How Photoperiodism Works in Flowering Plants

Photoperiodism is the influence of the relative lengths of day and night on the activities of an organism. Although the best known example of photoperiodism can be found in flowering plants and many other responses in both animals and plants are regulated by day-length, i.e. the duration of photoperiod.

In studying photoperiodism,  we observe that the influence of the photoperiod on flowering can be demonstrated by exposing certain plants to a brief flash of light in the middle of the night. If this treatment is continued for a sufficient number of days, flowering will be delayed. This is made use of by horticulturalists in guaranteeing a supply of plants like chrysanthemume and poinsettias at Christmas. Such plants can be made to flower early by giving them extra darkness. In some cases one long night is all that is necessary.

With certain other plants, petunias for example, the reverse is true: light treatment induces early flowering whereas dark treatment delays it. It should be noted that adjustment of the photoperiod, early and late varieties of plants can be made to flower simultaneously, thereby enabling plant breeders to cross them.

On the basis of their deferring responses to light and dark, flowering plants can be divided into three groups:

1. Those that require long days and short nights, long-day plants, for instance, petunias, spinach, radishes and lettuce. Long-day plants flower only when the light period exceeds a certain critical length in each 24-hour cycle. This varies, but on average is about 10 hours.

2. Those that require short days and long nights, short-day plants, for instance, chrysanthemums, poinsettias, cockleblur. Short-day plants flower only when the light period is shorter than critical length in each 24-hour cycle. For cocklebur this is 141/2 hours.

3. Those that are indifferent to day-length, day-neutral plants, for instance, tomato and cotton.

There is no hard and fast dividing line between long and short-day plants: all gradations between the two exist. Not surprisingly, long-day plants tend to flower in the summer, or to inherit temperature regions where days are long and nights are short. Short-day plants, on the other hand, tend to flower in the winter and spring, or to live nearer the equator. For both long- and short-day plants, the critical factor influencing flowering is not the length of the light period but the length of the dark period. Thus long-day plants can be induced to flower by nights that are shorter than a critical length, and short-day plants can be induced to flower by nights that are longer than a certain critical length.

What is Photoperiodism?

Photoperiodism is the ability of plants to detect the length of day and year based on wavelengths of sunlight. This knowledge of time of year triggers a variety of biological and behavioural changes in plants including flowering, bud dormancy, migration, hibernation, seasonal breeding, and sexual behaviour.

Plants are classified as short-day, long-day, or day-neutral based on their response to this external cue, which must overlap with internal circadian rhythms in order for flowering to occur.

What is Photoperiodism?

Photoperiodism is the innate ability of plants and animals to measure environmental day length by monitoring light conditions. This allows organisms to develop seasonal adaptations and is a key factor in plant growth and flowering, seed germination, fruiting, bud dormancy, and other life cycle events. In plants, this is primarily accomplished through the action of photosensitive pigments (phytochrome) in the HPG axis that respond to the length of the dark period by changing the expression and secretion of other hormones such as melatonin. The melatonin response is inversely correlated with the nightly duration of darkness, and thus serves as an endogenous “clock” that indicates the length of the dark period.

Photoperiodic responses to changes in day length are important for plants because they help to adapt to the environmental fluctuations that occur as the earth orbits the sun. For example, the flowering of many plants depends on photoperiodism and enables them to produce flowers during times when pollinators are most active. This synchronization of reproduction with the season helps to ensure that seeds are dispersed during optimal conditions.

In addition to influencing flowering, other photoperiodic responses include bud dormancy, bulb or tuber initiation, and the development of root systems. These responses to day length help species adapt to particular habitats and to synchronize their reproductive behaviors with other members of their populations.

In addition to plants, some animals display photoperiodic behaviors as well. For example, the breeding cycles of some birds and hamsters are regulated by their photoperiodism and depend on the length of the daylight to determine the appropriate time to reproduce. The testes of hamsters, for example, are smaller in short days and larger in long days, which is an indication of their photoperiodic behavior.

How Does Photoperiodism Work?

As the Earth rotates and the seasons change, the length of daylight varies. Plants that are photoperiodic use this natural cue to know when it’s time for them to flower.

Almost all plants can photosynthesize, which is the process of creating sugar molecules using light energy. However, some plants are able to do much more than that. They can also use this energy to regulate their growth and development, such as determining the season in which they will flower or fruit.

Photoperiodism is a type of circadian rhythm, which are patterns in gene expression and physiology that repeat on a 24-hour cycle. This type of response is triggered by external cues, such as changes in the length of daylight.

For plants to be able to tell the time of year, they must have a precise sensor for day length that is accurate enough to detect a progressive change over long or short periods of time. In addition, this sensor must be able to distinguish between different conditions of the environment, such as temperature.

