Shutterstock/Tatyana Soares
Mike Follows
Sutton Coldfield, West Midlands, UK
Plants have no calendar or brain, yet they can sense the arrival of spring through photoperiodism – their ability to measure day and night length. In many perennial plants, bud break is also strongly influenced by falling or rising temperatures, which interact with these light-based timing systems.
In many species, what matters most is the length of uninterrupted darkness. A light-sensitive pigment, phytochrome, switches to an active form in daylight and slowly reverts in darkness, allowing the plant to track night length as it shortens towards summer.
Flowering-related genes are activated only when external light conditions align with the plant’s internal circadian clock. In effect, the plant samples the day at a fixed internal time and asks: is it still light, or has night begun? When days become long enough, this timing allows a protein called CONSTANS to accumulate in leaf tissue.
CONSTANS activates the gene FLOWERING LOCUS T (FT), first identified in Thale cress (Arabidopsis thaliana). This produces a small, mobile protein signal, also called FT, that travels through the plant’s vascular tissue to the shoot apical meristem: the growing tip where new leaves and flowers form. There, FT binds with a partner protein, FD, triggering genes that shift the plant from leaf production to flowering or bud production.
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Plants have no brain, yet can sense the arrival of spring through their ability to measure day and night length
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For over a century, botanists predicted such a mobile flowering signal, calling it “florigen” before its discovery. FT is now recognised as a key component of this long-sought signal.
Seeds are also highly responsive to light cues. Through phytochrome, they detect light conditions that indicate whether they are near the soil surface and whether other plants are shading them. These signals help determine the right moment to germinate.
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