Effect of Light and Temperature on Floral Initiation in Cocklebur and Biloxi Soybean

In cocklebur, the conditions brought about during the photoperiod may be diminished to such a point that plants fail to initiate floral primordia when subsequently subjected to a 12-hour dark period. The methods employed are the insertion between the photoperiod and the dark period of the photoinductive cycle of a given number of short alternating periods of darkness and light or of a given number of hours of continuous light of low intensity. When Biloxi soybean was treated in the same manner there was no reduction in the number of plants flowering, although there was a decrease in the number of nodes with flowers. 2. In Biloxi soybean, short cycles of light and darkness, or continuous light of low intensity, when inserted following the dark period of each photoinductive cycle, reduced considerably the number of plants flowering as well as the average number of nodes per plant with flowers. This response appears rhythmical, and there are indications that utilization of cycles other than those under which the plant has been growing may bring about ambiguous results. 3. Effective short cycles consisted of either 3 hours of darkness and 3 minutes of light or 2 hours of darkness and 2 minutes of light. 4. The use of short alternating periods of darkness and light may be a more economical method of maintaining "short-day" plants in a vegetative condition than the common practice of exposing them to continuous supplementary illumination of low intensity. 5. The intensity of the light during these short periods apparently affects the response; and in cocklebur the temperature prevailing during the cyclic treatment had little, if any, effect. 6. A range of conditions imposed during the photoperiod of a photoinductive cycle can be directly correlated with the extent or degree of floral initiation shown by cocklebur subjected to the various treatments. 7. Following eighteen short cycles, cocklebur may regain the capacity of being induced and develop flowers if subjected to three photoinductive cycles consisting of photoperiods of 2 or more hours at 2000 foot-candles or 10 hours of 100 foot-candles and 14 hours of continuous darkness. Moderate to high temperatures during the photoperiod and an increase in the intensity and duration of the light favor establishment of the conditions brought about by the photoperiod as measured in these experiments.