Significance of Dark Reversion of Phytochrome in Flowering of Chrysanthemum morifolium

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Abstract
Flowering of chrysanthemum (Chrysanthemum morifolium Ramat.) is inhibited by a few hours of continuous or intermittent fluorescent or incandescent-filament illumination of low intensity. It is also inhibited by a 1-min fluorescent illuminance of a few hundred ft-c but not by one of several thousand from an incandescent-filament source. This difference in effectiveness of 1-min illuminances of fluorescent and incandescent light results from differences in their red-far-red energy ratios which are made even greater by chlorophyll screening in the leaf. Screening of red by chlorophyll tends to increase the amount of far red relative to red reaching phytochrome. Fluorescent light contains so little far red that screening does not cause a biologically significant change in the red-far-red ratio. In sunshine and incandescent-filament light, however, screening changes the ratio to such an extent that it can have striking biological consequences. The success of a 1-min fluorescent illuminance in preventing flowering shows that one conversion of phytochrome to Pfr each night is adequate, and the failure of a similar incandescent-filament illuminance indicates inadequate conversion of phytochrome to Pfr as a result of screening by chlorophyll in the red region of the spectrum. The peculiarly greater screening effects in chrysanthemum than in cocklebur are attributed to known differences in distribution of chlorophyll in their respective leaves.

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