492 RADIATION BIOLOGY 



promote germination changes in darkness to the other form (ibid.; Borth- 

 wick, Hendricks, Parker, et al., 1952). 



That the change of the pigment in darkness is the time-controlUng 

 reaction in photoperiodism is shown not only by the equivalence of the 

 action spectra but also by the effect on the critical night length of irradi- 

 ation in the region of 7350 A. Xanthium pensylvanicum in one experi- 

 ment, for instance (Borthwick, Hendricks, and Parker, 1952), required a 

 critical dark period of 8.5 hr for floral initiation. This was shortened to 

 less than 6.0 hr by irradiation in the region of 7350 A for 30 min just 

 before the start of the dark period and lengthened to 9.0 hr by irradiation 

 in the region of 6600 A. 



In short, during the day radiation maintains the pigment in the infra- 

 red-absorbing form, and at night it slowly returns to the red-absorbing 

 form. The reaction, although not yet tested on short-night plants, prob- 

 ably goes on in the same way in both long- and short-night plants. The 

 opposite response in flowering of these plants is determined by some step 

 later than the pigment reaction. 



THE PIGMENT AND THE PHOTOREACTION 



A simple form for the photoreaction and the time-controlled dark 

 reaction (heavy arrow) is 



red 



Pigment (6600 A max) + RX > pigment X (7350 A max) 4- R. 



far red 



This indicates that the photochemical change involves a reaction of the 

 pigment with another molecule. It is observed that the rate of the 

 photoreaction is independent of temperature, which requires the pigment 

 to be in continuous collision with the other molecule. A first-order rate 

 should be followed but the way in which this or any functional relation 

 can be tested on a population is under debate at this time (Eddy, 1953). 

 It is possible that the reversible photoreaction involves only the isomeri- 

 zation of a molecule, but this seems unlikely since any change in sensi- 

 tivity to red radiation is accompanied by a change in sensitivity in the 

 opposite direction to far-red radiation. 



There are model reactions of the expected type, thus methylene blue 

 can be reversibly photooxidized and reduced in the presence of ascorbic 

 acid or iron salts. 



These characteristics of the pigment, ascertained from response to red 

 and infrared radiation, are adequate to facilitate its isolation. The bear- 

 ing of further physiological observations on the nature of the pigment i?^ 

 of some interest, however, even though an unimportant alternative can- 

 not be decided except by isolation. 



Biologically active pigments of the types now known to absorb radi- 



