Photoperiod is the duration of light perceived by the plant. Light is perceived by the pigment phytochrome.
Phytochrome can exist in the red or far-red state. When exposed to red light (sunlight), phytochrome is converted to the far-red condition.
When exposed to far-red light or darkness, it reverts to the red condition. The time spent in the red condition tells the plant if it is under long or short days.
Phytochrome and photoperiod effects flowering, dormancy and germination in some plants.
(a) Empress tree (Paulownia) is a light sensitive seed that requires light to germinate. Examples of seeds germinated (c) with or (b) without light.
Photoperiod or daylength can be extended by using incandescent lamps, which are a good source of red light. This is commonly done to prevent flowering in short day plants like chrysanthemum and poinsettia. Stock plants used to collect cuttings may need daylength extension to prevent flowering and give a high number of vegetative cuttings.
Lighting may also be needed in the mist bed to prevent premature flower bud set. These lamps should be an outdoor type in case lamps get wet.
Blackout curtains can be pulled over a crop to simulate short days to induce or prevent flowering.
The opaque curtain has a white outer surface to reduce heat build-up under the curtain from solar radiation.