The McCree Curve is one of the most frequently referenced graphs in plant-lighting discussions.
It is often used to support statements such as:
“Plants mainly use red and blue light.”
or:
“Green light is inefficient for photosynthesis.”
Those statements are too simple.
The original McCree research was designed to investigate how the photosynthetic response of crop leaves changes with wavelength and to help define photosynthetically active radiation.
It was not designed to provide a universal grow-light spectrum recipe.
Understanding that distinction makes the McCree Curve much more useful.
Short Answer: What Is the McCree Curve?
The McCree Curve comes from research by plant scientist K. J. McCree on the wavelength dependence of photosynthesis.
McCree measured the photosynthetic response of leaves from 22 crop species across wavelengths from approximately:
350 to 750 nm
The research evaluated several related properties, including:
- action spectrum
- light absorptance
- spectral quantum yield of CO₂ uptake
The results showed that photosynthetic effectiveness varies with wavelength.
However:
The McCree Curve does not mean that photons outside its highest points are useless, and it does not tell growers exactly what spectrum a grow light should have.
It describes a particular aspect of leaf photosynthesis under defined experimental conditions.
What Did McCree Actually Measure?
The original study examined leaves from multiple crop species rather than a single plant.
McCree varied factors including:
- species
- variety
- leaf age
- growth conditions
- temperature
- CO₂ concentration
- monochromatic light intensity
- supplementary white light
- which side of the leaf was illuminated
The measurements were intended to help answer a fundamental question:
How does the photosynthetic response of a leaf depend on wavelength?
This is different from asking:
Which LED spectrum will produce the greatest crop yield in a greenhouse?
The first is a physiological measurement.
The second involves an entire plant and growing system.
Action Spectrum vs Quantum Yield
Two concepts associated with the McCree work are often confused.
Action Spectrum
An action spectrum describes how strongly different wavelengths drive a biological process — in this case photosynthesis.
The response depends partly on how much of the incoming light the leaf absorbs.
Quantum Yield
Quantum yield asks a slightly different question:
How efficiently does an absorbed or incident photon contribute to photosynthesis?
This matters because a wavelength can appear weak in an action spectrum partly because the leaf reflects or transmits more of that light.
Once absorption is considered, the interpretation can change.
That is one reason a simple image of a colored curve should not be used as a complete explanation of plant spectral response.
What Did the Classic Results Show?
McCree reported broad maxima in spectral quantum yield around approximately:
440 nm
and:
620 nm
with an additional shoulder around:
670 nm
The average blue-region peak was lower than the red-region maximum in those measurements.
This helped demonstrate that photosynthetic efficiency is not perfectly identical at every wavelength.
But the curve is broad.
It does not show that plants suddenly stop using photons between the red and blue regions.
Plants use photons throughout much of the visible spectrum.
Does the McCree Curve Mean Green Light Is Poor for Plants?
No.
This is probably the most common misunderstanding of the McCree Curve.
Leaves appear green because chlorophyll absorbs less green light near the leaf surface than it absorbs much of the red and blue light.
But:
less absorption near the surface does not mean no photosynthesis.
A substantial portion of green light enters deeper into the leaf.
That can be useful.
Green photons can penetrate farther into leaf tissue than strongly absorbed red and blue photons, allowing them to reach chloroplasts deeper within the leaf.
They can also penetrate farther into dense plant canopies.
Later research has shown that under moderate to strong white light, additional green light can drive photosynthesis efficiently in deeper leaf tissue.
So the statement:
“Plants don’t use green light.”
is incorrect.
Why Can Green Light Be Useful Deeper in a Leaf?
Consider what happens when strong light reaches the upper surface of a leaf.
Red and blue photons are absorbed strongly near the surface.
Under sufficiently intense light, some of the chloroplasts near that surface may already receive more light than they can use efficiently.
Green light is absorbed less strongly near the surface.
More of it can travel deeper into the leaf before being absorbed.
