146 THE FLOWERING PROCESS 



The results of this experiment seem incompatible with a phyto- 

 chrome, hour-glass kind of clock. Timing cannot be a function of 

 the time required for dark conversion of phytochrome if timing is 

 not influenced even when phytochrome conversion is not allowed to 

 go to completion. 



On the other hand, the results also seem nearly incompatible with 

 any other available scheme. If the flowering process is coupled with 

 the clock by phytochrome conversion, how can we have any timing at 

 all when phytochrome conversion is not allowed to go to completion ? 

 Is it possible that phytochrome has two actions : conversion of only 

 a small amount of pigment couples flowering to the clock (an on-off 

 reaction) but subsequent steps in the process will be quantitatively 

 inhibited (a modulated reaction) unless pigment conversion is 

 complete ? It is at least clear from Figs. 7-5 and 7-6 that in flowering 

 sensitivity to F-phytochrome changes during the dark period. Is it 

 possible that this sensitivity has nothing to do with starting or 

 stopping the timing mechanism ? Could the High Intensity Reaction 

 of Mohr be the controlling factor here? Such questions must be 

 investigated. 



7. Temperature Ejfects upon Timing 



It is well known that biological timing is relatively temperature 

 insensitive in most instances. That this is true in flowering is shown 

 by Fig. 9-5 wliich will be discussed in the next chapter. There is 

 some temperature effect, however. The critical dark period is 

 extended about 20 min by lowering the temperature from 30 to 15°C. 

 Dark conversion of phytochrome, on the other hand, is a metaboli- 

 cally driven, chemical process, quite sensitive to temperature 

 (Fig. 8-2). If the critical dark period consists of dark conversion of 

 phytochrome for up to about 2 hr, after which flowering is then 

 coupled to an endogenous, temperature resistant clock, then one 

 should be able to demonstrate an inhibitory effect of low temperature 

 if it is applied during the first part of the dark period, but not if it is 

 applied a few hours later. 



Results obtained previously by various workers seemed to indicate 

 that this was not so. In some cases at least, a 2-hr period of low 

 temperature had virtually no effect upon flowering regardless of when 

 it was given during a 16-hr dark period (Fig. 9-7 in the next chapter 

 is a good example). We decided to approach the problem in a different 



