506 RADIATION BIOLOGY 



similar in their photoperiodic mechanism but quantitatively distinct. It 

 Avould follow that, in biochemical work undertaken for the purpose of 

 isolating dark-period products peculiar to photoperiodism, the best source 

 might be a short-night plant subjected to long nights. 



INFLUENCE OF ENVIRONMENTAL FACTORS 

 ON PHOTOPERIODIC RESPONSE 



Photoperiodic stimulus is a very important factor in the ecology of 

 plants and animals. This aspect of the subject has been discussed by 

 AUard (1928, 1932), Baker (1938), Baker and Ranson (1938), Biinning 

 (1948), Garner (1936), Henfrey (1852), Marshall (1936), Rowan (1938), 

 and Sircar (1938). It Avill not be considered here in the many aspects of 

 speciation and type of response, but rather attention will be restricted 

 to the causative factors. 



The variation in night length with latitude and season is of interest 

 (Fig. 10-5). At 10° from the equator the maximum variation in night 

 length is about 65 min. Even within this small variation, effects that 

 seem to be photoperiodic have been reported in plumage change of birds 

 (Moreau et al., 1947), breeding habits of lions and other mammals (per- 

 sonal communication from C. E. Kellog, of the U.S. Department of Agri- 

 culture, based on information obtained in the Belgian Congo), production 

 of rice (Kerling, 1950; Sircar, 1938), and flowering of sugar cane (Sar- 

 toris, 1939). The rate of change of night length is also smaller near the 

 equator than in higher latitudes. This rate of change might be an impor- 

 tant factor for the intermediate plant sugar cane, in which flowering 

 appears to depend upon whether the critical period is approached from 

 long days of summer or short days of winter (personal communication 

 from G. 0. Burr, of the Hawaiian Sugar Planters Association, based on 

 observations on sugar-cane production in Hawaii). 



Diurnal and seasonal temperatures and changes in temperature might 

 interact both with the final physiological expression and with trans- 

 location from the region of perception. Effects of temperature, within 

 the range of biological activity, on the primary photoreaction are unlikely 

 unless the concentration of the initially absorbing compound is temper- 

 ature-dependent. Lack of dependence of the initial photoreaction on 

 temperature during a short interruption of a dark period with light was 

 established for Kalanchoe hlossfddiana (Harder et al., 1944). Similarly 

 Parker and Borthwick in unpublished work found that temperatures 

 between 15° and 40°C applied during dark-period interruptions had no 

 significant effect upon response of Biloxi soybeans. 



Effects of temperature variation on the over-all photoperiodic response 

 have been measured for floral induction of soybean and bulbing of onion. 

 These temperature effects are largely on the dark-period reaction. Floral 



