450 PLANT PHOTOPERIODISM 



The regulatory photochemical reactions fall into two general classes: 

 those in which the yellow pigments are activated by the blue to 

 produce the so-called straight growth effects and bending responses in 

 seedlings, and those activated by pigments having strong absorption 

 bands only in the red end of the visible spectrum. The blue and red 

 groups of reactions often produce very similar end results in terms of 

 their effect on stimulation or inhibiting cell elongation, and this has 

 led to confusion when white sources such as the incandescent or 

 fluorescent lamp have been used in experimental work. Also, when 

 monochromatic blue is used as the exciting radiant energy, it is 

 important to realize that it is impossible to distinguish between the 

 weak activity of the red-absorbing pigments in the blue, thus exciting 

 photomorphogenesis, and activation of yellow pigments such as the 

 flavins and carotenoids. The blue peaks of activity reported in this 

 symposium by Meijer (p. 101) and Wassink, DeLint, and Bensink (p. 

 Ill), and elsewhere, may very well be the direct result of yellow pig- 

 ment activation rather than any new photomorphogenic peak appear- 

 ing in the blue, as has been implied. 



The reversible red, far-red photomorphogenic reactions are capable 

 of inducing a wide variety of seemingly different and oftentimes 

 diametrically opposed responses in higher plants at all stages of 

 development from seed germination to flowering. After a red induc- 

 tion, any one of these responses can be reversed by activation with the 

 far red, as was first shown by the Beltsville group of Borthwick et al. 

 (1952). 



The red is capable of inducing germination in light-sensitive seeds 

 of higher plants (Flint, 1936) and the germination of fern spores, 

 (Mohr, 1956). In the seedling, the whole course of development is 

 altered by very low intensities of red energy. In many dicotyledonous 

 seedlings, the hypocotyl length growth is inhibited, whereas the 

 epicotyl is stimulated; the hypocotyl hook, when present, is caused to 

 straighten; and the rate of leaf development usually is greatly ac- 

 celerated. In monocotyledonous plants, the first internode behaves 

 much like the hypocotyl of the dicotyledon and is inhibited, whereas 

 the coleoptile behaves like the epicotyl and is markedly stimulated. 

 Leaf development likewise is accelerated in the monocotyledons. In 

 the course of development of the seedlings of many, but by no means 



