What you need to know
- 8.11phytochromeA plant pigment that exists in two forms, Pr and Pfr, which change into each other in red and far-red light. brings about plant responses, including flowering.
About 660 nm and 730 nm are the usual teaching wavelengths. Transcription detail is the next lesson. Do not call a long-day plant a plant that needs a long dark period.
By the end, you should be able to…
- Describe the red and far-red conversion of Pr and Pfr.
- Explain flowering in terms of night length.
- Predict the effect of a red flash and a following far-red flash.
Build your understanding
Two forms of the same pigment
phytochromeA plant pigment that exists in two forms, Pr and Pfr, which change into each other in red and far-red light. exists as Pr and Pfr. Pr absorbs red light, about 660 nm, and is converted to Pfr. Pfr absorbs far-red light, about 730 nm, and is converted back to Pr. Pfr is the biologically active form. In daylight both wavelengths are present, but the conversion to Pfr is effective, so Pfr builds up. In darkness, Pfr slowly converts back to Pr without needing far-red light.
A flash experiment uses this reversibility. Red light in the middle of the night makes Pfr. Far-red light immediately afterwards can cancel that flash by turning Pfr back into Pr. The last flash is the one that counts.
It is the length of the night
Flowering in many speciesA group of organisms that can interbreed to produce fertile offspring. depends on whether the dark period is longer or shorter than a critical length. A long-day plant flowers when the night is short enough that Pfr remains. It is really a short-night plant. A short-day plant flowers when the night is long enough for Pfr to fall. It is really a long-night plant. The day-length names are traditional. Write the night if you want the mark.
A red flash in the middle of a long night can stop a short-day plant flowering, because it recreates Pfr and splits the night into two short dark periods. The same flash can allow a long-day plant to flower even when the calendar night is long. Far-red after the red flash reverses the effect.
Not every plant, and not auxin
Day-neutral plants flower without this night-length switch. Do not force every species into the table. Phytochrome is a leaf pigment. The flowering response happens at the shoot tip. A signal moves between them. You do not need the name of that signal unless a question gives it.
auxinA plant growth substance made in shoot tips that promotes cell elongation. bends a shoot in hours. Phytochrome times flowering over days. Do not explain phototropism with Pr and Pfr.
Follow the process
Red light, about 660 nm, converts Pr to Pfr. Far-red light, about 730 nm, converts Pfr back to Pr. Pfr is the active form, and in a long night it slowly becomes Pr without a flash. A short-day plant flowers when the night is long enough for Pfr to fall. A long-day plant flowers when the night is short enough that Pfr remains. A red flash in a long night makes Pfr again and splits that night into two short dark periods. Far-red straight after the red flash removes that Pfr, so the last flash is the one that counts. Day-neutral plants do not use this switch. These wavelengths are the usual teaching values, not a measurement from one leaf.
NOT: Pr is not the active form, and a short-day plant does not need a short night.
- Pfr is the active form
- About 660 nm red makes Pfr
- About 730 nm far-red, or a long night, removes it
- The last flash is the one that counts
Two traps this diagram blocks
- Do not call Pr the active form.
- Do not say a short-day plant flowers because the day is short of red light.
From the diagram alone
Question
A short-day plant is given a long night interrupted by red light, with no far-red afterwards. Explain the effect on flowering. [3]
Show a model answer
The red flash converts Pr to Pfr and breaks the long night into two shorter dark periods. Pfr does not stay low for long enough, so the short-day plant does not flower.
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: a red flash in the middle of a long night stops a short-day plant flowering. Y: the flash converts Pr to Pfr and breaks the night into two stretches that are each too short. Z: Pfr does not stay low for long enough, so the long-night response fails. Incomplete: “Short-day plants need short days of Pr.” Improved: “Pfr is the active form. A short-day plant needs a long night so that Pfr can fall. The red flash puts Pfr back.”
Active form is Pfr
Red makes it. Far-red or a long night removes it.
Rewrite the names
Long-day plant: needs a short night. Short-day plant: needs a long night.
Avoid this misconception
The claim: A short-day plant flowers when the day is short because Pr is the active form that builds up in the light.
The correction: Pfr is the active form and it builds in the light. A short-day plant flowers when the night is long enough for Pfr to fall. The day name is about the companion light period, not about Pr being active.
How does a plant know the nights are getting longer?
Phytochrome switches between Pr and Pfr. Red light makes Pfr, the active form. In the dark, Pfr slowly turns back into Pr. A long night lets Pfr fall far enough for a short-day plant to flower.
What the model shows
A fictional short-day plant and long-day plant under different day lengths and night-break flashes, with the share of Pfr through the day.
What to look for
How long Pfr stays below the threshold, and what a red or far-red flash in the night does to it.
What it tells us
It is the length of the night that counts. A red flash splits a long night; far-red straight after cancels it. The last flash counts.
Be careful what you claim
Fictional plants and rates.
Try it in the model: press Play, then drag Hours of light down to 12.
Phytochrome and flowering
Set the day length and flash red or far-red light in the night, then play the days. Watch Pr and Pfr, and see which fictional plant flowers. Plants and rates are illustrative.
What you see
- A 24-hour schedule with the light period, the night and any night-break flashes, a fictional short-day plant and long-day plant that flower after three inductive nights, and the Pr ⇌ Pfr conversion lit by the current light.
