Time Relations and Endogenous Rhythms • 51 



FLOWERING IN LIGHT-DARK CYCLES OF DIFFERENT 

 LENGTHS; TEMPERATURE INTERACTIONS 



If a circadian rhythm regulates photoperiodic responses, 

 normal flowering should depend upon light-dark alternations of 

 about 24 hours. Schmitz (1951) using Kalanchoe and Schwabe 

 (1955a) using Kalanchoe, Xanthium, and an SDP variety of Im- 

 patiens balsamina, concluded against Bunning's theory on the 

 grounds that cycles with total lengths ranging from 15 to 50 hours 

 proved inductive, with any failures to flower attributable to the 

 length of either the dark or light periods but not to the periodicity 

 of the cycles. Schwabe also criticized the extensive use of leaf- 

 movements as indicators of the endogenous rhythm, since the 

 photoperiodic response is often insensitive to conditions which 

 may completely obscure the leaf movements. Calling attention to 

 the remarkable plasticity of both the endogenous rhythm and 

 Bunning's theory based on it, Schwabe questioned the value of the 

 latter in explaining photoperiodism and asked Bunning to "define 

 the sort of experimental result which he would regard as in- 

 compatible with it." 



In contrast to the results of Schmitz and Schwabe, cycle-length 

 experiments show clear quantitative effects on the flowering of 

 soybeans (Blaney and Hamner, 1957; Nanda and Hamner, 1958, 

 1959). Cycles totaling 24, 48, or 72 hours in length are far more 

 favorable to flowering than, for example, 36- or 60-hour cycles, 

 although neither of these most unfavorable cycles are completely 

 inhibitory. This certainly supports the concept of a circadian 

 rhythm in sensitivity to light and darkness. Finn and Hamner 

 (1960) have also published a group of experiments with Hyoscya- 

 mus in which the total length of the light-dark cycle appears to be 

 a major controlling factor. For example, with a 10-hour light 

 period, flowering was most rapid with a total cycle length of 18 

 hours (with an 8-hour dark period), slowest or absent with a total 

 cycle length of 24-30 hours (14- or 20-hour dark period), and faster 

 again with a 42-hour cycle length (32-hour dark period). Such 

 results may also be used to support a rhythm-based theory of 

 photoperiodism. 



Further experiments with soybeans (Blaney and Hamner, 1957) 



