26 • Photoperiodism: An Outline 



at 24° C or higher, but tended toward day-neutrality at about 

 13° C. Strawberry, Fragaria virginiana x chiloensis, shows a virtually 

 identical response (Went, 1957, Chap. 11). The requirement of 

 at least a flash of bright light for induction of Kalanchoe, men- 

 tioned previously, has been confirmed by Oltmanns (1960) at 20° 

 or 25°, but apparently is no longer present at 15° C, since Kalanchoe 

 will initiate flowers in total darkness at that temperature. 



The critical daylength for certain LDP is reduced at low 

 temperatures. Hyoscyamus niger grown at 28.5° C requires at least 

 1 1 Vo hours of light per day to flower, whereas at 15.5° the critical 

 daylength is reduced to 8 ]/ 2 hours (see Melchers and Lang, 1948). 

 However, the LDP Rudbeckia bicolor will flower at relatively high 

 temperatures (about 32° C) under photoperiods too short to permit 

 flowering under cool conditions; Rudbeckia speciosa, a similar 

 species, remains a true LDP under both conditions (Murneek, 

 1940). 



Most effects of this kind have been ascribed primarily to dark 

 period rather than light period temperatures (see Lang, 1952), 

 but unusual temperatures can modify both light and dark period 

 processes. Two of the early papers on Xanthium illustrate this 

 point. 



Long (1939) found that Xanthium required at least six cycles 

 of 9 hours light- 15 hours darkness for induction if the dark period 

 temperature was 5° C, even though the light periods were given 

 at 21° C. Further experiments showed that when plants were 

 grown at 21° light temperature and 5° dark temperature, the 

 ( ritical nightlength was increased to about 1 1 hours compared with 

 8% hours for plants held constantly at 21°. Long concluded that 

 "variations in temperature greatly affect the length of the critical 

 dark period," although his work has been cited, in a context to be 

 discussed later, as showing a "relatively temperature-independent 

 time measurement of nightlength" (Pittendrigh and Bruce, 1959). 



The light period processes in Xanthium also are temperature- 

 sensitive, at least when they are made relatively limiting (Mann, 

 1940). At least four hours of bright light (over 2000 foot candles) 

 are required for the optimum action of a subsequent dark period 

 if the light is given at 10° C, but only about one half hour of light 

 is required at 30° for the same effect. 



The sensitivity of light or dark periods to temperature changes 