This gives green photons access to chloroplasts that may otherwise receive less light.
Research by Terashima and colleagues showed that under moderate to strong white-light conditions, green light could drive photosynthesis very effectively because of this deeper penetration.
Therefore, spectral effectiveness is influenced not only by pigment absorption but also by:
where inside the leaf a photon is absorbed.
A Leaf Is Not the Same as a Plant Canopy
The distinction becomes even more important when moving from an individual leaf to an entire plant.
A dense crop canopy contains:
- upper leaves
- lower leaves
- overlapping leaves
- shaded leaves
- leaves at different angles
Strongly absorbed wavelengths may be captured primarily by upper leaves.
More penetrating wavelengths can travel farther into the canopy and contribute to photosynthesis in lower leaves.
This means a spectrum that looks less efficient in a simplified single-leaf experiment may behave differently when evaluated at the whole-canopy level.
That is one reason modern horticultural-lighting research should not be reduced to simply reproducing the peaks of the classic McCree Curve.
Does a PAR Meter Follow the McCree Curve?
No.
This distinction is extremely important.
A typical quantum PAR meter measuring conventional PPFD does not intentionally weight photons according to the McCree Curve.
PPFD normally counts photons approximately equally across the defined PAR waveband:
400–700 nm
In the measurement concept:
one 450 nm photon counts as one photon
and:
one 550 nm photon counts as one photon
and:
one 660 nm photon counts as one photon
assuming they fall within the instrument’s defined response range.
The meter is measuring photon flux density.
It is not attempting to predict the exact photosynthetic contribution of every photon.
Why Does PPFD Count Photons Equally?
This might seem strange.
If photosynthetic quantum efficiency varies somewhat with wavelength, why not give every wavelength a different weighting?
McCree examined this question.
In a later analysis, different methods for defining photosynthetically active radiation were compared against calculated leaf photosynthesis under natural and artificial light sources.
McCree concluded that measuring incident quantum flux from 400 to 700 nm produced systematic errors that were small enough to be acceptable for practical use.
This helped support the photon-counting approach that became widely used for PAR measurement.
The strength of PPFD is therefore not that every wavelength has an identical biological effect.
Its strength is that it provides a practical, reproducible measurement of photosynthetic photon quantity.
PPFD and Spectrum Answer Different Questions
Suppose two grow lights both produce:
500 µmol/m²/s
at canopy level.
Does that mean the plants will respond identically?
No.
The PPFD measurement tells you that the photon flux density within the defined measurement range is similar.
It does not tell you that their spectra are identical.
One fixture might contain relatively more:
- blue
- green
- red
- deep-red wavelengths
than the other.
Those spectral differences can influence plant responses.
Therefore:
PPFD measures photon quantity.
Spectral distribution describes where those photons occur by wavelength.
Both can matter, but they should not be confused.
Similar PPFD Does Not Mean Identical Plant Growth
It is scientifically reasonable to say that two spectra with the same PPFD can sometimes produce different plant responses.
But PPFD alone should not be blamed when that happens.
Spectrum can influence many plant processes beyond instantaneous photosynthesis.
These include:
- leaf expansion
- stem elongation
- pigmentation
- stomatal behavior
- photomorphogenesis
- flowering signals
- canopy architecture
The eventual biomass or morphology of a plant is therefore not determined by the McCree Curve alone.
Light intensity, DLI, photoperiod and environmental conditions also matter.
The McCree Curve Is Not a Grow-Light Recipe
A common mistake is to look at the high points of the curve and conclude:
“A grow light should emit only those wavelengths.”
That is not what the research demonstrates.
The original experiments were designed primarily to characterize photosynthetic spectral response and evaluate definitions of photosynthetically active radiation.
Modern plant production involves additional factors.
A grow-light spectrum can influence:
- photosynthesis
- plant form
- pigmentation
- flowering
- leaf temperature
- canopy penetration
Different crops and production goals may therefore benefit from different spectral distributions.
