456 PLANT PHOTOPERIODISM 



first class are those of altered rates of cell growth in stems and leaves, 

 and anthocyanin synthesis. The types of responses elicited by the 

 second class are photoperiodic in nature and involve flower bud 

 initiation, photoperiodic chlorosis, light-induced dormancy and prob- 

 ably light-stimulated seed germination. 



It will be noted that there are eight kinetic characteristics listed for 

 each class, the first five of which are the same. They involve the action 

 spectra, photoreversibility, threshold requirements, the rate-limiting 

 nature of the processes, and the general lack of reciprocity between 

 intensity and time. The last three properties are characteristically 

 different for the two classes. They include the graded versus ungraded 

 nature of the response, the response to increasing energy, and the time- 

 phasing requirements. 



Before proceeding with the experimental evidence upon which the 

 classification of the regulatory photoresponses are based, it is essential 

 to specify precisely the implications involved in the distinction between 

 graded and ungraded responses. In Fig. 7 are plotted hypothetical 

 curves of the two classes. The graded response begins to develop at 

 zero incident energy and increases in magnitude to a maximal or 

 saturation value. The rate of increase may be Hnear or some complex 



UNGRADED 



OR 

 THRESHOLD 



INCIDENT ENERGY , E 



Fig. 7. Hypothetical curves of the fundamental difference between 

 graded responses (open circles) and the ungraded, threshold or all-or-none 

 responses (closed circles). Eq is the threshold energy and £^. is the energy 

 just capable of saturating a threshold system. 



