410 



RADIATION BIOLOGY 



• CHLOROPHYLL 



FORMATION 

 O LIGHT ABSORPTION 



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monochromatic light— band width 5 m^— obtained by means of a large- 

 aperture grating monochromator. The light source was a high-pressure 

 mercury lamp. Illumination periods of 20-360 sec were used. The hght 

 intensity was measured with a thermopile, and the total energy (quanta) 

 incident on the leaves calculated. The extent of the pigment transfor- 

 mation was determined from spectroscopic measurements. The recipro- 

 cal of the number of quanta neces- 

 sary to produce 20 per cent 

 transformation was used as a meas- 

 ure of the effectiveness at each wave 

 length. The curves of effective- 

 ness vs. wave length are given in 

 Fig. 7-11 and compared with the 

 absorption of protochlorophyll in 

 methanol. 



For the normal sib the maxima 

 of effectiveness are at 650, 445, 593, 

 and 550 m^u, and the relative effi- 

 ciencies are in this order. How- 

 ever, for the albino white seedling-3, 

 the order is 445, 650, 593, and 550 

 m/i. The ratio of effectiveness for 

 the blue and red peaks is 0.66 for 

 the normal and 1.89 for the albino. 

 The lower effectiveness of blue light 

 for the normal sibs may be ascribed 

 to the screening effect of the carote- 

 noid pigments, which are abundant 

 in the normal seedlings but lacking 

 in the albino. It is difficult to un- 

 derstand Frank's results for the 

 high effectiveness of blue light rela- 

 tive to red in normal oat seedlings, 

 because it is evident from Fig. 7-1 

 that her seedlings contained an abundance of carotenoid pigments, which 

 absorb strongly in the blue. Frank attributed this anomaly to a particu- 

 lar geometrical arrangement of the pigments in the leaf. [For a discus- 

 sion of this point see Frank (1946), Koski e^ aZ. (1951), Aronoff (1950).] 



A comparison of the action-spectrum curves with the absorption- 

 spectrum curve (Fig. 7-11) shows that the shapes of the curves differ 

 but that the positions of the maxima are in agreement when allowance 

 is made for the spectral shift between solvent extracts and holochrome. 

 On a frequency basis the shift for methanol is 151.8 X 10" frequency 

 units per second and for ether, 199.3 X lO^^ (Koski et al., 1951). 



400 



7O0 



500 600 

 WAVE LENGTH, m/j. 



Fig. 7-10. The relative-effectiveness curve 

 for chlorophyll formation {Frank, 1946) 

 compared with the relative-light-absorp- 

 tion curve of protochlorophyll dissolved 

 in ether. The relative-light-absorption 

 curve of protochlorophyll has been ad- 

 justed so that the height of the chief ab- 

 sorption band equals the height of the 

 corresponding band in the relative- 

 effectiveness curve (Smith, 1948). 



