TIMING AND THE FLOWERING PROCESS 137 



sensitive to temperature. Of course, there are temperature effects in 

 the flowering process, as we discovered in Chapters 2 and 4. The 

 day-length response type itself may be a function of temperature, 

 but the critical dark period is influenced only very slightly compared 

 to what one might expect on the basis of chemical reactions. A 

 typical chemical reaction requiring 8 hr at 25°C might require 16 hr 

 at 15°C, but the critical night for cocklebur changed in one experi- 

 ment only from 8 hr 48 min at 25°C to 9 hr 4 min at 15°C (see 

 Fig. 9-5 in Chapter 9). 



Yet temperature independence is not completely incompatible 

 with the hour glass idea of timing. One simply has to postulate 

 temperature compensating reactions (see below). Although the 

 mechanisms become complicated, they are probably no more so than 

 Bunning's theory. It is interesting to note that circadian leaf move- 

 ments appear to become adjusted to temperature changes. Time 

 may be measured inaccurately when the plant is first placed in the 

 new temperature (the period is shortened or extended), but after a 

 few days everything is back on schedule, which seems to indicate 

 some sort of compensating system. 



2. The Dark Conversion of Phytochrome as the Timing Mechanism 

 Sterling Hendricks and other workers at Beltsville (1, 10, 19) have 

 suggested that the time of conversion of F-phytochrome to R-phyto- 

 chrome in the dark might account for timing in flowering. The 

 half-time for conversion of F-phytochrome to R-phytochrome in 

 many processes is in the neighbourhood of 30 min to 2 hr, as 

 illustrated in Fig. 8-2 for corn seedlings, which shows a decrease in 

 total phytochrome as well as F-phytochrome. With a 2-hr half-life 

 about 8 to 12 hr would be required to decrease the level of F-phyto- 

 chrome to 1.5 to 6% of its original concentration. If we assume that 

 the critical dark period length is determined by the time it takes for 

 F-phytochrome to decrease to some such level, then we have 

 essentially solved the problem of timing in the flowering process. 

 Perhaps anything above 1.5 to 6% of F-phytochrome is inhibitory 

 to the chemical reactions which occur in flowering (e.g. synthesis of 

 flowering hormone). When the phytochrome has reached this level, 

 then the other processes may be initiated. 



The Beltsville workers suggest that some degree of temperature 

 compensation could be achieved in such a system. Rate of dark 



