198 A. M. MAYER 



mechanisms, may therefore be possible. One action of thiourea 

 in breaking dormancy may then be the following: Normally, in 

 the initial stages of germination, the seeds are basing most of 

 their oxidative processes on the oxidation of glucose-6-phos- 

 phate, probably via the pentose shunt and their electron 

 transport system is based on transport via ascorbic acid or 

 phenols, with either a coupled oxidation with phenolase as the 

 oxygen carrier, or by direct participation of ascorbic acid 

 oxidase and DPNH oxidase. At a later stage of the germination 

 process these systems are replaced by the tricarboxylic acid 

 cycle, glycolysis and the cytochrome system for electron 

 transport. If the seeds are treated with thiourea, this whole sei 

 of changes is brought into operation with much greater rapidity, 

 because some of the other systems are depressed, and thus, it is 

 tempting to suggest that thiourea breaks dormancy by in- 

 directly calling into operation more rapidly the energy-releasing 

 processes in the seeds. 



To put forward this hypothesis at the present stage is some- 

 what premature, as the evidence is far from conclusive: it is, 

 however, a possible working hypothesis which can be tested 

 experimentally. It is by no means suggested that these are the 

 only changes which are induced by thiourea. That other changes 

 occur is becoming clear today, and it is possible that in order to 

 break dormancy, a number of biochemical events have to occur 

 simultaneously. There is some evidence that the hormonal 

 system is also altered by thiourea treatment. 



REFERENCES 



1 A. Poljakoff-Mayber, Palestine J. Botany, Jerusalem, Ser., 5 (1952) 180. 

 ~ A. Poljakoff-Mayber and A. M. Mayer, J. Exptl. Botany, 6 (1955) 28. 

 3 D. RiMON, Bull. Research Council, Israel, 6D (1957) 53. 

 '^ A. M. Mayer, Enzymologia, 16 (1954) 277. 



^ A. M. Mayer, A. Poljakoff-Mayber and W. Appleman, Physiol. Plan- 

 tar urn, 70(1957) 1. 

 « A. M. Mayer, Enzymologia, 19 (1958) 1. 

 ' A. M. Mayer, Proc. Xlth Intern. Botany Congr., II, 1959. 

 ^ A. Poljakoff-Mayber, Bull. Research Council Israel, 2 (1952) 239. 



