Action Spectrum for the Photoperiodic Control of Floral Initiation of Short-Day Plants

The purposes of this investigation were to obtain quantitative data on the photoreactions that prevent flowering of short-day plants, from which an action spectrum relating wave length to photoperiodic effectiveness of light could be derived, and to draw such inferences concerning the nature of the photoreactions as the action spectrum would permit. 2. Experiments designed to give the action spectrum made use of a specially designed prism spectrograph having a dispersion of 15 A. per cm. at 5000 A. At this wave length and with an effective slit width of 100 A. the energy was about 3000 ergs per sq. cm. per second with the slit illuminated by a carbon arc operated at 12 kw. input. 3. Plants investigated were soybean, Soja max (L.) Piper var. Biloxi, and cocklebur, Xanthium saccharatum Wallr. To facilitate irradiation, the foliar surface of the plants of each species at the beginning of an experiment was reduced to a single leaflet or leaf, respectively. 4. The experimental treatment was based on the fact that flowering of short-day plants receiving short photoperiods can be prevented by interrupting each of the accompanying long dark periods at or near the middle with a brief period of irradiation. In these experiments minimal energies required to prevent floral initiation were determined for many narrow regions of the spectrum. 5. The reciprocity law was tested and found to hold for the factors of time and intensity within the ranges used for dark-period interruption. Flowering response of both soybean and cocklebur depends on the total radiant energy used to interrupt the dark period. 6. Minimal energies necessary to prevent flowering depend upon the number of hours of darkness elapsing before the plants are irradiated but are constant over a 2-hour period beginning 30 minutes before the middle of the dark period. 7. Action spectra for soybean and cocklebur are similar in several respects. The limit of effectiveness of radiation for preventing flowering of both plants is near 7200 A., at the red end of the spectrum. Maximum effectiveness for both plants occurs over a broad region extending from about 6000 to 6800 A. A minimum of effectiveness of radiation for both plants occurs in the region of 4800 A., and effectiveness increases again at shorter wave lengths in the visible portion of the spectrum. 8. Ratios of energies required to prevent flowering in the regions of maximum and of minimum effectiveness were different for the two plants, being about 1:200 for cocklebur and 1:60 for soybean. 9. The possibility that chlorophyll may be the effective pigment in the photoperiodic reaction is examined.