Flowering 259 



inhibitory effects may be especially pronounced. In fact flower initia- 

 tion may be prevented in many species, including species sensitive to 

 photoperiodism or to vernalization and indeterminate species (Obsil, 

 1939; Leopold and Thimann, 1949; Bonner and Thurlow, 1949; 

 Leopold and Guernsey, 1953). In addition, some compounds which 

 antagonize auxin can have the reverse efl:ect, and quantitatively in- 

 crease flowering in many of the same species (Zimmerman and Hitch- 

 cock, 1942; Galston, 1947), or relieve the inhibitory effects of applied 

 auxins (Bonner and Thurlow, 1949). 



The quantitative effects of auxins on flower initiation are not 

 always inhibitory, for, as already mentioned, auxin applications can 

 promote initiation of some species as well. These promotive effects 

 appear to be most readily obtained with species in which flower ini- 

 tiation is photoperiodically induced by long days (Leopold and Thi- 

 mann, 1949; Claes, 1952; Liverman and Lang, 1953), although short- 

 day and indeterminate species may also be promoted by auxin after 

 experiencing conditions of low temperatures (Leopold and Guernsey, 

 1953). At relatively high concentrations, auxins have been found to 

 inhibit flowering of all species tested. 



The comparative effects of auxin applications to the foliage of a 

 species which is promoted and a species which is only inhibited are 

 shown in figure 106. 



The effects upon flowering of agents which antagonize auxin have 

 been studied in some detail, indicating that different agents of this 

 category may have very divergent effects. TIBA (2,3,5-triiodobenzoic 

 acid, an auxin synergist) was the first such compound found to alter 

 flowering (Zimmerman and Hitchcock, 1942). Increased flowering has 

 been obtained with this compound in each of the different photo- 

 periodic classes of plants. Both TIBA and DCA (2,4-dichloroanisole, 

 an anti-auxin) have been found to increase flower development in 

 cocklebur, and in fact the latter compound can actually overcome the 

 inhibitory effects of small quantities of auxin applied during photo- 

 induction of cocklebur (Bonner and Thurlow, 1949; Bonner, 1949). 

 Maleic hydrazide is another compound which can generally alter 

 flowering, but in contrast to TIBA and DCA this material generally 

 inhibits flowering. Since the first report of flowering inhibition by 

 maleic hydrazide (Naylor, 1950), it has been found to inhibit flower- 

 ing in each of the photoperiodic classes. It has been suggested that 

 the inhibiting effect of maleic hydrazide on flowering may be a result 

 of growth inhibition rather than a direct effect upon the flowering 

 stimulus (Klein and Leopold, 1953). 



Although the properties of auxins and auxin antagonists in 



