416 



RADIATION BIOLOGY 



chemical process is transformed to chlorophyll by photochemical 

 action. 



Since the immediate precursor of chlorophyll is protochlorophyll, it is 

 of interest to determine how protochlorophyll behaves in etiolated seed- 

 lings under intermittent illumination. 



The amount of chlorophyll formed from the protochlorophyll initially 

 contained in dark-grown leaves is too small to impart a green color to 

 the leaves. In order for protochlorophyll to be the precursor of chloro- 

 phyll in the over-all process of greening, it must be continually formed 

 and transformed. Protochlorophyll behaves in this manner (Scharfnagel, 



1931; Lubimenko and Hubbenet, 



7 - 



5 - 



- 4 



• CONTINUOUS, I7.I°C 

 O INTERMITTENT, I7,4''C 





1932). 



The rate of re-formation of proto- 

 chlorophyll has been measured by 

 Scharfnagel (1931), by Rudolph' 

 (1934), and by the authors. Scharf- 

 nagel irradiated etiolated corn seed- 

 lings for 10 min to transform the 

 protochlorophyll to chlorophyll and 

 then placed the plants in darkness. 

 After 1, 2, and 3 hr he extracted the 

 seedlings and found no re-formation 

 of protochlorophyll. After 10 hr 

 protochlorophyll was barely percep- 

 tible, and after 20 hr it was present 

 in significant amounts. By alter- 

 nating periods of light and darkness 

 he was able to demonstrate the 

 disappearance of protochlorophyll 

 in the light and appearance in the 

 dark. 



Experiments in the authors' labo- 

 ratory have also demonstrated the 

 rate of regeneration of protochlorophyll in the dark for normal and albino 

 corn seedlings. The results are presented in Table 7-5. 



The accumulation of protochlorophyll reaches a limiting value in seed- 

 lings that have been grown in the dark and also in dark-grown seedlings 

 that have been briefly illuminated and returned to the dark (Lubimenko 

 and Hubbenet, 1932). 



Polarized Light. Tessier (1783; see Guillemin, 1857) found that "la 

 matiere verte" is formed in red, yellow, blue, and violet Ught that has 

 been polarized by means of a Nicol prism. 



Photochemical Formation of Additional Organic Magnesium Compounds. 

 When etiolated barley leaves are illuminated at 0°C, their protochloro- 



40 60 80 100 



ELAPSED TIME, hr 



Fig. 7-16. The accumulation of chloro- 

 phylls a and b by barley seedlings in 

 continuous and intermittent illumination. 



