The correct timing of flowering is essential to maximize reproductive success in angiosperms.
And many flowering plants rely on the photoperiod (specifically, the relative night length) as an environmental signal to tell seasonal time. (To see how, please see previous posts about How Plants Tell Time and Why Plants Tell Time.)
As mentioned in the previous post, the so-called “flowering hormone”, historically known as florigen, is likely a small protein called FT.
Briefly, FT is produced in the leaves and is transported via the phloem to the shoot apical meristem (SAM). Here FT acts like a molecular “alarm-clock”, evoking a complex genetic scenario, which culminates in flower formation.
But what sets off this “alarm-clock”, i.e. the production of FT in the leaves?
Turns out the story involves red, far-red, and blue light, the length of the night, and the plant’s biological clock. (Please note: Why night length is more important than day length: animated explanation.)
First some caveats:
1. Most of this information is based on genetic research using the plant Arabidopsis thaliana. (Although specific genes and proteins vary, depending on plant species, it appears that the basic story presented below holds for most photoperiodic flowering plants.)
2. Arabidopsis is a so-called long-day (LD) flowering plant (in reality, a “short-night” plant, but don’t get me started). So, adjustments in the story need to be made for so-called short-day (SD) plants. (Yes, they really are “long-night” plants.)
3. In Arabidopsis florigen is likely the FT protein. In some SD cereals (such as rice), florigen is likely a protein called Hd3a, an ortholog of FT protein.
A Light-Sensitive, Flowering Alarm-Clock
The so-called biological clock in plants is set primarily in the leaves by phytochromes, which are sensitive to red and far-red light. They get help from blue-light-sensitive cryptochrome. These photoreceptors interact with “clock-genes” that cause some proteins in plant cells to cycle with a circadian rhythm.
One of these proteins regulates the gene that codes for florigen (FT in Arabidopsis and Hd3a in rice, for instance).
Thus, florigen cycles in the leaves also with a circadian rhythm.
Briefly, in LD (“short-night”) plants florigen apparently peaks not long after sundown, then slowly degrades during the night. If the nights are too long, the florigen level is below the threshold level to induce flowering at dawn, when the leaves begin to transport material to the SAM via the phloem. (Please note: florigen appears to be synthesized primarily by leaf vein cells adjacent to the phloem.)
Conversely, in SD (“long-night”) plants, the florigen apparently peaks long after sundown. So, if the night is too short, at dawn, the florigen hasn’t exceeded the threshold level to trigger flowering.
For more information, click on image below:

References:
1. Zeevaart, J.A.D. (2007) FT Protein, not mRNA, is the Phloem-Mobile Signal for Flowering. (see here)
2. Bäurle, I. and Dean, C. (2006) The Timing of Developmental Transitions in Plants. Cell, vol. 125, pp. 655-664 (see here)
3. Greenup, A., et al. (2009) The molecular biology of seasonal flowering-responses in Arabidopsis and the cereals. Annals of Botany, vol. 103, pp. 1165-1172. (see here)























