LIGHT AND THE PIGMENT 



103 



adjustment in time and intensity to obtain the 50 % flowering level. 

 Of course various combinations were tried in each experiment, but 

 the work had to be repeated many times before the action spectra 

 shown in Fig. 7-2 could be obtained, in which the quantity of light 

 required to inhibit flowering 50% is plotted as a function of the 

 wavelength. 



If such curves are inverted so that the least required energy is at 

 the top, as in Fig. 7-3, they are comparable to the absorption curve 

 of a pigment in which the amount of light absorbed is greatest at the 

 top (also shown for the photoperiodism pigment in Fig. 7-3). That 

 is, the wavelength which is effective at lowest intensities must be the 

 wavelength which is absorbed most by the pigment. 



560 580 600 620 640 660 680 700 

 WAVE LENGTH IN MILLIMICRONS 



700 720 740 760 780 800 

 WAVE LENGTH IN MILLIMICRONS 



Figure 7-2 

 Action spectra expressed as light energy required for half effect in a 

 number of systems. Red effects are for 50% promotion of germination 

 of lettuce seeds, 50 % inhibition of flowering in cocklebur, 50 % enhance- 

 ment of dark grown pea leaf elongation, and 50% promotion of 

 flowering in barley. The differences in the peaks are probably accounted 

 for by chlorophyll screening. The far-red reversal curve is a generalized 

 one for a number of systems. Original data are available in many papers 

 by the Beltsville workers (see, for example, 8, 10, 11, 18 and 19). 



