PREPARATION FOR RESPONSE TO PHOTOPERIOD 99 



during the flashing light treatment. James Liverman and James 

 Bonner at the California Institute of Technology provided supporting 

 evidence for this idea. They were able to make a long dark period 

 preceded by short dark periods effective by treating plants with 

 various sugars such as might be produced in photosynthesis. Anton 

 Lang (55), then at Los Angeles, has also successfully used reducing 

 substances such as ascorbic acid and glutathione. The interpretation 

 of these experiments is not completely straightforward, because the 

 reducing substances might themselves be used as energy sources, or 

 the sugars might be used to produce reducing substances. An 

 experiment will have to be devised to see whether flowering specifically 

 requires reducing substances or just any energy source. It does seem 

 clear that energy is required for operation of the flowering machine, 

 since respiration inhibitors will nullify an otherwise eff'ective dark 

 period. 



Specifically, production of ATP through respiration seems to be 

 essential to eff'ective action of the inductive dark period, since treat- 

 ment with dinitrophenol makes this dark period ineff'ective (see 

 Chapter 9). It has also been shown that photosynthesis is an important 

 part of the flowering process in long-day plants. 



Actually, the experiments described above are not as easy to 

 interpret now as they were a number of years ago. We shall see in 

 Chapter 8 that a brief light flash such as those effective in separating 

 the short dark periods in Hamner's experiment may not reset the 

 timing mechanism of the flowering process, while a longer exposure 

 to high intensity light might so reset the flowering clock. Thus 

 Hamner's original experiment could also be interpreted on the basis 

 of the timing considerations which are discussed in Chapter 8. The 

 short flashes might inhibit flowering to a degree determined only by 

 the timing mechanism, and the long exposure to sunlight might reset 

 this timing mechanism. But how can we relate this to the results 

 with sugar and reducing substances, which seem to explain every- 

 thing on the basis of photosynthesis and respiration ? What relation- 

 ship can sugar and reducing substances have to the timing mechanism ? 

 Some new experiments will have to be performed to straighten out 

 some of these problems. 



In spite of these difficulties, it seems amply clear (based on the 

 respiration inhibitor evidence at least) that the flowering machine 

 must have an energy source for eff'ective operation. 



