BIOCHEMICAL CHANGES IN SEED DORMANCY 195 



hardly affected, free fatty acid formation is slightly depressed 

 and volatile fatty acid formation is somewhat depressed. In this 

 case, therefore, dormancy breaking with thiourea and dormancy 

 induction with coumarin have opposite effects only on one part 

 of the system, that is, the free volatile fatty acid. Although this 

 fatty acid was not identified with certainty it seems probable 

 that it is either acetic or lactic acid. In either case this would 

 make it probable that its origin is closely connected with 

 respiratory mechanisms. It seems more than doubtful in any 

 case, that the changes observed are directly related to dormancy 

 breaking or induction. 



Seeds which germjnate in the dark in water show an increasing 

 oxygen uptake which starts almost immediately after the seeds 

 are placed in the water. A number of respiratory enzymes are 

 active even before the seeds are imbibed. These enzyme systems 

 can be shown to exist if the dry seeds are extracted with water: 

 the possibility that they become activated during extraction with 

 buffer cannot be excluded. Those systems which have been 

 shown to be active in the dry seeds, keeping the above limita- 

 tions in mind, are a cytochrome oxidase, a number of dehydro- 

 genases, a DPNH oxidase, catalase, peroxidase and a phenolase 

 system, as well as what is possibly an ascorbic acid oxidase^' ^. 

 Some of these enzymes increase greatly in activity as germination 

 proceeds while others, particularly the glucose phosphate dehy- 

 drogenase^o^ the phenolase and the DPNH oxidase^-^, do not, but 

 even show a decrease. In this respect it is worthwhile to recall 

 that during germination the number of cells in the seed or 

 embryo increases. Therefore, the failure of an enzyme system to 

 increase as germination proceeds implies that the average 

 amount of enzyme per cell is decreasing. It is of course also 

 quite possible that in some parts of the embryo the system does 

 not develop at all while in others it maintains its original 

 activity. 



It has been shown that during normal germination, the tri- 

 carboxylic acid cycle activity of mitochondria prepared from 

 the seeds is very low initially, and then increases as germination 



References p. 198 



