoted by the auxin 



