Photoperiodism

Photoperiodism is the response of an organism to changes in the photoperiod, the relative lengths of day and night. In plants, photoperiod controls flowering, and in some species also dormancy and tuber formation.Away from the equator, day length changes with the seasons in a regular, predictable way, so it is a reliable signal of the time of year. Using it means that all the plants of a species in an area flower at the same time, which improves cross-pollination, and that flowering happens when pollinators are active and seeds can develop before winter.The dark period is what mattersExperiments showed that plants actually measure the length of the uninterrupted dark period, not the length of the day. Each species has a critical night length. If a long night is interrupted by a short flash of light (a night break), the plant responds as if the night were short.Plants measure night length using phytochrome. During the day P_fr builds up; during the night it slowly converts back to P_r. The amount of P_fr left at the end of the night therefore reflects how long the night was. A flash of red light during the night converts P_r back into P_fr, which explains the effect of a night break.FlorigenPhotoperiod is detected by the leaves, not the buds. The leaves then produce a flowering signal, called florigen, which travels in the phloem to the shoot apex and switches it from making leaves to making flowers.Plants are classed as long day, short day or day neutral.