the synthesis of flowering hormone 173 



Chemical Substances Which Influence 

 Flowering12 



Effective compounds may be grouped according to the time 

 during the inductive dark period when they are most effective in the 

 inhibition of flowering (we have found a few compounds which 

 promote flowering of cocklebur slightly, but most effective ones are 

 inhibitory). 



1. Compounds Affecting Time Measurement 



We have already discussed in previous chapters the effect of the 

 cobaltous ion on flowering. As can be seen in Fig. 9-11, it inhibits 

 flowering only when it is applied during the early hours of the dark 

 period before the critical night. This is the time-measuring phase of 

 the flowering process, and our previous discussions (Chapters 7 

 and 8) indicated that cobaltous ion inhibits time measurement. 



2. Compounds Affecting Flowering Hormone Synthesis 



Table 9-1 lists a number of compounds which are effective when 

 they are applied during the inductive dark period and which are 

 ineffective when they are applied after the end of the inductive dark 

 period. Most of these compounds become progressively less effective 

 as the end of the critical dark period is approached. This is what 

 we would expect if they were inhibiting flowering hormone synthesis. 

 Compounds that act this way may be further subdivided into at 

 least three groups. 



A. Respiration inhibitors. Japanese scientists, using the Japanese 

 morning glory, have found a number of known respiration in- 

 hibitors which inhibit during the last part of the dark period. 

 I have shown that 2,4-dinitrophenol acts in the same way. Most 

 of the respiratory reactions continue in the presence of dinitro- 

 phenol, but no ATP is formed. Thus flowering hormone synthesis 

 must require the energy of respiration in the form of ATP — 

 which is what one might expect. 

 In Chapter 6 we reviewed the evidence indicating that energy 



^2 Effects on flowering of some of the compounds discussed in this section have 

 not been described in previous publications. I am indebted to my colleague, 

 Dr. Cleon Ross, and my former graduate student, Dr. Walter Collins, for much 

 of this work. 



