Action Spectrum for Photoperiodic Control of Floral Initiation of a Long- Day Plant, Wintex Barley (Hordeum vulgare)

Wintex barley grown with an 11.5-hour photoperiod and a 12.5-hour dark period remained vegetative. If the dark period was interrupted with a brief period of irradiation of sufficient intensity, spikelet formation was stimulated. 2. By use of this technique of interrupting the dark period, quantitative data on the photoreaction that promoted flowering were obtained, and action spectra relating wave-length to photoperiodic effectiveness of light were derived. 3. The most effective time to apply the dark-period interruptions was the 2-hour period beginning 6.5 hours after the start of the dark period. Within this time and with the intensities used, the reciprocity law held. Energy required to promote flowering, if applied continuously throughout the 12.5-hour dark period, was about tenfold greater than if applied within the 2 hours near the middle of the dark period. 4. The action spectrum for the production of spikes in barley was very similar to the action spectra for the prevention of floral initiation in soybeans and in cocklebur. The long-wave-length cut-off beyond 7200 A, the positions of maximum effectiveness between 6000 and 6600 A, and the region from 5000 to 5600 A, in which effectiveness changed rapidly with small changes in wave-lengths, coincided very closely. Minimum effectiveness occurred near 4800 A for all three plants. 5. The elongation of barley stems was closely correlated with spike development. 6. The action spectra indicated that essentially the same pigment was involved in transferring energy to the photoperiodic reaction both in long-day and in short-day plants. 7. A working hypothesis based on the assumption that flowering both in long-day and in short-day plants is controlled by the same substance and that effectiveness is due to optimum concentration is discussed.