The Reaction Controlling Floral Initiation

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Abstract
The reversible photoreaction and an accompanying dark reaction demonstrated for lettuce seed were found also to control floral initiation of cocklebur (Xanthium saccharatum). All leaves except one were removed from young plants, which were placed on 12-hr. dark periods and 12-hr. light periods for 2 or 3 cycles. The dark periods were interrupted by radiation in various wave-length bands between 4000 and 10,000 A, after which the plants were returned to 8-hr. dark periods and 16-hr. light periods. The terminal meristem was dissected 12 days later. Under these conditions the plants remained vegetative if flowering was not previously induced by long nights. Radiation at wave-lengths greater than 7200 A, was ineffective in suppression of floral initiation of cocklebur. If the plants were irradiated first with unfiltered radiation from an incandescent-filament lamp adequate to prevent floral initiation, then on the spectrograph with radiation in the region 7215 to 7450 A, floral initiation occurred.o Application of various wave-lengths from 6820 to 8300 A showed that max. response with respect to promotion of flowering was in region of 7215 to 7450 A, which was closely equivalent to results for lettuce seed germination. The various action spectra for regulation of floral initiation depend on the integrated absorptions of the two compds., Pigment and Pigment X, and while closely similar, are not identical. For floral initiation of cocklebur, as sensitivity to inhibition by red radiation increased, sensitivity to promotion of flowering by infrared decreased. In another test the critical length of the dark period required for floral initiation of cocklebur was changed. The light period for a group of vegetative plants was extended for 30 min. with red radiation (using a 4500[degree] white fluorescent lamp filtered by several layers of red cellophane) or infrared radiation (using sunlight through a 5.0 mm. thick Corning red-purple ultra filter). Various lots of these plants were given dark periods of 7 durations from 7 to 10 hrs. at 30-min. intervals. Control lots were sub-jected to the same dark periods as those irradiated in the red. It was found that red radiation applied at the end of the light period lengthened the critical dark period from 8.5 to 9 hrs. whereas infrared shortened it to less than 7 hrs. These results are explained on the basis that red radiation at the start of the dark period displaces the reaction to the right, lengthening the required dark period, whereas infrared displaces the reaction to the left, shortening the required dark period.

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