20 • Photoperiodism: An Outline 



breaks" during main light periods have essentially no effect on 

 the process. 



The evidence reviewed above should make clear the reason lor 

 emphasizing duration and timing of light (and darkness) rather 

 than total energy in the definition of photoperiodism. It has 

 also resulted in the term "critical nightlength" replacing "critical 

 daylength" in some reviews and articles on the subject, in order 

 to stress the relative importance of light and dark periods. However, 

 as will be shown, light also plays a role, although perhaps less 

 important, in the normal time requirements of photoperiodism, so 

 that the second terminology is only slightly more accurate than the 

 first. Either will be used, as occasion demands. 



Ancillary evidence for the more crucial role of the dark periods 

 has also been derived from experiments in which temperature is 

 varied, some of which will be considered elsewhere. 



REQUIREMENTS FOR HIGH-INTENSITY LIGHT 



The effects of brief or prolonged exposures to low-intensity 

 light, nullifying dark periods, will be considered in detail in the 

 next chapter. Meanwhile, after setting up generalizations that dark- 

 ness plays the major role in photoperiodism and that the total 

 light energy during a treatment or cycle is relatively unimportant, 

 it is now necessary to consider what role, if any, is played by the 

 high-intensity light periods which, at least in nature, normally 

 alternate with dark periods. 



1. Short-Day Plants: Early work with SDP soon showed that 

 in spite of the critical role of the dark periods, the main light 

 periods also had to include at least a certain amount of high- 

 intensity light for optimum (lowering to occur in many plants. 

 An elegant demonstration of this was given by Hamner (1910). 

 using Xantliium. 



It was obviousl) not reasonable to study the effect of a dark 

 period preceded by a dark period, since the two together simply 

 add up to a longer one. Hamner made use ol the light-break tech- 

 nique, however, in the following manner. Xanthium plants can be 

 kept vegetative on cycles of 3 minutes Light-3 hours darkness. 

 After a lew such cycles, a single dark period of 12 hours, which 



