530 



RADIATION BIOLOGY 



40 



30-48 hr in the dark the seeds become negatively photoblastic. After 

 5 days they are completely skotodormant. In lettuce seed, too, when 



darkness is more prolonged than 8 hr, 

 desensibilization sets in until the 

 sensibility is nil and the seeds are 

 skotodormant (Evenari, unpublished 

 work; see Fig. 11-3). 



Other seeds {Epilobium, Eschschol- 

 zia) behave similarly, but with tobacco 

 seeds desensibilization, which sets in 

 after about 40 hr of darkness, never 

 reaches the zero point (Bihlmeier, 

 1927). Desensibilization is influenced 

 by temperature. For tobacco seed it 

 is greatest at 31°C and less pro- 

 nounced at 28° and 25°C {ibid.). 



Both positively and negatively 

 photoblastic seeds can be made photo- 

 dormant by prolonged illumination 

 in blue, germination-inhibiting light 

 (e.g., Ehason and Heit, 1940); this 

 indicates that photo- and skoto- 

 dormancy are essentially the same 

 phenomenon that is produced when 

 seeds, ready to germinate, are kept 

 for longer periods in conditions 

 unfavorable to germination. This 

 photodormancy can be broken by low- 

 temperature treatment. 



2 4 6 8 24 



SOAKING TIME, hr 



Fig. 11-3. Germination index of Grand 

 Rapids lettuce seed soaked in water 

 in darkness for different lengths of 

 time (2-24 hr), illuminated with 

 5 sec X 250 ft-c (curve a) and 

 10 sec X 250 ft-c (curve h) of white 

 Hght, and then returned to darkness 

 at 20°C. The germination index is 

 the number of seeds germinated after 

 the illumination related to the number 

 of seeds germinated in the dark con- 

 trol, taken as 1.0. The peak of 

 photosensitivity is reached after 8 hr. 

 After 24 hr of darkness the seeds are 



more or less skotodormant. • . , i j j.u 



pretreatment was more prolonged, the 



photosensibility remained at the same level. A pretreatment at 10°C 

 brought about a sensibilization that after a duration of 32 days turned 

 into a slight desensibilization. After pretreatments at 22°, 28°, and 40°C, 

 the maximum sensibilization was attained after 2 days. Longer pretreat- 

 ments cause a strong desensibihzation. 



3-2. PRETREATMENT WITH 

 DIFFERENT TEMPERATURES 



Tilly (1934-1935) treated positively 

 photoblastic seeds with different tem- 

 peratures before giving them a 

 standard illumination. A pretreat- 

 ment at 3°C caused a pronounced 

 sensibilization, reaching its peak after 

 a pretreatment of 8-16 days. If 



