scientific glossaryPhotoperiod · brief definitionOrigin: Yolia Secure Educational CatalogExact route: /library?path=glossary/photoperiod COMPLETE THEMEPhotoperiod · brief definition Name and classification Photoperiod is an environmental light variable, defined as the relative duration of the light and dark periods in a 24-hour cycle. It is classified within the environment and phenology, and in many plants it acts as a signal of the time of year. What is it It is the proportion between hours of light and hours of darkness that the plant receives each day. It is not the same as the total amount of light: two crops can receive the same light energy with different photoperiods, and the plant responds to both separately. Where and how it was studied It was discovered at the beginning of the 20th century when it was noticed that certain plants flowered depending on the length of the day. It is studied in growth chambers that vary the hours of light and darkness, and with genetics in model species, where photoreceptors and clock genes were identified. Why it matters for cultivation In sensitive species it can decide when flowering occurs, thus affecting the length of the cycle and the timing of harvests. Even in species indifferent to the photoperiod, the rhythm of light and darkness regulates the daily metabolism, so it is not advisable to alter it without reason. Photoreceptors — mechanism Phytochromes and cryptochromes absorb red, far-red and blue light, and change shape depending on the spectrum. That information is integrated with the circadian clock. In sensitive species, the coincidence between light and a phase of the internal rhythm activates or stops proteins that induce flowering. Mechanism of action The internal clock generates a rhythm of about 24 hours that is synchronized with sunrise and sunset. When the light signal coincides with a certain phase of this rhythm, genes are activated that produce a mobile signal towards the meristems, which go from vegetative to reproductive growth. Where it occurs in the plant Light is perceived above all in the leaves, where the signal of the day is integrated. From there, a signal travels through the phloem to the apex of the stem, the meristem, which responds by reorganizing its development. Photoreceptors also act in stems and seedlings. How it is regulated It is regulated by the circadian clock and the comparison between its phase and external light, in addition to the age of the plant and the temperature. In some species, a brief interruption of night counts as light, showing that continuous darkness is the key variable. What triggers/affects it Under natural conditions it changes with latitude and season. Indoors it is defined by the lighting program. Extraneous light at night, timer failures, or power outages can alter it without being noticed, and the spectrum and intensity of that light also plays a role. What other processes does it interact with? It interacts with temperature, which modulates the response, with hormonal regulators such as gibberellins, and with nutritional status and age. It also intersects with pre-flowering, the phenological window and the amount of light accumulated each day. Relevant differences between species in the catalog There are short-day, long-day and neutral-response species. Many fruiting vegetables tend to be less sensitive, while others flower depending on the length of the day. The exact answer depends on the species and variety, so it is worth reviewing each case in the catalog. How it is observed in practice It is observed by comparing the moment of flowering with the recorded hours of light, and by changes in stem elongation, growth habit and movement of leaves or flowers. A log of the space's lighting schedule is the most reliable way to know. What happens if there is a sudden alteration Sudden or irregular changes can delay or advance flowering, produce incomplete flowers or cause the plant to return to vegetative growth. They also generate stress due to imbalance in the internal rhythm. The severity depends on the species and the stage in which they occur. How Yolia Uses It Yolia considers the photoperiod along with the phenological stage before interpreting changes in plant behavior. It does not treat it as isolated data. Warns that sudden changes can affect the internal rhythm and that altering it without reason can confuse processes regulated by those rhythms. Common interpretation errors A common mistake is to think that more hours of light are always better. Another is to confuse photoperiod with intensity or total light. It is also assumed that all species respond the same, when many are neutral to the photoperiod and others depend on it. Evidence and limits of current knowledge The molecular basis is well described in model species and in some important crops. In other species less is known and extrapolation is cautious. The critical values of light hours vary depending on the variety, and in poorly studied crops the information is limited. Related topics and modules It is related to circadian rhythms, phenology, pre-flowering, flowering, the phenological window, light and its spectrum, night temperature and lighting programming in indoor spaces, where the on and off schedule defines the daily pattern. Related topicsPhotoperiod