Many flowering plants use a special protein called phytochrome to sense day length. Phytochrome is composed of two molecules, Pr and Pfr. During the day, Pr is active and converts to Pfr, which absorbs far-red light. During the night, Pfr slowly reverts back to Pr. The ratio of Pfr to Pr is then used by the plant to determine the length of the day.

Other types of organisms, such as animals, are also able to respond to changes in the duration of daylight. For example, birds can be induced to migrate out of their usual breeding and migration patterns by artificially manipulating the duration of darkness.

What Plants are Photoperiodic?

Plants have a variety of growth and development responses to the amount and wavelength of light they receive. These include phototropism (growth towards or away from the light) and photoperiodism, which regulates flowering and other developmental processes in response to changes in day length.

Different species of plants have specific day length requirements to trigger the transition from vegetative to reproductive phases. Botanists group these plants into categories based on their preferred day length for floral induction, with some species needing long days to bloom and others requiring short days. These categories are used to classify the plants and provide the appropriate growing conditions for each type of plant.

In addition to affecting flowering, photoperiodism also affects other developmental and growth processes, such as bud dormancy and seed germination. The ability to perceive day length and respond accordingly helps plants adapt to their environment by synchronizing their growth with seasonal changes in the environment.

For example, most kalanchoes, with the exception of one species, bloom in winter, so they are considered to be short-day plants. The kalanchoes that do bloom in summer are classified as being long-day plants.

In addition to flowering, photoperiodism is responsible for a variety of other biological and behavioural changes in plants and animals. These changes include changes in fur and feather colour, migration, entry into hibernation, sexual behaviour and even the resizing of body parts. Knowledge of day length is also crucial for animal species that live at high latitudes, as it allows them to time their breeding so that their babies will be born at a season when food and other resources are available. For example, polar bears use photoperiodism to time their breeding so that their young are born during the warm summer months when it is safe for them to survive.

What Plants that are Day-Neutral?

Plants have evolved a way to measure the time of day and night by looking at light wavelengths. The phytochromes Pr and Pfr are photoreceptors that can absorb different wavelengths of light. When sunlight enters the leaf, the phytochromes convert to their active form of Pfr and remain in this state until dawn. During the day, sunlight is comprised of mostly red wavelengths and a small amount of far-red wavelengths. This gives the plant a sense of the length of the day by detecting the ratio of Pr to Pfr.

Most plants have either an obligate or a facultative response to photoperiodism. Obligately photoperiodic plants require a specific critical day length to start flowering. Examples of these plants are chrysanthemums, poinsettias and certain soybeans. Facillatively photoperiodic plants flower regardless of the length of the day. African marigolds, cosmos and zinnia are some of the common short-day plants.

In contrast, long-day plants don’t have a critical day length and flower naturally when they are exposed to the right conditions. Green grams, cotton, rice and some varieties of wheat are considered long-day plants.

Finally, day-neutral plants do not respond to the length of the day or darkness and instead bloom based on other factors. These plants are able to grow in a variety of climates and can be found in many gardens, restaurants, grocery stores and hospitals. Examples of day-neutral plants include roses, tomatoes and cucumbers. In general, it is best to keep plants with an obligate or a facultative photoperiodic response in a protected environment such as a greenhouse. This will prevent the plant from getting too tall and causing it to lose its ability to bloom.

What Animals that are Photoperiodic?

Photoperiodism is a biological response to a change in the proportions of light and dark in a 24-hour daily cycle. It allows plants to anticipate seasonal changes in their environment and coordinate their life functions with them. For instance, the timing of flowering in many plants is controlled by photoperiodism. Other phenotypes that are determined by photoperiodism include flower color, stem length and dormancy in many plants, seed germination, bulb formation, and the formation of spores in aquatic plants.

Animals that are photoperiodic, also known as seasonal animals, display changes in physiology and behaviour to adapt to predictable yearly changes in their environment. Photoperiodism influences seasonal changes in body mass, food intake and reproduction. Typical animal models for the study of photoperiodism are long-day breeding seasonal rodents such as hamsters, voles and sheep. Laboratory rats such as Fischer 344 (F344) are also suitable for studying photoperiodic responses, as they have been shown to respond to altered day length by altering their physiology and behaviour.

A major challenge in understanding photoperiodism is determining the precise time-measurement mechanism used by plant and animal organisms to assess and interpret gradual changes in day length and use them as an anticipatory cue for seasonal functions.

It is critical that this mechanism must be accurate and insensitive to unpredictable variations in the environment. For example, a change in day length must be accurately measured as it crosses the plant’s growing point and must not be affected by fluctuations in temperature.

The precise timing-mechanism of photoperiodism remains a mystery but it is likely to involve the plant pigment phytochrome, which has been linked to biological clock activity in plants and to the photoperiodic control of flowering.