122 



THE FLOWERING PROCESS 



8-0 

 7-0 

 6-0 



H 5-Oi- 



4-0-- 



S 3-0 



2-0 

 1-0 



4 Minutes 



il luminotion time 



y^ 



^O-A-^O 



I 64 Minutes 



/ illunnination time 



X 0-24 microjoules/cmysec 



ntensity about 



680 



•Oh— ^F^ 



700 720 740 760 780 800 

 WAVE LENGTH IN MILLIMICRONS 



Figure 7-10 



Action spectra for far-red reversal of red inhibition of flowering in 



Chenopodium rubrum, showing effects of either 4 or 64 min of far-red 



illumination. Data from Hendricks and Borthwick Austrahan 



paper (19). 



If we assume that F-phytochrome is the active, inhibitory form of 

 the pigment while R-phytochrome has no effect, inspection of the 

 absorption curves in Fig. 7-3 helps us understand the results shown 

 in Fig. 7-10. When plants are illuminated at 720 m^t, for example, 

 most of the pigment is converted to R-phytochrome, but R-phyto- 

 chrome itself absorbs some light at 720 m/x, and thus some 

 F-phytochrome is also produced by 720 m/x light. Perhaps 5 to 10% 

 of the pigment is in the F-phytochrome form even when the plant is 

 illuminated with far-red light. If the illumination time is short, this 

 small percentage will be removed by the normal dark conversion in 

 a short time following illumination. If, however, the illumination 

 time is long, the small percentage of F-phytochrome will have time 

 to carry out its inhibitory act, and far-red will be inhibitory rather 

 than promotive. From Fig. 7-10 it would appear that the percentage 

 of F-phytochrome remaining is significantly high to wavelengths at 



