PHOTOKEACTIVATION 483 



11. PHOTOKEACTIVATION IN HIGHER PLANTS 



Bawden and Kleczowski (1952) found that the first-formed leaves of 

 Phaseolus vulgaris, if isolated from the plant and kept in darkness after 

 exposure to ultraviolet light, acciuired a bronze color owing to death of the 

 cells of the upper epidermis. If, following ultraviolet treatment, the 

 leaves were illuminated with daylight, the bronze color was not produced 

 and the majority of the cells appeared intact. 



12. CONCLUSIONS AND SUMMARY 



1. Photoreactivation is a very widespread phenomenon, affecting 

 changes of seemingly different nature produced by ultraviolet in organisms 

 of different levels of organization. The fact that most of the known bio- 

 logical effects of ultraviolet irradiation have been photoreactivated seems 

 to suggest that they all have a common step. 



2. The effect of photoreactivation is almost completely specific for 

 damages produced by ultraviolet radiation. The susceptibility of dam- 

 ages produced by different ultraviolet wave lengths to photoreactivation 

 is little known, and more complete determinations would be desirable. 



3. The site at which the photoreactivable damage is produced is a 

 nucleoprotein, as shown by the results obtained with bacteriophages 

 (Sect. 3-3), plant viruses (Sect. 4), echinoderm sperm (Sect. 9) and yeast 

 (Sect. 7) ; it is probably nucleic acid, since Hershey and Chase (1952) have 

 shown that at least most of the protein part of the bacteriophage does not 

 enter the bacterium it infects. In this respect there seems to be a corre- 

 lation between photoreactivability and the nucleic acid content of viruses 

 (Sect. 3-4 and Sect. 4). 



4. The mechanism of reactivation requires substances of "cytoplasmic" 

 nature. The action spectra obtained seem to indicate that different pig- 

 ments are responsible for the absorption of the photoreactivating light in 

 different organisms (Sects. 3-8 and 5-5). This result is surprising in view 

 of the universal nature of the phenomenon. However, the action spec- 

 trum of bacteriophage T2, of E. coli B/'r, and of Streptomyces griseus 

 spores could all be due to light absorption in the same substance, for 

 example, in a flavin, provided that the ciuantum yield varied with the 

 wave length in a different wa}^ in different organisms. 



5. The type and sequence of reactions involved in photoreactivation 

 are rather clearly understood in bacteriophages (Sect. 3-5), and presum- 

 ably the same results will be found true for other organisms. The light 

 acts on a pigment in efiuilibrium with a nonpigment substance; the prob- 

 ability of photoreactivation is proportional to the concentration of the 

 product of the light action on the pigment. 



6. The killing effect of ultraviolet is reverted l)y one (}uantum of the 



