420 



RADIATION BIOLOGY 



mation in etiolated corn seedlings illuminated at — 6°C. But in none 

 of these experiments was the rate of formation determined at different 

 temperatures and at different light intensities. This has been done by 

 Koski and Smith (1948-1949; cf. Koski, 1949). The results (Fig. 7-14) 

 show that the rates are nearly identical at 5° and 18°C and at hght intensi- 

 ties of 30, 120, and 250 ft-c. 



Etiolated leaves that have had their original protochlorophyll trans- 

 formed to chlorophyll re-form protochlorophyll when returned to the 



240 



16 20 24 28 

 TEMPERATURE . 



Fig. 7-19. Curves showing the total production of chlorophyll at different tempera- 

 tures after 2, 8, 16, 24, 48, and 72 hr of exposure of wheat (Triticum ferrugineum) 

 seedlings to hght. (Lubimenko and Hubbenet, 1932.) 



dark at 19°, but not at 0°C (Smith and Koski, 1947-1948). They do 

 not transform protochlorophyll to chlorophyll after they have been heated 

 to a temperature of 90°-95°C (Koski and Smith, 1948). 



5. INFLUENCE OF THE AMBIENT ATMOSPHERE ON THE FORMATION 

 AND ACCUMULATION OF CHLOROPHYLL 



Oxygen. Certainly in the higher plants oxygen is necessary for green- 

 ing. Boehm (1865) determined that, in atmospheres of pure nitrogen, 

 hydrogen, or carbon dioxide, etiolated plants did not green. They 

 greened only when oxygen was present. Correns (1892), Friedel (1902), 

 and Trebitz (1905) have all demonstrated that greening is regulated by 

 the partial pressure of the oxygen surrounding the plant and that there 

 is a threshold oxygen pressure below which they will not green. Indi- 

 viduals of the same species vary in this regard (Correns, 1892; Liro, 1908). 



