PHOTOPERIOD: IN PLANTS AND ANIMALS - BIOLOGY - 2025

The photoperiod is the amount of light and dark in a 24-hour cycle. In the area of ​​the equator - where latitude takes a value of zero - it is constant and equitable, with 12 hours of light and 12 hours 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.

The photoperiod affects the germination of the seeds. Source: pixabay.com

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 other characteristics.

Thanks to the presence of special pigments, called phytochromes, plants are able to detect the environmental changes that occur in their surroundings.

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 slow down in the fall seasons, where the photoperiod is 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.

Why does the photoperiod occur?

As we move away from this area, the times of light and dark change in response to the tilt of the earth's axis toward 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, days are longer in summer and shorter in winter.

Advantages of responding to the photoperiod

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.

Photoperiod in plants

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:

Flowering

Historically, plants have been classified as long-day, short-day, or neutral plants. Plant mechanisms for measuring 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 and short-day plants

Long-day plants flower more quickly only when exposure to light lasts for a 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 in this category.

In contrast, short-day plants require lower light exposures. For example, some plants that bloom in late summer, fall, or winter are short-day. Among these, chrysanthemums, the Christmas flower or star and some varieties of soy stand out.

Latency

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 reduced day length in fall as a warning for cold.

In this way, they can develop a dormant state that will help them cope with the freezing temperatures to come.

In the case of liverworts, they can survive in the desert because they use long days as a signal to go into dormancy during arid periods.

Combination with other environmental factors

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 usually 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).

Photoperiod in animals

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 melatonin secretion.

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.

References

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