SEED DORMANCY AND DORMANCY BREAKING 183 



to tackle the nature of the enzymatic action involving this 

 pigment through which germination and growth process in 

 general are controlled. 



Among the many data available on light mechanism in the 

 lettuce seed, I shall mention only two: First, that light exerts an 

 influence not only on the imbibed, but also on the 'dry' seed^^. 

 This influence, which is strongly dependent on the relative 

 humidity of the air, is probably related to the internal water 

 content of the seeds. Seeds stored at a relative humidity of 

 60-80% in the light and subsequently germinated in the dark 

 gave a higher germination percentage than those stored at the 

 same relative humidity in the dark. No such effect of light could 

 be found in seeds stored at 20-30% relative humidity. This 

 indicates that the relatively low water content of the dormant 

 seed has to be raised only slightly in order to make the seed 

 respond to external influences. 



Secondly, the reversibility of the Red-Far Red reaction in the 

 lettuce seed depends on the time interval between the two 

 illuminations. After 10 min of darkness, Far Red cannot any 

 longer reverse the effect of red^^. This shows that only the first 

 steps in dormancy breaking are influenced by light. Once the 

 reaction chain has started, light, at least in lettuce, does not have 

 any influence on further events. 



Considering the changes induced by these light mechanisms 

 which finally cause resumption of growth, one of the most 

 important factors is, of course, water uptake. It might be 

 imagined that dormant and non-dormant seeds would take up 

 different amounts of water when imbibed, i.e. their water uptake 

 would differ in lisht and darkness. This, however, is not the 

 case, and water uptake under both these conditions proceeds 

 almost indistinguishably until growth starts in the light-treated 

 seeds, after approximately 14 h; i.e., the light treatment does not 

 change water uptake in the seeds. 



Since a good deal of work has been done on chemical inhibi- 

 tory and promotory agents for germination, let me say a few 

 words on the effect of some of these substances in lettuce seed 



References p. 190 



