138 



THE FLOWERING PROCESS 



5-0 



4-On 



3-0 



2-0 - 



1-0 



Maize seedling (in vivo) 



O 



Total phytochrome : 3°C 



Difference curv^ 



R- phytochrome _^>^r- — ' 



^^- ^ ""^F-phytochrome (1/2 time) 

 ^^^ ""^^^decoy curve 



12 3 4 5 



HOURS IN DARKNESS AFTER RED IRRADIATION 



Figure 8-2 

 In vivo measurement of total phytochrome and of F-phytochrome in 

 maize shoots, showing changes with time following a saturating illu- 

 mination with red light. At 3°C, only total phytochrome is shown. 

 The F-phytochrome curve is a calculated half time decay curve, chosen 

 to approximate the experimental points. Data from Hendricks and 

 Borthwick (19). 



conversion of F-phytochrome is decreased at lower temperatures 

 (Fig. 8-2), but then it is less effective at lower temperatures, com- 

 pensating in some measure for the longer time that it is present. At 

 extreme temperatures such a system would be expected to fail, and 

 time measurement does fail at very low or very high temperatures. 



As noted in the last chapter, extracted F-phytochrome is not 

 converted in the test tube in the dark to R-phytochrome. Apparently 

 this conversion requires the presence of the enzymatic systems 

 normally available in the plant cells. Thus the actual decay time for 

 F-phytochrome would be determined by the enzyme systems of the 

 plant under discussion. 



An impressive evidence in favor of the idea that timing in general 

 is a matter of F-phytochrome decay times, is the observation that the 

 clock which controls leaf movement seems to be controlled by 

 phytochrome. The leaf movement cycle is initiated when the plants 

 first are illuminated with light following a dark period. Lars Lorcher 

 (61), one of Bunning's students, showed that one hour of red light 

 initiates the cycle in beans, and the effect can be reversed by far-red. 



