Quote:

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    *  Regulation of Photoperiodic Flowering by Arabidopsis Photoreceptors
    * Author(s):Todd Mockler, Hongyun Yang, XuHong Yu, Dhavan Parikh, Ying-chia Cheng, Sarah Dolan and Chentao Lin
    * Source: Proceedings of the National Academy of Sciences of the United States of America, Vol. 100, No. 4 (Feb. 18, 2003), pp. 2140-2145
    * Published by: National Academy of Sciences
    * Stable URL: http://www.jstor.org/stable/3138498

Abstract
Photoperiodism is a day-length-dependent seasonal change of physiological or developmental activities that is widely found in plants and animals. Photoperiodic flowering in plants is regulated by photosensory receptors including the red/far-red light-receptor phytochromes and the blue/UV-A light-receptor cryptochromes. However, the molecular mechanisms underlying the specific roles of individual photoreceptors have remained poorly understood. Here, we report a study of the day-length-dependent response of cryptochrome 2 (cry2) and phytochrome A (phyA) and their role as day-length sensors in Arabidopsis. The protein abundance of cry2 and phyA showed a diurnal rhythm in plants grown in short-day but not in plants grown in long-day. The short-day-specific diurnal rhythm of cry2 is determined primarily by blue light-dependent cry2 turnover. Consistent with a proposition that cry2 and phyA are the major day-length sensors in Arabidopsis, we show that phyA mediates far-red light promotion of flowering with modes of action similar to that of cry2. Based on these results and a finding that the photoperiodic responsiveness of plants depends on light quality, a model is proposed to explain how individual phytochromes and cryptochromes work together to confer photoperiodic responsiveness in Arabidopsis.

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Here also is a graph from a study done in the late forties involving floral repression at differing spectra:


The grey band indicates the blue area of the spectrum.