A KINETIC ANALYSIS OF PHOTOPERIODISM^ 



R. B. WITHROW 

 Smithsonian Institution, Washington, D. C. 



When a new physiological phenomenon is clearly delineated, the first 

 information as to mechanism is nearly always based on kinetical ex- 

 periments; i.e., one begins by measuring the rate of the biological 

 response in relation to the intensity of physical and chemical factors. 

 Although photoperiodism has been with us as a scientific phenomenon 

 for nearly 40 years, the experimental research is still largely at the 

 kinetic level. By this we infer that nearly all our quantitative informa- 

 tion has been derived from observations on the effects of singly variant 

 factors such as light, temperature, nutrients, and chemical growth- 

 regulating agents. The next phase, which is just now beginning, will 

 be biochemical, and the final phase will consist of an integration of 

 our information to form a detailed physicochemical description of the 

 overall process. 



At the present time, we have not identified any pigment which can 

 be assigned the role of the photoreceptor. We can only guess as to the 

 biochemical reactions which are first mediated by light. Our knowledge 

 of how growth is coupled to the endogenous rhythm is even less com- 

 plete; we know little, if anything, as to the mechanism or cellular site 

 of this low-frequency oscillatory phenomenon. While we have no direct 

 proof that endogenous rhythms are closely related to photoperiodism, 

 the evidence indicates that they probably are an integral part of it. We 

 are about where photosynthesis would be if the only measurable 

 criterion of the photoreaction were the rate of increase in dry weight 

 of plant parts. 



Ecologically one might define photoperiodism as a mechanism 

 evolved by both plants and animals for measuring seasonal time. The 

 need arose primarily because climatic conditions during certain seasons 

 were unfavorable for growth, reproduction, or even survival, while 



1 Published with the approval of the Secretary of the Smithsonian Institution. 



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