Changes in Buds and Seeds in Response to Chilling 103 



ing involve primarily the germination promoter, nevertheless there 

 is good evidence that an inhibitor plays an important part in the 

 dormancy of the seed. Indeed, the responses of the seed appear to in- 

 volve interaction between the promoter and the inhibitor, and the 

 hypothesis is suggested that dormancy is due to the presence of the 

 inhibitor and that emergence from dormancy involves the accumula- 

 tion of the promoter to a level which overcomes the effect of the in- 

 hibitor. Since the greater part of the inhibitor present in the seed is 

 contained in the endosperm, whereas the promotor is confined to the 

 embryo, it would seem that a build-up of promoter is necessary to 

 enable the embryo to overcome the inhibitory effect of the endosperm. 



Since gibberellins are able to break the dormancy of various rest- 

 ing organs, including the unchilled embryos of F. excelsior, the ques- 

 tion arises whether the germination promotor present in the chilled 

 embryos is a gibberellin. Several pieces of evidence suggest that this 

 is not the case; for example, the very small amount of tissue required 

 to be extracted for its detection contrasts markedly with the relatively 

 large amounts of tissue generally required for the detection of gibber- 

 ellins. Moreover, the embryo promoter is capable of removing the 

 stunting of seedlings derived from unchilled embryos, whereas gibber- 

 ellic acid does not have this effect. 



Whatever the nature of this promoter may prove to be, it would 

 seem very probable that it functions as such in the intact seed, for the 

 great merit of the work with Fraxinus seeds is that detection of the 

 germination promoter was carried out with embryos of the same spe- 

 cies, so that there is strong presumptive evidence that the in vitro ex- 

 perimental results are equally applicable in vivo. 



If these results with seed of F. excelsior prove to be of more gen- 

 eral application to buds and other resting organs, then it would seem 

 that the reduced inhibitor level, reported to occur in the buds of sev- 

 eral species in response to chilling, is not the only or even the pri- 

 mary cause of emergence from dormancy of these buds. It is thus de- 

 sirable to investigate whether there is any accumulation of dormancy 

 breaking substances during the chilling of buds. The existence of 

 such substances in buds of Tilia and Fraxinus has, in fact, already 

 been reported (22). 



SUMMARY 



Dormancy can be envisaged as being due either to a lack of cer- 

 tain essential growth factors or to the presence of active growth inhib- 

 itors. Studies of the changes in the levels of growth inhibitors in rest- 

 ing buds have shown a correlation between the state of dormancy and 

 the level of inhibitors, and suggest that the effect of winter chilling 

 in removing dormancy is to reduce the level of inhibitor. 



