BIOCHEMICAL CHANGES IN SEED DORMANCY 197 



normally in peas germinated in coumarin"- ^^ Thus it appears 

 that coumarin does not act by directly suppressing som^e 

 specific enzyme. There are, however, indications that it may act 

 indirectly through its effect on release of inorganic phosphorus 

 from phytin^ and also on ATP metabolism^^ and may thus in 

 some way control respiration, or rather the energy supply to the 

 seeds, by interfering with the rate-determining or controlling 

 steps during oxidation of substrates. 



The changes in the oxidative system brought about by treat- 

 ment of the seeds with thiourea are far more widespread. 

 Poljakoff-Mayber and Evenari^^ showed that the rate of oxida- 

 tion of tricarboxylic acid cycle intermediaries was much more 

 rapid by mitochondria isolated from seeds germinated in 

 water. Thus it seems that the tricarboxylic acid cycle enzymes 

 become active much more quickly in the presence of thiourea. 

 Preliminary results indicate similar effects for the cytochrome 

 system., or at any rate for cytochrome oxidase. In contrast to 

 these systems which become more rapidly active, a number of 

 systems are inhibited. Oxidation of ascorbic acid and of DPNH 

 by extracts of lettuce seeds is completely prevented when the 

 seeds are germinated in thiourea. The systems responsible for 

 this oxidation are situated in the soluble part of the cell^^' i^. 



The oxidation of phenolic substrates is altered by germination 

 in thiourea (Table II). Extracts of seeds germinated in thiourea 

 do not show any blackening even on prolonged standing. These 

 extracts still contain an active phenolase which is not itself 

 inhibited by thiourea^^. However, the oxidation of quinol, 

 apparently by a coupled oxidation, is completely inhibited both 

 by in vivo and in vitro treatment. 



It is possible that such coupled oxidations and also ascorbic 

 acid oxidase and DPNH oxidase, normally mediate part of the 

 electron transport in the seeds. There is at present little evidence 

 to show that in such electron transport systems oxidative 

 phosphorylation occurs. The disruption of such a system by 

 thiourea, leading to the more rapid entry into operation of the 

 normal Krebs cycle and cytochrome system as energy-providing 



References p. 198 



