PHOTOPERIODISM

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Plant Power: How Plants Sense the Arrival of Spring

Plants do not have brains or calendars, but they track seasonal changes using a phenomenon called photoperiodism—their ability to measure day and night length.

• The Light Switch (Phytochrome): Plants contain a light-sensitive pigment called phytochrome. It switches to an active form during daylight and slowly reverts back in darkness, allowing the plant to actively track the length of the night.

• The Internal Clock: Flowering and budding genes activate when these external light signals perfectly align with the plant's internal circadian clock.

• The Chemical Signal (CONSTANS & FT): When days reach a specific length, a protein called CONSTANS accumulates in the leaf tissue. This activates the gene FLOWERING LOCUS T (FT), which produces a small, mobile protein signal (historically predicted by botanists as "florigen").

• The Trigger to Bloom: This FT protein travels up through the plant's vascular tissue to the shoot apical meristem (the growing tip). There, it binds with a partner protein called FD, triggering the genetic shift that tells the plant to stop making leaves and start producing buds and flowers.

• Germination: Phytochromes also detect if a seed is near the soil surface or shaded by other plants, telling the seed exactly when it is safe to germinate.