408 



RADIATION BIOLOGY 



mination then causes destruction of the chlorophyll. These facts are 



pictured in Fig. 7-8 (Smith, 1949a). 



When leaves containing various 

 amounts of protochlorophyll were illu- 

 minated at low temperatures, they pro- 

 duced chlorophyll in direct proportion to 

 the protochlorophyll they had contained. 

 This is demonstrated in Fig. 7-9. 



The transformation of protochlorophyll 

 to chlorophyll a is quantitative. During 

 illumination the protochlorophyll de- 

 creases and the chlorophyll a increases in 

 such a way that the sum of the two re- 

 mains constant even up to as high as 80 

 per cent conversion (see Table 7-4) 

 (Koski et al, 1951 ; Koski, 1950). These 

 results bring cogent evidence that chlo- 

 rophyll a is formed from protochloro- 

 phyll, molecule for molecule. 



Dependence of Chlorophyll Formation 

 on Wave Length of Light. There is gen- 

 eral agreement among all investigators 

 that the infrared radiation is ineffective 



in chlorophyll formation (Wiesner, 1877; Sayre, 1928; Guillemin, 1857; 



Reinke, 1893). The longest wave length effective is somewhat in doubt, 



but Sayre places this at 680 m/x, and Wiesner at less than 716 m^. The 



Table 7-4. The Constancy of Total Pigment during Transformation 



OF Protochlorophyll to Chlorophyll a in Dark-grown Corn Leaves 



after Irradiation for Various Periods of Time 



7 9 II 13 



PROTOCHLOROPHYLL, gXIO^g dry wt. 



Fig. 7-9. The relation of the quan- 

 tity of chlorophyll formed in dark- 

 grown barley seedlings by illumi- 

 nation for 2 hr at low temperatures 

 to the quantity of protochlorophyll 

 initially present. Temperatures: 

 0°C; 4.5°C; below 6 or 7°C (ice 

 bath). {Smith, 1948.) 



long-wave-length limit is in the neighborhood of 700 m/z, perhaps little 

 longer than 680 m.fx. 



All investigators agree that the visible spectrum is effective in the for- 



