78 PHOTOCONTROL OF GROWTH 



Table II. Seed of Betula puhesceiis, Effect of Various Periods 

 of Blue Irradiation Before and After Exposure to Red 



Percentage 



Treatment Germination 



1. 8 hr red only 32.6 



2. 8 hr red, preceded by blue: {a) PA hr 37.8 



3. 8 hr red, preceded by blue: {h) 3 hr 52.5 



4. 8 hr red, preceded by blue: (t) 12 hr 67.0 



5. 8 hr red, preceded by blue: (d) 24 hr 70.6 



6. 8 hr red, followed by blue: (a) 12 hr 16.5 



7. 8 hr red, followed by blue: (b) 24 hr 13.1 



8. 8 hr red, followed by blue: (c) 48 hr 19.2 



Source, blue fluorescent tubes with blue Perspe.x and copper chloride 

 screen. Intensity, 80 /nw/cm^. 



given before the red very strongly promoted germination. Further 

 experiments indicated that relatively long periods (about 12 hr) of 

 exposure to blue are necessary to obtain maximum stimulation or in- 

 hibition. Experiments with the interference filters indicated that the 

 most effective spectral region for these effects lies in the region of 452 

 m/A. 



In order to explain the promotive and inhibitory effects of blue in 

 lettuce seed, Borthwick et al. (1954) have postulated that the photo- 

 receptors for the red and far-red responses must have absorption 

 regions in the blue which overlap. This hypothesis would explain why 

 the response of the seed is so dependent upon the duration of exposure 

 and the period of imbibition, since these two variables can be en- 

 visaged as affecting the equilibrium between the promotive and in- 

 hibitory processes. The Beltsville group was unable to obtain any 

 appreciable reversal of the effects of red by blue, however. It is now 

 clear that, in order to obtain photoreversal, the period of irradiation 

 by blue must be about 4 hr, whereas much shorter periods are effective 

 with far red. 



The greater period of exposure required with blue may result from 

 the fact that, even at long imbibition periods, there may still be some 

 promotive effect of blue and that the balance is only decisively toward 

 inhibition when longer periods of irradiation are used. Further, it is 

 known that light transmission by the seed coat of lettuce is very much 

 less for blue than for far red (Evenari, 1956), and it is possible that 

 shorter periods of irradiation with blue would be more effective at 



