PHOTOMORPHOGENESIS IN DIFFERENT 

 SPECTRAL REGIONS 



G. MEIJER 



Philips Research Laboratories, N. V. Philips' Gloeilampenfabrieken 



Eindhoven, Netherlands 



PHOTOPERIODISM 



In photoperiodism it is well known that the red part of the visible 

 spectrum has the greatest influence on the flowering of long-day and 

 short-day plants, whether this hght is supplied for a number of hours 

 to supplement a short day or used as an interruption of the correspond- 

 ing long dark period (Withrow and Biebel, 1936; Parker et al., 1950). 

 Moreover, the effect of a night break by red light can be nullified by 

 a near infrared (far red) irradiation (Borthwick et al, 1952). This 

 red-near infrared antagonism has been found to exist also in a great 

 number of other light-controlled processes, e.g., seed germination, 

 elongation, and pigmentation. 



By growing plants in light of different spectral regions, it has been 

 shown by Stolwijk and Zeevaart (1955) that the inclusion of a certain 

 amount of violet, blue, or infrared in the long day is necessary to 

 obtain a long-day effect in Hyoscyamus niger. A long-day treatment in 

 red light exhibited only a slight long-day effect, i.e., a flower-inducing 

 effect; in green light no flower initiation at all was obtained. 



In a number of experiments we have shown that this blue and near 

 infrared influence on the long-day efl'ect is not restricted to Hyoscya- 

 mus niger (Meijer, 1957; Meijer and van der Veen, 1957). By using 

 a combination of colored fluorescent tubes with suitable filters, high- 

 intensity light of different spectral regions was obtained: red, green, 

 blue, and blue in combination with infrared. The infrared was emitted 

 by the blue fluorescent tubes and transmitted by the blue filter. It was 

 found that Salvia occidentalis, an obligate short-day plant, does not 

 flower in a long day in red or in blue light. In a long day in green 



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