8.1.8Topic 8 · Grey Matter

Phytochrome — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

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8.1.8 Phytochrome — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe how the light receptor phytochrome exists in two interconvertible forms and how it controls germination, flowering, and shade avoidance.Phytochrome is a light-sensitive pigment protein found in the leaves, …

active form
photoperiodism
Short-day plants
long-day plants
day-neutral plants
uninterrupted dark period

Section 1 of 8What is phytochrome?

Phytochrome is a light-sensitive pigment protein found in the leaves, stems and seeds of plants. It allows plants to detect the quality (wavelength) and duration of light and so to respond to the time of day and year. It exists in two forms that are interconverted by light of different wavelengths:

Pr absorbs red light (about 660 nm) and is converted to P_fr. It is physiologically inactive.

Pfr

Pfr absorbs far-red light (about 730 nm) and is converted back to P_r. It is the active form that triggers responses.

Sunlight contains more red than far-red light, so during the day most of the phytochrome is converted to P_fr.

What is phytochrome?

In darkness, P_fr slowly reverts to P_r (dark reversion), and P_fr is also broken down more rapidly than P_r is.

The ratio of P_fr to P_r therefore provides information on light and darkness.

P_fr acts by moving into the nucleus and affecting the transcription of genes, so it works as a transcription factor activator.

Section 2 of 8Control of germination

Many small seeds, such as lettuce, need light to germinate; they contain phytochrome and remain dormant in P_r form in the dark.

Light that exposes the seed to red light converts P_r to P_fr, which stimulates the synthesis of gibberellins.

Gibberellins stimulate the production of amylase, so stored starch in the seed is mobilised to supply sugars for respiration and growth of the embryo.

A flash of red light stimulates germination, but a flash of far-red light given immediately afterwards reverses the effect and prevents it.

The final flash decides the outcome: in a sequence of red and far-red flashes the seeds germinate only if the last flash is red.

This ensures that seeds germinate only when light conditions are suitable, for example when they are near the surface of the soil and not buried too deeply to grow to the light.

Section 3 of 8Control of flowering

Many plants flower in response to day length (photoperiodism). Short-day plants (such as chrysanthemum and poinsettia) flower when the night is longer than a critical length; long-day plants (such as spinach and clover) flower when the night is shorter than a critical length; day-neutral plants flower regardless.

1Uninterrupted dark period

What matters is the length of the uninterrupted dark period, not the day.

A short flash of light in the middle of a long night prevents a short-day plant from flowering because it converts P_r to P_fr,

so the plant "reads" the night as short.

A far-red flash directly after reverses this effect.

The P_fr:P_r ratio is thought to control the production of a flowering signal in the leaves, which moves in the phloem to the shoot apex.

Control of flowering

In short-day plants P_fr inhibits flowering, and in long-day plants it promotes it.

The first week of the long night allows P_fr to fall below the threshold.

Growers use this: by controlling the light period in greenhouses they can bring plants into flower out of season.

Section 4 of 8Shade avoidance and other roles

Leaves absorb red light for photosynthesis but transmit and reflect far-red light, so light under a canopy has a higher far-red to red ratio.

Phytochrome detects this change, as more P_fr is converted to P_r.

A seedling in shade responds by elongating its stem more rapidly, growing taller to reach the light (etiolation-like response),

and decreasing branching and leaf expansion.

Shade avoidance

This shade avoidance response gives a competitive advantage.

Phytochrome also controls the unrolling of leaves, chloroplast development, and the breaking of dormancy in buds.

Feature	P_r	P_fr
Absorbs	Red (660 nm)	Far-red (730 nm)
Activity	Inactive	Active
Dominates	In the dark and in far-red light	In daylight (red-rich)

Section 5 of 8Interpreting experiments on photoperiod

Experiments on flowering usually involve plants given different light and dark periods, with a graph or table of the number of plants that flower.

Interpreting experiments on photoperiod

To interpret them, work out the length of the continuous dark period in each treatment, not just the total hours of light.

For example, a short-day plant with a critical night length of 10 hours flowers when given 8 hours of light and 16 hours dark,

but not when the 16-hour night is broken by 5 minutes of light, as that gives two nights of 8 hours.

Grafting experiments showed that a signal moves from leaves to the bud,

so the leaf is the site of detection;
- covering a single leaf with black paper is enough to induce flowering in a short-day plant, even if the rest of the plant is kept in light.

A concise way to remember the logic: red light gives P_fr, P_fr is active, far-red or darkness gives P_r, and the last wavelength of light decides the plant's response. Link this lesson with IAA and gibberellins, because phytochrome signals often act through changes in plant growth factors.

Section 6 of 8Exam-style questions

Lettuce seeds were exposed to alternating flashes of red (R) and far-red (FR) light. State, with a reason, whether the seeds germinate after R, FR, R, FR. [3]3 marks
Explain why a short-day plant fails to flower if a flash of red light is given in the middle of the night. [3]3 marks
Suggest how phytochrome helps a seedling growing in the shade of a taller plant. [3]3 marks

Section 7 of 8Key terms

Term	Meaning
Phytochrome	Light-sensitive plant pigment that exists in Pr and Pfr forms
Pr	Inactive form that absorbs red light at about 660 nm
Pfr	Active form that absorbs far-red light at about 730 nm
Photoperiodism	Response of an organism to the relative lengths of day and night
Short-day plant	Plant that flowers when the night is longer than a critical length
Long-day plant	Plant that flowers when the night is shorter than a critical length
Shade avoidance	Increased stem elongation in response to a high far-red to red ratio
Gibberellin	Plant growth factor that stimulates germination and stem elongation

Section 8 of 8Exam technique

Saying that P_r is the active form: P_fr is active.

Saying that plants measure the length of the day: they measure the uninterrupted night.

Saying far-red light is stronger than red light in sunlight: it is relatively less in direct sunlight.

Saying that phytochrome is a hormoneit is a photoreceptor pigment that affects gene expression.

Mixing up short-day and long-day plantsthey are named for the effect of the night length on flowering.

Last updated: 18 October 2026

Section 1 of 8What is phytochrome?

Section 2 of 8Control of germination

Section 3 of 8Control of flowering

Section 4 of 8Shade avoidance and other roles

Section 5 of 8Interpreting experiments on photoperiod

Section 6 of 8Exam-style questions

Lettuce seeds were exposed to alternating flashes of red (R) and far-red (FR) light. State, with a reason, whether the seeds germinate after R, FR, R, FR. [3]3 marks

Explain why a short-day plant fails to flower if a flash of red light is given in the middle of the night. [3]3 marks

Suggest how phytochrome helps a seedling growing in the shade of a taller plant. [3]3 marks

Section 7 of 8Key terms

Section 8 of 8Exam technique

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