32 • Photoperiodism: Attempts at Analysis 



wavelengths 4000-4500 A (Angstrom units), and red, 6200-6800 

 A— is also the light most strongly absorbed by chlorophyll solutions. 

 That is, the action spectrum for photosynthesis resembles the ab- 

 sorption spectrum of chlorophyll solutions. 



In principle, this seems simple enough; in fact, the accurate 

 determination and evaluation of absorption and action spectra is a 

 complex, still-developing branch of physics and chemistry, as well 

 as biology; for some references, see articles in Hollaender (1956) 

 and Withrow (1959). For present purposes, however, it should be 

 evident that the action spectra for light-break elfects in various 

 plants might indicate whether or not these effects are mediated by 

 the same pigment and what that pigment might be. 



Much of the work on this question has been done by Garner 

 and Allard's successors, a group at the U.S. Department of Agricul- 

 ture, Beltsville, Maryland, and many reviews by the original 

 workers are in the literature (see, for example, Borthwick, Hen- 

 dricks, and Parker, 1956; Borthwick, 1959; Hendricks, 1958, 1959). 

 Their procedures are basically simple, though not technically easy. 

 Stating the situation more quantitatively than before, an action 

 spectrum can be represented either as a graph of varying responses 

 brought about by equal energies of light of given wavelengths, or 

 as a graph of the energy which must be given at each wavelength 

 to cause a particular degree of response. Thus it is necessary to 

 measure the effect of each wavelength chosen at several energv 

 levels, and on a considerable number of plants; this requires light 

 of considerable intensities but in relatively pure wavelength bands 

 spread out over considerable areas. For this purpose, the Beltsville 

 group built a large spectrograph, in which high-intensity white 

 light could be passed through a prism and projected as a spectrum. 

 They then took advantage of the fact that in the plants chosen 

 photoperiodic treatments need only be given to a single leaf if 

 the other leaves were removed. The single leaf could be placed so 

 as to receive light of a particular color and energy at the optimal 

 time for dark period interruptions; after main such experiments, 

 the relative effectiveness of the various colors can be calculated. 

 (See Figs. 3-1 and 3-2.) 



From 1946 on, action spectra for light-break responses were 

 obtained in both SDP and LDP, including Xanthium, Biloxi soy- 

 bean, Hyoscya/nu.s. and Wintex barley. All these spectra seem 



