Photoperiodism
The physiological response of plants (and other organisms) to the relative length of light and dark periods in a 24-hour cycle. Controls flowering, bulbing, dormancy, and other developmental transitions in many vegetable and fruit crops.
Photoperiodism is the mechanism by which plants respond to the relative lengths of light and dark periods in each 24-hour cycle. It’s the biological system that allows plants to track season by measuring day length rather than temperature - a more reliable indicator because temperature fluctuates unpredictably while daylength changes predictably with the calendar.
The term encompasses the entire system: the photoreceptor molecules that detect light, the signaling pathways that transmit photoperiod information, and the developmental responses (flowering, bulbing, dormancy) that result.
The Phytochrome System
The primary photoreceptors for photoperiodism are phytochromes - pigment proteins that exist in two interconvertible forms:
Pr (phytochrome red): Absorbs red light (660nm) and converts to Pfr.
Pfr (phytochrome far-red): The active signaling form. Absorbs far-red light (730nm) and converts back to Pr, or slowly reverts to Pr in darkness.
During the day, red light from sunlight converts Pr to Pfr, which accumulates. At night, Pfr slowly reverts to Pr in the absence of light. The plant’s circadian clock measures how long this reversion takes - giving the plant a way to measure night length.
For practical purposes: the plant is measuring the length of uninterrupted darkness, not day length. A brief light interruption during the night (even a few minutes) can reset the measurement, preventing a short-day response in short-day plants or allowing continued vegetative growth in long-day plants.
The Three Photoperiod Response Types
Short-day plants flower when night length exceeds a critical threshold (typically 11-13 hours of darkness). Strawberries (June-bearing types), chrysanthemums, and short-day onions are examples. In temperate gardens, these flower in fall when nights lengthen.
Long-day plants flower when night length falls below a threshold (when days exceed 14-16 hours of light). Spinach, lettuce, long-day onions, and most grains are long-day plants. In temperate gardens, these transition to reproductive growth in late spring and early summer.
Day-neutral plants flower regardless of photoperiod. Most warm-season vegetables - tomatoes, peppers, cucumbers, squash - are day-neutral.
Practical Applications
Why onion selection by latitude matters: Long-day onion varieties are calibrated to bulb when summer daylength reaches 14-16 hours, which occurs only at northern latitudes. Planting them in the South produces non-bulbing or poorly bulbing results.
Why spinach bolts in summer: The increasing daylength past spinach’s critical threshold triggers the shift from vegetative to reproductive growth. Slow-bolting varieties have elevated thresholds; all will eventually respond to extreme summer day lengths.
Why strawberry flowering depends on planting type: June-bearing varieties measure the short days of fall and winter to prepare flower buds that open in spring. Day-neutral varieties produce flowers regardless of daylength, enabling continuous production.
Night pollution effects: Light from streetlights, security lights, or grow lights during the dark period can disrupt photoperiod responses in sensitive crops. A strawberry bed directly beneath a bright security light may have impaired flower bud initiation in fall.
Photoperiodism is also why growers can manipulate plant development with artificial lighting: extending days with grow lights delays flowering in short-day plants and accelerates it in long-day plants. This is commercially exploited in chrysanthemum and cannabis production.
See also: Short-Day, Long-Day, Day-Neutral, Photoperiod