266 - Multicellular Planfs 



which are relatively insensitive to dark-light 

 alternations, but are sensitive to other sea- 

 sonal changes, particularly temperature. 



The experimental evidence upon which 

 this classification is based is very extensive. 

 Here it will be sampled only sparsely, with 

 mere mention of a few pioneer workers, 

 namely }. A. D. Lcevaart, James Bonner, 

 and H. A. Borthwick. 



If a short-day plant, grown under experi- 

 mentally controlled lighting conditions, is 

 exposed to less than a certain span of dark- 

 ness (usually about 12 hours), it will not pro- 

 duce any floral buds. If, on the other hand, it 

 is exposed to three or lour long nights, it 

 will initiate flowering, provided the dark 

 periods are not interrupted. Even a very brief 

 exposure to light, interrupting the dark 

 period for only 2 to 3 minutes, suffices to 

 abolish the flowering reaction. The leaves, 

 apparently, serve as the site of induction, 

 since only the leaves — in some cases only a 

 single leaf — need be exposed to the appro- 

 priate period of darkness. Long-day species, 

 on the other hand, display an opposite be- 

 havior. To flower, these plants must be pro- 

 vided with a relatively short night, and the 

 flowering tendency is damped off if the dark 

 period is interrupted by just a few minutes 

 of exposure to light. 



An analysis of the wavelength of light util- 

 ized in these experiments led to the conclu- 

 sion that certain wavelengths are critical. 

 Red light of 660 iri/i wavelength was most 

 effective in damping off the flowering reac- 

 tion when used to interrupt the periods of 

 darkness. Moreover, it was found that infra- 

 red light, of wavelength 730 m^. was exceed- 

 ingly effective in canceling out the effects of 

 a previous illumination with red (660 ni/x) 

 light. 



Wavelength studies also indicate that a 

 receptor pigment, with an absorption maxi- 

 mum in the red range (660 m^), is important 

 and that a reversible photochemical reaction 

 serves as a trigger for the photoperiodic re- 

 sponses. Recently, this pigment has been 

 identified and named phytochrome, although 



it has not yet been obtained in fully purified 

 form. Phytochrome is a photosensitive pro- 

 tein compound. It can be shifted back and 

 forth between two forms, phytochrome 660 

 (P oco ) and phytochrome 730 (P 730 )> according 

 to the wavelength of the light to which it is 

 exposed. Exposure to red light shifts it from 

 P,j ( ;o to P 730 ; whereas exposure to infrared 

 has the opposite effect. The physiologically 

 active form appears to be phytochrome 730. 

 The concentration of this form, maintained 

 throughout a critical period, serves as a 

 trigger for the various photosensitive re- 

 sponses of the plant. At the end of a day, 

 all the phytochrome of the leaves has been 

 converted to the P 730 form; but gradually 

 during the darkness P 730 reverts spontane- 

 ously to Poori- Whether or not a response is 

 triggered depends, apparently, upon how 

 much of and how long the active (P 73(1 ) form 

 of phytochrome is present at the site of 

 stimulation. 



Although many unresolved areas remain 

 in our understanding of photoperiodic phe- 

 nomena, one current working hypothesis may 

 be presented as follows: 



Phytochrome 660 



Red component of 

 white light. 

 Photochemical 



reaction. 



Spontaneous reversal 



in dark. 

 Photochemical reversa 

 by infrared. 



Phytochrome 730 



Active form; stimulates in long-day plants; 



inhibits in short-day species. 



Precisely how the flowering stimulus is 

 transmitted from the leaves, where the photo- 

 chemical timing device is localized, to the 

 growing points of the stem, where the floral 

 buds are formed, is still not known. Un- 

 doubtedly one or more substances, perhaps 

 hormones of the gibberellin type, are liber- 

 ated when the photochemical mechanism is 

 tripped, and these substances are conveyed 

 by the vascular elements from the leaves to 

 the buds. In any event, flower induction can 

 be observed in a plant that has not received 



