536 



RADIATION BIOLOGY 



Fat Metabolism. Gardner reported in 1921 that embryos of various 

 positively photoblastic seeds became more acid when incubated in Hght 

 than when incubated in darkness [confirmed by Fassbender (1925) and 

 Kummer (1932)]. Since all these seeds contain mainly fat as reserve 

 substance, he concluded that light seems to activate lipolytic enzymes. 

 In grass seeds a relation exists between photoblastism and the content of 

 neutral fats and free fatty acids. Seeds containing large quantities of 

 free fatty acids germinate equally well in light and in darkness. Those 

 containing neutral fats are positively photoblastic (Ralski, 1924). Kum- 

 mer (1932) reports for different strains of the positively photoblastic 

 Holcus lanatus a parallelism between germination and the acid number 

 of fats (see Table 11-1). 



Table 11-1. 



Helation between Photoblastism and Acidity Number of Fats 

 FOR Different Strains of Holcus lanatus 

 (After Kummer, 1932.) 



But light does not appear to influence fat hydrolysis directly, because, 

 in the positively photoblastic Poa and in the negatively photoblastic 

 Bromus, fat hydrolysis is fastest in light and darkness, respectively. So 

 wherever the germination conditions are best, fat hydrolysis proceeds 

 fastest (Fassbender, 1925; Zeiher, 1936). 



Enzyme Activity. Early in the history of photoblastic research, dif- 

 ferent workers were of the opinion that light influences germination by 

 activating or inactivating enzyme systems. As papayotin and trypsin 

 stimulate germination of positively photoblastic seeds in darkness, it was 

 thought that proteolytic enzymes are involved (Lehmann, 1913; Lehmann 

 and Ottenwalder, 1913). Siegel (1950) found that by temperature pre- 

 treatment of Digitaria (see Sect. 3-2) the autolytic activity was signifi- 

 cantly increased, whereas the activity of amylolytic enzymes decreased 

 strongly. 



There is today no experimental evidence allowing us to relate enzymes 

 and photoblastism, since the rise of catalase and peroxydase activity 

 observed in tobacco seeds after illumination is undoubtedly the conse- 

 quence of better germination and not its cause (Schroeppel, 1933). We 

 may mention here the observation made by Gassner and Franke (1934- 

 1935) that, when positively photoblastic Lythrum and Poa seeds are made 

 skotodormant, their content of soluble nitrogen remains constant, whereas 



