PHOTOPERIODIC CONTROL OF FLOWERING 



277 



lopoo 



BfiOO. 

 o 



8 





2pOC 



300 



S200_ 



4000 4400 4800 5200 5600 

 W»VE LENGTH IN ANGSTROM UNITS 



5000 5400 5800 6200 G600 7000 

 WAVE LENGTH IN ANGSTROM UNITS 



Fig. I. Composite action spectra for Wintex barley, cocklebur, and soy- 

 bean. Left, in "blue-violet" region of spectrum; right, in "red" region of 

 spectrum. Action spectra for short-day plants, cocklebur and soybean, give 

 energy required to suppress floral initiation when applied as a dark- 

 period interruption. Action spectrum for Wintex barley gives energy re- 

 quired to initiate spike development and stem elongation when applied at 

 middle of a 12.5-hr dark period (Borthwick, Hendricks, and Parker, 

 1948). 



activity of these two opposed reactions results in a very rapid change 

 of response with wavelength in a narrow spectral region near 7000 A. 

 This wavelength region of rapid change is identical for the various 

 kinds of plant tested, the apparent differences in the curves not exceed- 

 ing the errors inherent in measurement of the response. 



The principal differences in the action spectra of the four plants 

 listed occur at the blue end of the spectrum. Response to radiant 

 energy in this region was found in soybean and cocklebur, but in 

 barley it was very low, and in henbane it was not clearly detected. 

 Responses of soybean, cocklebur, and barley were at a minimum 

 between 4400 and 4800 A, and they increased again at shorter wave- 



