PHOTOPERIODISM 493 



ation by electronic transitions in the far red or near infrared have of the 

 order of 20 or more conjugate double bonds (Strain, 1949). The most 

 Hkely ones for consideration here are cychc and open-chain tetrapyr roles. 

 The positions of the red and infrared maxima for physiological action 

 are in agreement with either type of pigment, and the distinction between 

 - the two classes must depend on the absorption in the region 4000-5000 A ; 

 the cyclic tetrapyrroles such as the porphyrins have a high absorption in 

 this region owing to the so-called "Verdet bands" (Rabinowitch, 1944), 

 and the open-chain tetrapyrroles such as the prosthetic group of phyco- 

 cyanin (Svedberg and Katsurai, 1929) have low absorptions. 



The action spectra for photoperiodism and for seed germination show 

 relatively low physiological response in the blue compared with the red 

 and infrared parts of the spectrum. From seed germination it is apparent 

 that the absorptions of the two pigment forms overlap in the region below 

 5000 A. The details of this response, however, show that the approach 

 to photoequilibrium between the two forms recjuires an irradiance at least 

 tenfold greater than that near 7000 A, where the infrared and red absorp- 

 tions overlap and after appropriate times of imbibition prior to irradiation 

 lead to the same germination as that effected in the blue region. 



Low physiological response, however, might accompany a high absorp- 

 tion if the quantum efficiency for chemical change were low. The alterna- 

 tives then are that (1) the pigment is an open-chain tetrapyrrole or (2) the 

 pigment is a cyclic tetrapyrrole with low effectiveness of the Verdet bands 

 for physiological action. 



RECIPROCITY AND ENERGY RESPONSE 



Information about the photoreactions and the biological response 

 can be obtained from reciprocity experiments in which the variation in 

 response with intensity of radiation is measured for constant energy. 

 Further information is given by variation in response with change in 

 energy. 



Reciprocity holds in simple photoreactions from short times limited 

 by rate of energy supply to times of the order of the half-life for back- 

 reaction if a product of the system reverts in darkness and is not limited 

 by a nonphotosensitive reactant. If a biological response is involved, 

 reciprocity will hold only over a period in which biological change, such 

 as growth, is small. Thus reciprocity might be expected to hold in the 

 photoperiodic systems from fractions of a second to an hour or more, as 

 has been observed for control of floral initiation of soybean and cocklebur 

 (Parker et al, 1946) and for leaf lengthening in pea (Parker et al., 1949). 



In seed germination, reciprocity is limited by the rate of change of 

 sensitivity of the seed to radiation and by the rate of germination. Devi- 

 ations from reciprocity for germination of light-sensitive lettuce seed and 



