Several stages of plant development occur at certain times of the year.

The flowering season, for example, is characteristic for each species: it is common to hear that such a plant blooms in August, another in September, and so on.

How do plants know when to bloom?

The environmental stimulus that plants use most often is the photo period, that is, the relationship between the duration of days (light period) and nights (dark period). The physiological response to this relationship is called photoperiodism.
According to the way photoperiodism affects flowering, plants can be classified into three main types: short day plants, long day plants and indifferent plants.

Short Day Plants

Short-day plants are those that bloom when the length of the night (dark period) is equal to or greater than a certain value, called the critical photoperiod. Short-day plants bloom in late summer, autumn or winter.

Long day plants

Long-day plants bloom when subjected to periods of darkness shorter than the critical photoperiod. Plants of this type, lettuce being an example, bloom in late spring or summer. For some plants a single exposure to the inducing photoperiod is sufficient to flourish, while others require several successive days of suitable photoperiods.

Some plants respond to the photoperiod only after receiving some other type of stimulation. Winter wheat, for example, will not bloom unless exposed for several weeks to temperatures below 10 ° C. This need for cold to bloom or a germinating seed is common to many temperate plants, called vernalization. If, after vernalization, winter wheat is subjected to shorter inducing periods than the critical photoperiod, it will flourish.

Indifferent plants

There are plants that bloom regardless of the photoperiod. In this case, flowering occurs in response to other stimuli. Tomato and string bean are examples of indifferent plants.

Phytochromes and Light Perception

The fact that plants respond to light stimuli means that they are able to perceive light. The photoreceptor involved in photoperiodism, as well as many other types of light response, is phytochrome, a blue-green protein.

Phytochrome Types

Phytochrome exists in two interconversible forms, one inactive, called phytochrome R, and another active, called phytochrome F. Phytochrome R Red, red) becomes phytochrome F far-red, long red) when absorbing red light of wavelength in the range of 660 nanometers. Phytochrome F, in turn, becomes phytochrome R by absorbing red light of wavelength in the range of 730 nanometers (longer wavelength red).

Sunlight contains both wavelengths (red and long red). Therefore during the day the plants present the two forms of phytochrome (R and F), with predominance of phytochrome F. At night, the more unstable phytochrome F becomes spontaneous phytochrome R. Depending on the length of the dark period , this conversion may be total, so that the plant after a long period of darkness may only have phytochrome R.

Role of phytochrome in flowering

In short-day plants phytochrome F is a flowering inhibitor. Short-day plants bloom at seasons when the nights are long, because during the prolonged period of darkness phytochrome F spontaneously becomes phytochrome R, no longer inhibiting flowering. A brief exposure of light (about 10 minutes) during the dark period is sufficient to prevent short-day flowering of plants, as during this period phytochrome R is converted to phytochrome F.

In long day plants phytochrome F is an inducer of flowering. So long day plants only bloom if the dark period is not too long, so that there is no full conversion of phytochrome F to R. Already in seasons that nights are long, long day plants do not bloom, because all phytochrome F is converted to phytochrome R, which does not induce flowering.

Phytochromes and germination

Phytochromes are also involved in other plant physiological processes, including seed germination.
Seeds of various plant species need to be exposed to light to germinate. This is because germination is induced by phytochrome F, formed during the period of light exposure.