Phytochrome: Definition, Meaning, Examples, Function,Types & Forms

What is Phytochrome?

Phytochrome is a photoreceptor pigment of plants, absorbing chiefly the red and far-red wavelengths of visible light and is involved in the control of most numerous particularities of plant growth and development. It controls photomorphogenesis, a development process in plants by which form and structure are determined by light. It is needed for the germination of seeds, elongation of stems, and the expansion of leaves and regulates the time of flowering. This way it will help plants adapt to the light environment to have appropriate growth and reproduction as well.

Structure of Phytochrome

The structure of phytochrome is described below-

Chemical Structure

The phytochromes are a family of photoreceptors made up of a protein moiety linked to a light-absorbing chromophore called phytochromobilin.

Active and Inactive Forms

The two interconvertible forms of the phytochromes are the red light-absorbing form.

• Pr (inactive) – absorbs red (660 nm)

• Pfr (active) – absorbs far-red (730 nm)

Pr ↔ Pfr Conversion Mechanism

This mechanism controls several light-dependent processes in plants.

Under Red Light

• Pr to Pfr

• Promotes germination, leaf expansion, flowering (in LDP)

Under Far-Red/Shade

• Pfr to Pr

• Promotes elongation (shade-avoidance response)

Dark Reversion

• Pfr slowly reverts to Pr in darkness

Types of Phytochromes

The types of phytochrome is described below-

Phytochrome A (PhyA)

This phytochrome is most sensitive to continuous far-red light. It plays a critical role in seedling development under shade from the canopy.

Phytochrome B (PhyB)

PhyB normally absorbs red light and performs different regulatory activities such as seed germination and shade avoidance.

Phytochromes C, D, E

The phytochromes C, D, and E have partly overlapping functions with PhyA and PhyB, thus giving an idea about the fine-tuning effects on the response of plants toward light.

Functions of Phytochrome

The functions of phytochrome are described below-

Seed Germination

It refers to how phytochromes perceive the light signals for seed germination, where Pr is converted into Pfr which will break dormancy and promote growth.

Stem Elongation

Etiolation and De-Etiolation: In dark situations, phytochromes are in the Pr form and promote etiolation, i.e., greater elongation of the stem. On exposure to light, Pr is converted into Pfr, and de-etiolation is followed by normal growth of the stem.

Leaf Expansion and Chlorophyll Development

Phytochromes regulate the expansion and unfolding of leaves. So, therefore, it is done by regulating the size and shape of leaves according to light.

Flowering Regulation

The role of phytochromes as major regulators of time to flowering allows the plant to have a way it can be capable of flowering at the right moment through regulating the length of a day, photoperiod.

Shade Avoidance Response

Plants' response to the changes in light quality created by shading because of their phytochromes is by elongation of stems and leaves to grow above other plants to receive more light.

Differences Between Pr and Pfr

The difference between Pr and Pfr is given in the table below:

Applications in Agriculture

The difference between Pr and Pfr is given in the table below:

• Manipulating flowering time

• Controlling seed germination

• Shade-tolerant crop development

• Greenhouse light management

Phytochrome NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Types of Plants based on photoperiodic response

• Long-day plants vs Short-day plants vs Day-neutral plants

• Mechanism of Photoperiodic flowering

Practice Questions for NEET

Q1. Which of the following plants belong to long short Day plants (LSDP)?

1. Hyoscyamus niger

2. Bryophyllum cestrum

3. Oryza sativa

4. Zea mays

Correct answer: 2) Bryophyllum cestrum

Explanation:

Long short Day plants (L-SDP) -

Plants require long photoperiods for floral initiation and short photoperiods for blossoming. When they get more than 12 hours of light per day—typically 14–16 hours—they bloom. Another name for them is short night plants. Plants flower between summer and autumn. Eg. Bryophyllum Cestrum

Hence, the correct answer is option 2) Bryophyllum cestrum.

Q2. Photoperiodism is the

1. Response of plants to the changing environmental conditions

2. Response of plant to the low temperature treatment expressed in the form of flowering

3. Response of plants to the photoperiod expressed in the form of flowering

4. Response of plants to the quality of light expressed in the form of seed germination

Correct answer: 4) Response of plants to the photoperiod expressed in the form of flowering

Explanation:

The response of plants to the photoperiod expressed in the form of flowering is called photoperiodism. Photoperiodism is controlled by photoreceptor proteins like phytochromes and cryptochromes, which sense the duration of light and dark periods. Plants are classified based on their flowering response into short-day, long-day, and day-neutral plants. This mechanism helps plants synchronize their reproductive cycles with favorable environmental conditions.

Hence, the correct answer is option 3) the response of plants to the photoperiod expressed in the form of flowering.

Q3. Long day plants produces flowers when they exposed to

1. Any duration of light

2. Light period longer than a critical day length

3. Light period longer than 12 hours

4. Light period shorter than a critical day length

Correct answer: 2) Light period longer than a critical day length

Explanation:

The critical photoperiod is the length of time that must be exposed to light for some plants to flower. This varies among species and is important for their reproduction.
Key Terms
Long-Day Plants: These plants need a light period longer than a certain number of hours (the critical photoperiod) to begin flowering. They tend to bloom in late spring or early summer when days are longer.
Examples: Some common long-day plants are:
Spinach
Lettuce
Some Wheat Types
Light exposure past the critical photoperiod causes these plants to begin producing flowering hormones, causing the transition from vegetative growth to flowering. Knowledge of critical photoperiods is critical for agriculture as it determines the timing of planting schedules and crop yields.

Hence, the correct answer is option 2) Light period longer than a critical day length.

Also Read:

• MCQs on Photoperiodism in Plants

• Auxin Biosynthesis

• Factors Affecting Plant Growth

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