Photoperiodism

Photoperiodism in Plants

Plants respond to light in various ways including:

• Photomorphogenesis: the growth and development of plants in response to light by which plants can take full advantage of available light and space.

• Phototropism: the ability of plants to grow towards or away from the direction of light.

• Photoperiodism: the ability of plants to determine the time of day and year by detecting wavelengths of sunlight.

Photoperiod Definition

How do plants determine the time of day and year? A change in the length of a day is the environmental cue that tells a plant what time of the year it is.

How Does Photoperiodism Work?

A plant's ability to detect light in its environment is critical to its competitiveness and survival. Plants can detect and respond to blue, red, and far-red wavelengths of light through photoreceptors. These photoreceptors are made up of chromoproteins, each of which consist of a protein and a light-absorbing pigment called chromophore.

Phytochromes are chromoproteins that detect red and far-red light. Phytochromes are classified into two types:

• Pr (phytochrome red) which can absorb red light.

• Pfr (phytochrome far-red) which can absorb far-red light.

When Pr absorbs red light, it transforms into Pfr, and when Pfr absorbs far-red light, it transforms back into Pr. The absorption of red or far-red light changes the structure of the chromophore, which influences the conformation and activity of the phytochrome protein to which it is linked.

Put simply, red light initiates phytochrome activity whereas far-red light inhibits it. The two types of phytochrome, known collectively as the phytochrome system, function as a "biological switch".

Note that unfiltered, full sunshine includes significantly more red light than far-red light. This means that at dawn, phytochromes in a leaf would convert to the active Pfr and stay in that state until dusk. When there is no more sunlight, the Pfr gradually reverts back to Pr form, so when the night is lengthy, as it is in winter, the Pfr form completely reverts. If the night is short, as it is in the summer, a significant amount of Pfr may remain by the time the sun rises.

A plant can determine the length of the day/night cycle by detecting the Pr/Pfr ratio at dawn. Furthermore, by retaining this information for several days, a plant can compare the length of the night before with several other preceding nights.

• If it detects shorter nights, the plant determines that spring is approaching.

• If it detects longer nights, the plant determines that autumn is approaching.

Detecting the length of day or night along with temperature and water availability enables plants to determine the time of year and modify their physiology accordingly.

Responses to photoperiod include:

• Flowering

• Seed germination

• Bulb dormancy

• Color change (due to pigments called anthocyanins) in leaves.

Photoperiod Effect

Plants are affected by the length of day or night in varying ways. Here we will talk about descriptors you might encounter alongside photoperiodism: long-day, short-day, and day-neutral plants.

We will then talk about night length and how this is a more useful approach to understanding photoperiodism.

Short-day Plants

Researchers began to understand how plants detect seasons by observing a mutant variety of tobacco called the Maryland Mammoth which grew tall but was not able to produce flowers during summer. It finally bloomed in December, but under controlled conditions.

So what conditions caused this variety to bloom? Researchers experimented with different variables including temperature, moisture, and mineral nutrition. They found that the Maryland Mammoth only flowered if the photoperiod was less than 14 hours.

Because the Maryland Mammoth required a photoperiod shorter than critical length in order to flower, it is considered a short-day plant. Other examples of short-day plants include chrysanthemums and some varieties of soybeans. Short-day plants tend to flower in late summer, fall, or winter when the days are shorter.

Long-day Plants

In contrast to short-day plants, long-day plants flower only when the photoperiod is longer than critical length. These tend to flower in late spring or early summer. Examples of long-day plants include radishes and irises.

Day-neutral Plants

Not all plants use the phytochrome system in the aforementioned ways. Unlike short-day and long-day plants, day-neutral plants are not affected by daylength. Regardless of photoperiod, these plants flower when they reach a certain level of maturity. Examples of day-neutral plants include tomatoes, rice, and dandelions.

Critical Night Length

Researchers used to believe that the amount of daylight a plant received determined whether or not it produced flowers. Experiments suggested something else: turns out, the length of darkness that a plant was exposed to proved to have a more significant effect than the length of day.

By observing cockleburs–a “short-day” plant that flowers only when the photoperiod is less than 16 hours (and the night length is at least 8 hours long)– researchers found that flowering is controlled by night length rather than day length. Their observations can be summed up as follows:

• If the day length is interrupted by a brief exposure to darkness, the cocklebur will flower.

• If the night length is interrupted by a brief exposure to light, the cocklebur will not flower.

They noted that cocklebur is not actually responding to day length, rather, it is responding to the length of continuous darkness. The same was observed in other “short-day” plants.

This means that short-day plants can be more accurately described as long-night plants, whereas long-day plants can be described as short-night plants. Note that long-day and short-day plants are not set apart by an absolute night length. Instead they are distinguished based on whether they set a maximum or minimum night length:

• A long-day (or, short-night) plant has a maximum number of hours of darkness.

  • Night length cannot exceed a specific number of hours.

• A short-day (or, long-night) plant has a minimum number of hours of darkness.

  • Night length cannot go below a specific number of hours.

While they could be misleading, the terms short-day and long-day just happened to be more well-established in the world of Botany.

Importance of Photoperiodism in Plants

Photoperiodism helps plants detect seasonal changes. This is important because it enables plants to adapt to these changes, increasing their chances of survival and reproduction.

Photoperiodism Examples

Seasonal flowering as a response to change in day or night length helps plants adapt to their local environment. For example, some plants flower when their pollinators are most active to increase reproductive success.

Plants may also reproduce during springtime so that their offspring have enough time to develop before facing the harsh conditions of winter, which increases their survival rate.

Ground-level woodland plants may also flower in early spring so that they are able to complete seed production before the leaf canopy fully emerges and limits the available light.

Besides flowering, plant responses to photoperiod also include bud dormancy. For example, when certain trees and perennial plant species in northern latitudes detect shortening day length, they respond by inducing cold hardiness and bud dormancy as preparation for the incoming freezing winter temperatures.

Desert plants like liverwort, on the other hand, use longer days as a signal to go into dormant state so they have a higher chance of survival during the arid summer months.