← Responses of plants and animals to their external environment

4 Photoperiodism: flowering triggered by day length

Every recent paper has put a photoperiodic plant in Question One or Two — poinsettia, kōwhai, or a parasite like dodder that flowers whenever its host does. The mechanism is the same phytochrome system every time.

Short-day vs long-day plants

  • A short-day plant (SDP) flowers when the uninterrupted NIGHT is LONGER than its critical night length — so it flowers in autumn/winter as nights lengthen (poinsettia is the standard example).
  • A long-day plant (LDP) flowers when the uninterrupted night is SHORTER than its critical night length — so it flowers in spring/summer as nights shorten.
  • The mechanism runs on night length, not day length, despite the name — this is why interrupting a long night with a flash of light can stop a short-day plant flowering.
  • The phytochrome pigment converts between an inactive form (Pr) in darkness and an active form (Pfr) in light; the balance between them, tracked over the uninterrupted dark period, is what the plant actually measures.
Worked example Forcing a short-day plant to flower out of season Poinsettia is a short-day plant. Explain how a grower could make it flower during a New Zealand summer (long nights are needed, but summer nights are short). Extend the plant's effective night by covering it to create an artificially long, uninterrupted dark period each day. That artificial night must exceed the poinsettia's critical night length for the phytochrome system to register it as "night", allowing Pfr to convert back to Pr uninterrupted. Alternatively, interrupting an already-long summer night with red light partway through would prevent SHORT-day flowering (converts Pr to Pfr) — so for forcing flowering the grower needs the opposite: an uninterrupted extended dark period. ✓ Cover the plant to extend the uninterrupted dark period past its critical night length, so the phytochrome system registers a "long night" even during a short-night summer. ✕

It is the night length that is measured, not the day length

Despite the name "photoperiodism", the critical measurement is the length of the UNINTERRUPTED DARK period, not the light period. A flash of red light in the middle of a long night resets the clock as if the night had ended — this is exactly why interrupting the night (not the day) is the tool growers use to control flowering.

Practise just this