- A graph of the share of phytochrome as Pfr through one day, with the threshold the plants respond to and the stretch where Pfr is low.
How to use it
- Set the hours of light and a night break, then play the days or drag Day.
What it shows
- Red light (about 660 nm) converts Pr to Pfr, and far-red light (about 730 nm) or darkness converts Pfr back to Pr. Pfr is the active form (8.11).
- Flowering depends on the length of the uninterrupted night: short-day plants are long-night plants, and long-day plants are short-night plants. The last flash counts.
Limits of the model
- The plants, the 10 h critical night and the threshold are fictional. Real plants also use a circadian clock, and the signal from leaf to shoot tip is not modelled.
- Day-neutral plants do not use this switch. Pfr changing transcription is the next lesson (8B 7).
Guided tour · step 1 of 8
- In Light schedule, just below these steps, keep Hours of light at 16 h (an 8 h night).
- Press Play and run to day 4.
Red light in the day converts Pr to Pfr, the active form. In an 8 h night Pfr falls but never stays below the threshold, so the long-day plant flowers. Really it is a short-night plant.
Light schedule
Questions
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Which wavelength converts Pr to Pfr, and which form is active?
Show suggested answer
Red light, about 660 nm, converts Pr to Pfr. Pfr is the active form.
Practice question
Retrieval and explanation · suggested answer
Why does a long-day plant fail to flower if the night is too long?
Show suggested answer
Pfr falls during the long night. The plant needs the night to be short enough for Pfr to remain.
Practice question
Retrieval and explanation · suggested answer
A short-day plant is given a long night with a red flash in the middle, then darkness again. What happens to flowering, and how could far-red light reverse that?
Show suggested answer
The red flash makes Pfr and breaks the long night, so the plant does not flower. Far-red immediately after the red flash converts Pfr back to Pr, restoring the effect of a long night, so it can flower.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Plants of one species flower only when nights are shorter than 10 hours. Nights are 14 hours, but each night is interrupted by red light, and no far-red follows. Will they flower? What is the ordinary name for this flowering type? Assume the species uses phytochrome in the usual way.
Show a hint
The interruption changes the night the plant measures.
Explore the reasoning
They are long-day plants, because they need a short night. The red flash splits the 14-hour night, leaving Pfr, so the plant responds as if the night were short and it flowers. Without the flash, a 14-hour night would not let them flower. The 10 hours is a teaching critical night, not a value to memorise for all species.
Recall this lesson
Lesson recall questions
Question 1 of 5: Explain, in causal order, how phytochrome can change transcription.
Plant growth regulators
Practice question
Retrieval and explanation · suggested answer
Explain, in causal order, how phytochrome can change transcription.
Lesson: 8B 7: Phytochrome and Transcription
Build the sequence from memory first. Then reveal the model order.
Tap the steps in the order you recall. Tap a placed step to send it back.
- 01Next step
- 02Waiting
- 03Waiting
Tap to place
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Flowering is a transcription decision
8B · Phytochrome and FloweringMedlinePlus Genetics describes signals acting through transcription factors that bind DNA and change transcription. Phytochrome’s Pfr is such a signal in the plant, even though that human-genetics page…Specification 8.11Read & use the scienceClose article
Why this matters
MedlinePlus Genetics describes signals acting through transcription factors that bind DNA and change transcription. Phytochrome’s Pfr is such a signal in the plant, even though that human-genetics page never names it. The night-length rules on this page are how much Pfr the plant has. The next lesson is what Pfr does to genes. 8.11: phytochrome controls flowering responses. A scan or growth response is indirect evidence: it cannot alone identify every signalling step or prove the proposed cause.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 8.11 Phytochrome, auxin and gibberellin effects on plant responses and transcription.
Use the science
Why is a correct night-length answer still incomplete if it never mentions that Pfr is active?
Explore a possible response
The plant is not measuring darkness with a clock you can see. It is the amount of Pfr, which falls in a long night and is rebuilt by red light. Leaving out the active form misses the mechanism.
Explore the source
- MedlinePlus Genetics: can genes be turned on and off?US National Library of Medicine
A night-time flash can change a flowering signal
8B · Phytochrome and FloweringA study in tomato found that short interruptions of darkness and changes in red:far-red light altered flowering time and expression of flowering-related genes. The phytochrome B1 mutant changed…Specification 8.11Read & use the scienceClose article
Why this matters
A study in tomato found that short interruptions of darkness and changes in red:far-red light altered flowering time and expression of flowering-related genes. The phytochrome B1 mutant changed that response, giving evidence that a light receptor was involved. This is richer than saying “red light makes plants flower”: species, photoperiod and gene-regulatory context all matter. The 8.11 connection is that phytochrome can influence transcription, which later changes a developmental outcome. Researchers counted flowering and measured RNA; neither measurement alone is a photograph of phytochrome binding a promoter. The paper’s gene names are not required recall.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 8.11 Phytochrome, auxin and gibberellin effects on plant responses and transcription.
Use the science
Why does a mutant that loses a night-break response strengthen the case that phytochrome is involved?
Explore a possible response
Changing the receptor changes the outcome under the same light treatment. That links the receptor to the response, although other pathway components still need investigation.
Explore the source
- Phytochrome and tomato floweringPeer-reviewed research via NIH PubMed Central