384 GROWTH FACTORS AND FLOWERING 



BASIC EXPERIMENTS WITH GROWTH REGULATORS 



The effects of a series of growth regulators upon the flowering of 

 Xanthiiim have been used as effective tools in the study of the partial 

 processes. Three basic kinds of experiments have been done in the 

 first phases of this study. 



1. Prepared cocklebur plants (Sahsbury, 1955, 1957b; Sahsbury 

 and Bonner, 1956) are treated just previous to induction by a single 

 14- (or 16-) hr dark period with a wide concentration range of the 

 growth regulator in question. After 9 days the apical meristems are 

 examined and floral stage is plotted as a function of growth regulator 

 concentration. Vegetative condition of the plants is also noted. This 

 appears to be an effective means of surveying for growth regulators 

 which influence the flowering process, although it is conceivable that 

 compounds which might influence flowering when applied at some time 

 other than just before induction might be ineffective when applied at 

 that time. It is also somewhat difficult to observe promotion of flower- 

 ing by this method, although not impossible. In using this basic experi- 

 ment, the following growth regulators have been found to inhibit 

 flowering (Salisbury, 1957b): 2,4-dichlorophenoxyacetic acid (2,4- 

 D), 2,2-dichloroproprionic acid (Dalapon), maleic hydrazide, indole- 

 acetic acid, naphthaleneacetic acid, 2,4-dinhrophenol, and cobaltous 

 chloride. Gibberellic acid has been found to promote flowering. 



2. In the second kind of experiment, growth regulators are applied 

 to plants at various times previous to, during, and following a single 

 inductive dark period. With this technique, it is possible to determine 

 which phase of the flowering mechanism might be influenced by a 

 particular growth regulator. This is illustrated by Fig. 1, which is a 

 composite of 13 experiments (Salisbury, 1957b) performed at Colo- 

 rado State University. Data are shown as relative floral stage (usually 

 measured 9 days after induction) plotted against the time of appli- 

 cation of a particular chemical. The translocation experiment (defolia- 

 tion) is carried out in conjunction with the time of application experi- 

 ments, and this is plotted against the floral stage of plants at the time 

 the apical buds are examined. The apical buds of some untreated 

 plants are examined at various times after the inductive dark period to 



