540 RADIATION BIOLOGY 



starts processes that bring about germination, and light counteracts this 

 (ibid.). Kipp (1929) elaborated upon this idea. In the seed there is an 

 equilibrium oxidation-reduction reaction, which is changed by light and 

 different oxygen concentrations. Light pushes the equilibrium toward 

 reduction; oxygen, above the amount needed for respiration toward oxi- 

 dation. An enzyme that is indispensable for germination is reduced and 

 so activated or oxidized and inactivated. Negatively photoblastic seeds 

 can germinate only when the equilibrium is changed toward oxidation. 



Leggatt (1948) is of the opinion that the inhibitor of germination is 

 carbon dioxide, which cannot escape from the seeds so long as the coats 

 are impermeable to it. He thinks that under the influence of carbon 

 dioxide a type of zymase appears which produces little alcohol and much 

 acetaldehyde, which, by the way, was earlier considered by Maze (see 

 Evenari, 1949) as a germination inhibitor. This brings us to the question 

 of photodormancy, since Leggatt (1948) found that, when lettuce seeds 

 are made photodormant by illumination with blue inhibiting light, they 

 contain less alcohol than seeds illuminated with white light. Accord- 

 ingly he thinks that photodormancy is brought about in the same way 

 as the germination inhibition caused by carbon dioxide. It may be men- 

 tioned that Wieser (1927) had tried to explain skotodormancy by sup- 

 posing that, when seeds are put to germinate in darkness, germination 

 starts. But, owing to lack of oxygen, intramolecular respiration sets in. 

 Substances are produced which inhibit germination and make the seed 

 unable to react later to the photochemical effect produced by light. 



Biinning (1948) thinks that light starts two diametrically opposed 

 processes in germinating seeds: one inhibiting and one stimulating. This 

 opinion is strongly supported by the fact that there are inhibiting and 

 stimulating wave-length zones that are nearly identical in position and 

 effect on positively and negatively photoblastic seeds. Possibly there 

 are even two different photoreceptors, carotene for the inhibiting and 

 chlorophyll for the stimulating wave lengths. It is of interest that there 

 seems to be a similar dual effect of light upon stomatal movement, one 

 set in motion by blue light (absorbed by carotene?) and one brought 

 about by red light (absorbed in chlorophyll?) {ibid., p. 361). In connec- 

 tion with this we may mention that Flint and Moreland (1943) found for 

 Hymenocallis occidentalism whose integuments contain macroscopically 

 visible chlorophyll, dependence of germination upon the photosynthetic 

 activity of the integuments. This, in their opinion, explains the obser- 

 vation that in a carbon dioxide atmosphere the seeds germinated well in 

 light but did not germinate in darkness. 



A point difficult to explain is the fact that photoblastism is a function 

 of temperature. Gassner (1911a,b), for example, beUeves that at low 

 temperatures light inhibits the germination of Chloris, which is positively 



