PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



223 



trum for germination of Grand Rapids lettuce 

 seeds. Relatively low intensities of red light 

 were most effective in promoting germination, but 

 seeds exposed to far-red light germinated even 

 less than dark controls. In 1952, Borthwick. 

 Hendricks, Parker, Toole, and Toole (6) tested 

 to see if exposure to far-red light would over- 

 come previous exposure to red. It did. When 

 lettuce seeds that had properly imbibed water 

 were illuminated by an alternating series of red 

 and far-red light, they germinated when the last 

 exposure was red and failed to germinate when 

 the last exposure was far-red. Action spectra 

 (fig. 3) for many photomorphogenetic processes 

 were similar to those for promotion of germina- 

 tion by red light, so it seemed reasonable to test 

 these other phenomena for photoreversibility. In 

 a large number of cases, far-red light did over- 

 come the effects of red light. Apparently ex- 

 posure of some pigment to red light converted it 

 to a form that most efficiently absorbed far-red 

 light, and conversions might occur metabolically 

 in the dark : 



Orange-red 

 (660 nm) 



Pr 



Pfr 



(730 nm) 

 Far-red 

 Synthesis Metabolic conversion Destruction 

 The feature of photoreversibility at once sug- 

 gested an approach to isolation of this pigment 

 but at the same time posed a difficult problem 

 {10). The idea was to search for a pigment sys- 

 tem that was converted from one form to another 

 by exposure to red or far-red light, but deter- 

 mination of red light absorption required ex- 

 posure to red light, and this converted the pig- 

 ment to the form that most effectively absorbs far- 

 red light. The problem was solved in 1950 by 

 construction of a special spectograph in which 

 beams of red and far-red light were alternated 

 16 times per second, and differences in absorp- 

 tion were measured. The pigment system was 

 named photochrome. Its detection (12) in vivo 

 (dark-grown corn coleoptiles) was followed by its 

 detection in vitro (ground-up coleoptiles, cauli- 

 flower florets, and so forth), and finally its puri- 

 fication and virtual isolation. The pigment proves 

 to be a protein with an allophycocyanin chromo- 

 phore and a molecular weight estimated at about 



300 400 500 600 



WAVELENGTH (nm) 



700 



800 



Figure 3. — Absorption spectra of both forms of photo- 

 chrome (13), and action spectra for several physiologi- 

 cal responses (26, Jil, 56). 



60,000 grams per mole. The phytochrome system 

 has been studied now in many plant responses, 

 including germination, etiolation, phototaxis. 

 opening and closing of leaflets, entrainment (this 

 is in doubt), pigment formation, plant form, sun 

 and shade leaves, flower formation, and winter 

 dormancy. 



5. The High Energy Reaction (36, 37).— It 

 was noticed that several photomorphogenetic 

 events were elicited by illumination with high 

 light intensities applied for extended times, as 



