460 



PLANT PHOTOPERIODISM 



-2 -I I 2 3 4 5 



INCIDENT ENERGY {log A^i/cm^) 



Fig. 8. Logarithmic-energy response of various photomorphogenic sys- 

 tems. Data for Avena were taken from Weintraub and Price (1947); bean 

 seedling data, Klein et al. (1956, 1957). 



effectiveness of the energy increases over 100-fold between 2.5 min of 

 exposure and a 20-hr exposure. The same type of phenomenon is 

 evidenced in the flowering response of the long-day plant Nobel 

 spinach (Withrow and Withrow, 1944). The criterion of the effective- 

 ness of the incident energy was taken as the time required for flower 

 buds to appear. The most effective condition was continuous irradia- 

 tion, and even 270 flashes per night were less effective. When the 

 energy was distributed in the form of five equally spaced flashes per 

 night, it was approximately equal to 225 ft-c-min, or about 50% 

 efficiency. 



In both the photomorphogenic and photoperiodic systems, then, 

 it would appear that continuous irradiation is more efficient than any 

 form of intermittancy and that the consequences of the photochemical 

 reaction die out rather rapidly and require continuous renewal if the 

 system is to be pushed at its maximum rate. 



The photoperiodic systems have a second type of reciprocity failure 

 which is related to the time-phase requirement for the system. This 

 requirement is apparently superposed upon the need for continuously 

 activating the system and considerably complicates the picture. A 

 typical response is shown jn Fig. 12 for a short-day plant, Xanthium, 

 in which a single flash of incandescent lamp energy is applied at 



