Photoperiod
/ˈfoʊtəˌpɪriəd/
Definition
Photoperiod refers to the length of light and darkness a plant receives in each 24-hour cycle. It is a biological timing cue, not just the number of daylight hours, because many plants respond especially to uninterrupted darkness. This signal helps regulate flowering, dormancy, tuber formation, and other seasonal responses. Plants are often grouped as short-day, long-day, or day-neutral according to how their growth responds to photoperiod.
Biological Role
- Flowering signals: In many species, photoperiod helps switch plants from leafy growth to flowering. Long-day plants bloom when days are above a critical length, while short-day plants flower when nights are long enough.
- Night length matters: Plants often respond more strongly to uninterrupted darkness than to daylight hours alone. Brief light exposure during the night can delay or prevent flowering in some short-day species.
- Seasonal timing: This cue helps plants match flowering, dormancy, and bud set with predictable seasonal change. That timing can improve pollination success and reduce damage from cold or drought.
- Horticultural control: Growers use blackout cloth or supplemental lighting to manage flowering in crops such as chrysanthemum and poinsettia. This allows scheduling for markets or more reliable production under greenhouse conditions.
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