542 RADIATION BIOLOGY 



5-3. SUGGESTIONS FOR FURTHER WORK 



Summing up, we would like to suggest the following lines of approach 

 for future research on photoblastism : 



1. Isolation and identification of the photoreceptor or receptors 

 involved. 



2. Research into the enzyme systems involved, with special stress upon 

 photoactivation or inactivation of enzymes along the lines worked out by 

 Galston (1950) and Galston and Baker (1949). 



3. Research into the changes of permeability of artificial membranes 

 brought about by light [see Brauner and Brauner's studies (1940) on the 

 photoeffect of membrane permeability]. 



4. Clarification of the relation between respiration, light, and special 

 inhibitors hke coumarin. 



5. Research into the specific role played by coats, endosperm, and 

 embryo in normal germination, dormancy, and photoblastism. 



6. Research into the mechanism of the interrelation between temper- 

 ature and photoblastic reaction. 



6. ADDENDUM 



After this manuscript was ready for publication, some new contribu- 

 tions to the problem of photoblastism were made. Poljakoff-Mayber 

 (1951, 1952) tried to elucidate the influence of light and coumarin on 

 the metabolism of germinating lettuce seeds. Evenari (1952) investi- 

 gated the interaction of light, temperature, and coumarin on the germi- 

 nation of different varieties of lettuce seeds. 



It could be shown that under certain conditions all varieties are photo- 

 sensitive, even those whose germination generally would be considered 

 independent of fight. The interaction of the three factors investigated 

 is explained by the change of thermosensitivity caused by light and treat- 

 ment with coumarin. 



Borthwick and coworkers (1952) proved that the germination of lettuce 

 seeds stimulated by red light can be inhibited by infrared radiation and 

 that this inhibition again can be suppressed by red fight, and so on. This 

 brings the authors to the f ofiowing scheme of light action : 



Seeds dormant Seeds germinating 



red 



Pigment + RX . pigment X + R (11-1) 



infrared, dark 



Pigment X -f- R . pigment -\- RX. (11-2) 



This means that in this reversible photoreaction two pigments are 

 involved as well as other reactants. The percentage of germination 

 under given conditions depends upon the integrated absorption of light 

 by the two pigments. 



