22 • Photoperiodism: An Outline 



The interpretation of high-intensity light requirements in SDP 

 as basically photosynthetic is not entirely secure. Kalanchoe bloss- 

 feldiana is an SDP incapable of flowering in continuous darkness. 

 It will, however, initiate flowers if it receives one one-second flash 

 of light in every 24 hours (see Harder, 1948; Schwabe, 1959). 

 Although COo is indeed required during the light flash, it is not 

 likely that a great deal of photosynthesis takes place during that 

 time, so that a more specific requirement is at least suggested. 



Even the generalization that the photoperiodic responses of SDP 

 are generally promoted by at least some exposure to high-intensity 

 light does not hold for the widely studied Perilla. Using Perilla 

 crispa, de Zeeuw (1953) found that the critical daylength becomes 

 longer (dark requirement becomes shorter) as the main light period 

 intensity is lowered; with sufficiently low light intensities, flower 

 initiation occurs under continuous light. A set of experiments on the 

 complex interactions of bright and dim light periods on Kalanchoe 

 has been published by Krumwiede (1960), who also provides a 

 thorough bibliography on the question. It seems clear that probably 

 more factors than photosynthesis are involved in the effects of 

 bright light. 



2. Long-day Plants: Since, in general, the longer the light 

 period the better for flowering in LDP, analyses of the kind de- 

 scribed above have attracted little interest. A number of LDP are 

 nevertheless known to flower more rapidly in either continuous 

 light or long photoperiods if at least part of each light exposure 

 is at high intensity (see Bonner and Liverman, 1953). Much of the 

 work on the main light periods of LDP, like some of that on SDP, 

 has been on the effects of various light colors, and will be considered 

 in the next chapter. 



MUTUAL INTERACTIONS OF LIGHT AND 

 DARK PERIOD LENGTHS 



Extremely complex interactions between light and dark period 

 lengths have been observed in both LDP and SDP, to the extent 

 that the critical values of either light or dark periods are markedly 

 aflected by the lengths of the complementary periods. 



Claes and Lang (1947) studied the effects of various light and 

 dark schedules on the rapidity with which the LDP Hyoscya?nus 



