Action Spectrum for the Photoperiodic Control of Floral Initiation of the Long-Day Plant Hyoscyamus niger

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Abstract
Vegetative plants of H. niger can be induced to flower by appln. of small amts. of radiant energy near the middle of dark periods that would prevent flowering if uninterrupted. Interruptions of 12-hr. dark periods with energies corresponding to about 100 ft.-candle-min. of light were effective in causing initiation of flower primordia. Min. energy to cause a given effect was relatively constant if the interruption was made during the 2-hr. period following the middle of the dark period. The action curve for floral initiation of Hyoscyamus niger was readily established in the region from 7300 A to 5600 A, Limitation of energy from 5600 A to 4000 A made it impossible to establish a continuous response curve for this region, but sufficient points were obtained to establish the limits of effectiveness. This curve was essentially the same as those for control of flowering in soybean, cocklebur, and barley and for regulation of leaf size in etiolated peas. Similarity of the action curve for control of initiation of flower primordia in Hyoscyamus to the absorption of phycocyanin was noted, and the possibility that the effective pigment for the photoperiodic reaction might be some type of straight-chain tetrapyrrole was pointed out.