HISTORY OF RESEARCH IN 

 PHOTOPERIODISM 



by 



A. E. MURNEEK 

 University of Missouri, Columbia, Missouri 



Introduction : — In their growth and development plants are affected by many 

 environmental factors. Some of these are under the control of man and form a basis 

 for cultural practices in crop production. One of the latest major external factors 

 discovered is the length of day, technically known as the photoperiod. The vast 

 majority of plants probably are sensitized to various degrees, through natural or 

 artificial selection, and react to recurrent diurnal light periods. This response to the 

 photoperiod is called photoperiodism. 



While the day length may modify any and all structural parts of a plant, our 

 interest, from the beginning of the disclosure of this phenomenon till today, is centered 

 primarily on its influence on the formation of flowers and development of fruits and 

 seeds. This is but natural, for this particular function of the plant is closely associated 

 with, if not basic for, the successful production of many economic crops. 



The influence of length of day is modified, and sometimes counteracted, by other 

 environmental factors, particularly temperature. For this altered or dual effect the 

 term thermophotoperiodic induction has been proposed. In cases where the development 

 of a plant is affected by diurnal alterations in temperature, somewhat analogous to that 

 in other plants by photoperiod, one may possibly speak of thermoperiodicity. 



Variability in response to the photoperiod of various species and varieties points 

 clearly to a genetic mechanism in operation in bringing about this phenomenon. Active 

 selectional processes, in this respect as in many others seem to be at work in the in- 

 tricate economy of living organisms towards adaptability to the environment. In the 

 case of domesticated species, man often participates as an operating agent in the selec- 

 tion, preservation, propagation and multiplication of many photoperiodically adapted 

 varieties and strains. If necessary, through selection and hybridization, it is possible 

 to impart desirable characteristics in this respect to plants hitherto not possessing them. 



Photoperiodism, in relation to vegetative growth and sexual reproduction, is but a 

 phase of the general problem of plant growth and development. The early investiga- 

 tors in this field were Klebs in Germany (1903-1918) and Lysenko in the U.S.S.R. 

 (1928- ). Klebs initiated research work on developmental physiology of plants and 

 demonstrated by various experimental procedures that structural changes, with special 

 reference to reproduction, are not of an unalterable nature and, therefore, not due en- 

 tirely to an "'internal hereditary rhythm." They may be modified drastically, or even 

 suppressed, by changing certain environmental factors, such as light, temperature or 

 nutrition. Lysenko, while known chiefly for his discovery and application of seed 

 vernalization to hasten reproduction, has postulated the now popular theoretical con- 

 cept of "phasic development of plants." Both the temperature and the photoperiod 

 constitute two of the most important environmental factors affecting growth and de- 

 velopment, as conceived by Lysenko and later elaborated by his co-workers. A de- 

 tailed discussion of contributions of the Russian investigators and others on this subject 

 will be found in Whyte's chapter on "History of Research in Vernalization." 



Terminology : — To prevent a possible confusion in the use of certain 

 technical terms and phrases in connection with the discussion of photo- 

 periodism and related subjects, it is thought desirable to present herewith 

 a Hst of the more popular terms with their definitions. 



