Photoperiodism And Vernalisation

Plants need the right signals from their environment to flower. Two major signals are the length of day/night (photoperiodism) and exposure to low temperature (vernalisation). These processes bring about flowering at the right season and improve survival and reproduction.

What is Photoperiodism?

Photoperiodism is the response of plants to the relative length of day and night, especially in relation to flowering. In 1920, scientists Garner and Allard performed experiments on variants of soybean and tobacco and concluded that the day length period has effects on the flowering of these plants.

Types of Photoperiodism

On the duration of the photoperiod, plants can be classified into five categories.

Short Day Plants (SDP) or Long Night Plants

A relatively short (8-10 hours) daylight period and a continuous dark period (14-16 hours) are required.
The dark period is critical. Inhibition of flowering occurs with a brief exposure to red light during the dark period; the effect is reversed by a subsequent exposure to far red light.
Interruption of the light period with red light does not inhibit flowering.
Prolonged dark period induce early flowering.

Examples: Biloxi variety of soybean, Maryland Mammoth variety of tobacco, etc.

Long Day Plants (LDP) or Short Night Plants

A longer daylight period (14-16 hours) and a shorter dark period (8-10 hours) are required.
The light period is critical. Brief exposure to light during the dark period does not have any inhibitory effect on flowering.
Prolonged light period stimulates early flowering.

Example: Hyoscyamus niger (Henbane), Beta vulgaris (sugar beet), etc.

Day Neutral Plants

Do not exhibit any sensitivity to light period.
Effective photoperiod ranges from 5-24 hours.

Example: Tomato, cotton, sunflower, etc.

Long Short Day Plants

These are SDPs that first need a long day period during early growth.

Example: Bryophyllum, etc.

Short Long Day Plants

These are LDPs that first need a short day period during early growth.

Examples: Wheat, rye, etc.

Photoperiodic Induction

Photoperiodic induction is the initiation of flowering under the influence of the photo-inductive cycle. Plants may require multiple inductive cycles for flowering; an appropriate photoperiod in a 24-hour cycle constitutes one inductive cycle. If a plant which has received the requisite number of inductive cycles is subsequently placed under an unfavourable photoperiod, it will still flower.

Flowering Stimulus

Flowering stimuli are produced in leaves and translocated to apical and lateral meristems, where flower formation is initiated. This flowering stimulus is a flowering hormone known as Florigen. The flowering stimulus is similar in long-day plants and short-day plants.

Significance of Phytochrome in Photoperiodism

Between the late 1940s and to early 1960s, scientists Sterling Hendricks and Harry Borthwick discovered the phytochrome pigment.
Phytochrome is a proteinaceous pigment present in the cytoplasm of plant cells, and has two components—chromophore and protein. The pigment is present in roots, stems, petioles, leaf blades, vegetative buds, flowers, seeds and developing fruits of higher plants.
Phytochrome exists in two photochemically interchangeable forms.

Red light-absorbing form (Pr) and far red light-absorbing form (Pfr)

Pr absorbs red light (660-665 nm) to convert to Pfr.

Pfr absorbs far-red light (730-735 nm) to convert to Pr.

In the dark, the Pfr form gradually changes into Pr form.

Explanation of the effect of phytochrome on the flowering of a short-day plant

Experimental observations show that brief exposure to red light during a critical dark period inhibits flowering in a short-day plant, and a subsequent exposure to far-red light can reverse this inhibitory effect.

During the daytime, the Pfr form of phytochrome accumulates in the plants, which has an inhibitory effect on the flowering of short-day plants.
During the critical dark period in short-day plants, this form gradually changes into the Pr form, stimulating flowering.
Brief exposure to red light will convert this Pr form again into Pfr form, inhibiting flowering.
The inhibitory effect can be reversed by subsequent far-red light exposure, because the Pfr form will again be converted back into Pr form after absorbing far-red light (730-735 nm).

Similarly, prolongation of the critical light period or the interruption of the dark period results in the accumulation of the Pfr form, thus stimulating flowering in long-day plants.

Importance of Photoperiodism

Helps in hybridisation experiments.
Plants can be classified by photoperiodism, which helps in the accurate farming of the flowering plants.
Physiological preconditionings could be implemented to induce flowering.

What is Vernalisation?

Vernalisation (Lt. vernalis, of the spring) was explained and termed by T.D. Lysenko in 1928. It is a process of physiological preconditioning to induce flowering in higher plants through exposure of plants (or seeds) to low temperatures. The shoot tips, embryo tips, and other meristematic regions of a plant readily perceive the stimulus of cold treatment for vernalisation, which is supposedly mediated by the hypothetical hormone vernalin.

Vernalisation only prepares the plant to perceive flowering stimuli and does not induce flowering. It is usually practised in regions where a plant does not grow naturally, and accelerates the process of plant breeding.
Plants that grow in a temperate climate (low winter temperatures) could be induced to initiate the flowering process in spring rather than autumn, to ensure reproductive development and seed production. Thus, winter varieties of crops can easily be converted to spring varieties.
The positive effects of vernalisation are reversible by subsequent exposure to high temperature—a process known as devernalisation.

Summary

Photoperiodism = response of plants to day length. It is controlled by phytochrome and the hormone-like stimulus florigen. Plants are classified as short-day, long-day, day-neutral, long-short-day, or short-long-day plants. Correct light/dark cycles trigger flowering; interruptions with red/far-red light can alter the response.

Vernalisation = low temperature treatment that prepares plants for flowering. The effect is reversible (devernalisation).

Both processes are vital for crop breeding, hybridisation, and agricultural planning.

Frequently Asked Questions (FAQs)

Q1. What is hybridisation?

Hybridisation is the process of combining the characteristics of two genetically different individuals to improve the quality of a crop. As photoperiodism plays an important role in determining the period of flowering of plants, it can affect the hybridisation effort between two plants having different photoperiods.

Q2. How does a plant perceive the stimulus for photoperiodism?

Plants receive the light stimulus through their leaves. The phytochrome pigment acts as the photoreceptor; the ratio of the two interchangeable forms of phytochrome triggers the synthesis of the florigen. Florigen then initiates the formation of floral buds after it is transported to the apical meristem.