166 A. POLJAKOFF-MAYBER 



of hormones have some role in regulating dormancy in seeds. 

 There are also other substances found in the seeds but not as yet 

 identified, which take part in regulating the dormancy phe- 

 nomenon and germination processes. These substances are 

 either germination inhibitors or germination promoters, and 

 they are the products of normal metabolic processes in the 

 ripening and in the germinating seed. Similar substances regulate 

 the dormancy and sprouting of buds. 



As has already been explained in previous lectures, various 

 dormant seeds will not germinate unless their dormancy is 

 broken by special temperature or light treatment. In some cases 

 such treatment may be avoided and the seeds may be induced to 

 germinate by treating them with plant hormones. Such seeds, 

 for example, are lettuce seeds [Lactuca sativa L.) variety Grand 

 Rapids. These seeds, for several years after harvesting, will not 

 germinate in the range of temperatures of 23-28 "" unless 

 illuminated, when fully imbibed, for a certain period of time 

 with red or white light. This light requirement diminishes with 

 increasing period of storage, and five to six years after harvesting 

 the seeds will germinate in the dark to the extent of 80% or 

 more. Irradiation with Far Red reverses the eff'ect of red and 

 usually also lowers the percentage of germination in the dark. 



The red light treatment breaks the dormancy of the seeds and 

 forces them to germinate. A similar effect may be achieved by 

 soaking the seeds in a solution of gibberellin''' ^' ^. The mecha- 

 nisms of the two treatments are not identical as the effect of 

 both of them is additive (Table I) and the Far Red completely 

 reverses the red effect but only partially the gibberellin effect^. 



Besides in lettuce, gibberellin has been shown to be effective 

 in breaking the dormancy of Arabidopsis'^^, Kakmchos^^ and 

 many other species^-- ^^. 



A closer interaction between the effects of light and a plant 

 hormone in breaking dormancy was shown for kinetin^^' ^^. 

 Kinetin greatly increases the sensitivity of the seeds to red light. 

 In the presence of kinetin, extremely small amounts of light 720 

 ft.c.s. (foot-candle-seconds) are sufficient to bring about maximal 



