The photoperiod is the amount of light and dark in a 24-hour cycle. In the area of the equator - where the latitude takes a value of zero - it is constant and equitable, with 12 hours of light and 12 of darkness.
The response to the photoperiod is a biological phenomenon where organisms modify some of their characteristics - reproduction, growth, behavior - depending on the variation of light, the seasons and the solar cycle..
Generally, the photoperiod is usually studied in plants. It seeks to understand how variations in the lighting parameter modify germination, metabolism, flower production, the interval of the dormancy of the buds, or another characteristic..
Thanks to the presence of special pigments, called phytochromes, plants are able to detect environmental changes that occur in their environment.
According to the evidence, the development of the plants is affected by the number of hours received. For example, in countries with marked seasons, trees tend to reduce their growth in the autumn seasons, where the photoperiod becomes shorter..
The phenomenon extends to the members of the animal kingdom. The photoperiod is capable of affecting its reproduction and its behavior.
The photoperiod was discovered in 1920 by Garner and Allard. These researchers showed that some plants modify their flowering in response to changes in the length of the day.
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As we move away from this area, the times of light and dark change in response to the tilt of the earth's axis towards the sun..
When we move from the equator to either of the poles, the differences between light and dark are more marked - particularly at the poles, where we find 24 hours of light or darkness, depending on the time of year..
In addition, the annual rotation of the earth around the sun causes the photoperiod to change throughout the year (with the exception of the equator). Thus, the days are longer in summer and shorter in winter..
The ability to coordinate certain developmental processes with a particular time of year where there is a high probability that conditions will be more favorable confers a number of advantages. This occurs in plants, animals, and even certain fungi.
For the organisms it is advantageous to reproduce at times of the year where the juveniles do not have to face the extreme conditions of a winter. This, undoubtedly, will increase the survival of the offspring, providing a clear adaptive advantage to the group..
In other words, the mechanism of natural selection will favor the diffusion of this phenomenon in organisms that have acquired mechanisms that allow them to probe the environment and respond to changes in the photoperiod..
In plants, the length of days has marked effects on many of their biological functions. Below we will describe the main processes that are affected by the length of day and night:
Historically, plants have been classified into long-day, short-day, or neutral plants. The mechanisms of plants for the measurement of these stimuli are very sophisticated.
Currently, it has been determined that a protein called CONSTANS has a significant role in flowering, activated to another small protein that moves through the vascular bundles and activates a developmental program in a reproductive meristem and induces flower production..
Long-day plants flower more quickly only when exposure to light lasts for a certain number of hours. In these types of plants, flowering will not occur if the duration of the dark period is exceeded by a particular value. This "critical value" of light varies depending on the species.
These types of plants bloom during spring, or early summer, where the light value meets the minimum requirement. Radish, lettuce and lily are classified within this category.
In contrast, short-day plants require lower light exposures. For example, some plants that bloom in late summer, fall, or winter have short days. Among these, the chrysanthemums, the flower or Christmas star and some varieties of soy stand out..
Latency states are useful for plants, as they allow them to cope with unfavorable environmental conditions. For example, plants that live in northern latitudes use the reduction in the length of the day in autumn as a warning of the cold..
In this way, they can develop a dormant state that will help them cope with the freezing temperatures that are to come..
In the case of liverworts, they can survive in the desert thanks to the fact that they use long days as a signal to enter dormancy during arid periods..
Many times the response of the plant is not determined by a single environmental factor. In addition to the duration of light, temperature, solar radiation and nitrogen concentrations are often decisive factors in development..
For example, in plants of the species Hyoscyamus niger The flowering process will not occur if it does not meet the requirements of the photoperiod, and also of vernalization (minimum amount of cold required).
As we saw, the length of day and night allows animals to synchronize their reproductive stages with favorable times of the year..
Mammals and birds usually reproduce in spring, in response to the lengthening of the days, and insects usually become larvae in the fall, when the days get shorter. Information regarding the response to photoperiod in fish, amphibians and reptiles is limited.
In animals, photoperiod control is mostly hormonal. This phenomenon is mediated by the secretion of melatonin in the pineal gland, which is strongly inhibited by the presence of light..
Hormonal secretion is greater in periods of darkness. Thus, the photoperiod signals are translated into the secretion of melatonin.
This hormone is responsible for activating specific receptors located in the brain and in the pituitary gland that regulate the rhythms of reproduction, body weight, hibernation and migration..
Knowledge of the response of animals to changes in the photoperiod has been useful for man. For example, in livestock, various studies seek to understand how milk production is affected. So far it has been confirmed that long days increase said production.
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