BIOCHEMICAL CHANGES IN SEED DORMANCY 193 



the early stages of germination. However, while in germinating 

 seeds glucose appears as the sucrose disappears, this accumula- 

 tion of glucose is completely blocked in seeds whose germination 

 is prevented by coumarin (Table I). 



TABLE I 



THE EFFECT OF THIOUREA ON THE METABOLISM OF LETTUCE SEEDS AND SEEDLINGS 



Seeds germinated in thiourea (1250 p. p.m.) at 16' in the dark 



Metabolite Activity effect of thiourea Reference 



Sucrose utilisation Practically no effect 1 



Fat utilisation Slight inhibition 2,8 



Glucose accumulation Practically no effect 1 



Free fatty acid liberation Stimulation in later stages of growth 2 



Volatile fatty acids Slight stimulation 2 



Phytin breakdown No effect 6 



The principal storage material in lettuce seeds are lipids. The 

 major decomposition of the lipids occurs at a fairly late stage of 

 germination, but small amounts are probably already broken 

 down at earlier stages. Both coumarin and thiourea cause 

 rather extensive changes in the metabolism of the fats. Coumarin 

 in general prevents the breakdown of lipids in the seeds-' ^ and 

 at the same time the appearance of free fatty acids is also 

 delayed or prevented while volatile ones increase. These latter 

 need not necessarily and probably in fact do not arise from the 

 lipids directly. 



The enzyme systems which are known to be active in the 

 seeds in lipid breakdown are two lipases, one having maximal 

 activity at acid pH and the other maximal activity at neutral 

 pH. Neither of these enzymes show an increase in activity in 

 seeds treated with coumarin (Table II). In contrast, thiourea 

 inhibits the acid lipase, which in normal germination begins to 

 increase in activity only at a fairly late stage of the germination 

 process, while the neutral lipase activity rises initially and then 



References p. 198 