There is no single McCree-based spectrum that is automatically optimal for every plant.
What About Red Light?
Red photons are highly relevant to photosynthesis and can be delivered efficiently by modern LEDs.
Red light therefore plays an important role in horticultural lighting.
But that does not mean:
more red always equals better growth.
Plant morphology can change substantially when the red, blue, green and far-red proportions change.
Spectrum should therefore be evaluated as part of the entire lighting strategy rather than by maximizing one part of the McCree Curve.
What About Blue Light?
Blue photons also contribute directly to photosynthesis.
In addition, blue light is involved in several regulatory responses, including effects on stomata and plant morphology.
Increasing the blue fraction of a spectrum can produce different morphological responses from increasing red light, even when PPFD remains unchanged.
Again, these effects go beyond what a basic photosynthetic action spectrum alone describes.
What About Far-Red Light?
Traditional PAR and PPFD are generally defined around:
400–700 nm
Far-red wavelengths above 700 nm therefore fall outside conventional PPFD.
However, modern plant research has demonstrated that far-red photons can interact with shorter wavelengths and contribute to photosynthesis under some conditions.
Far-red light is also important in plant signaling through phytochrome.
This is another example of why the classic 400–700 nm framework is extremely useful for measurement but should not be interpreted as a statement that wavelengths immediately outside that range have no biological effect.
Why Modern Research Has Revisited the McCree Curve
The McCree studies remain foundational.
But researchers have continued investigating spectral responses using newer measurement methods, narrow-band LEDs and different experimental conditions.
Modern reviews have pointed out several limitations when people attempt to use the original curve as a universal description of whole-plant spectral performance.
These include differences in:
- light intensity
- bandwidth of the test light
- background illumination
- leaf versus whole-plant response
- photon absorption
- canopy structure
The correct interpretation is therefore not:
“The McCree Curve is wrong.”
It is:
“The McCree Curve answers a specific scientific question and should not be asked to answer every horticultural-lighting question.”
How Should Growers Use the McCree Curve?
Use it as a foundation for understanding that:
wavelength matters.
But do not use it as a replacement for actual lighting measurements.
For practical growing, several measurements may be useful.
PPFD
Use PPFD to measure instantaneous photosynthetic photon flux density at the canopy.
DLI
Use DLI to determine how much photosynthetic light accumulates during an entire day.
Spectrum
Use spectral information when the wavelength distribution itself matters.
Photoperiod
Consider how many hours of light the plant receives.
Growing Environment
Temperature, CO₂, humidity, water and nutrients can all influence how effectively plants use available light.
The McCree Curve is one piece of this larger system.
Can a PAR Meter Tell You Whether a Spectrum Is Good?
Not by itself.
A PAR meter tells you how much photon flux is reaching the measurement location within its specified wavelength range.
It does not provide a complete spectral analysis.
For example, two fixtures may both measure:
600 µmol/m²/s
while having different spectral distributions.
A PAR meter can tell you that their PPFD is similar at that location.
It cannot tell you from that number alone whether one contains more blue, green or red light.
For that, spectral measurement is required.
Why Visual Brightness Is Also Not Enough
Human eyes are not reliable PAR meters.
Our visual system weights wavelengths according to human brightness sensitivity and adapts strongly to changing light levels.
A grow light that appears brighter to the eye does not necessarily produce proportionally greater PPFD.
Likewise, purple or red-heavy grow lights can appear visually unusual while still delivering substantial photon flux to plants.
This is why quantitative measurement is preferable when comparing plant-light environments.
Frequently Asked Questions
What does the McCree Curve measure?
It describes the wavelength-dependent photosynthetic response of plant leaves based on classic experiments by K. J. McCree involving multiple crop species.
How many plants were studied in the original McCree research?
McCree measured leaves from 22 crop species while investigating action spectrum, absorptance and quantum yield across approximately 350–750 nm.
Does the McCree Curve show the best spectrum for grow lights?
No.
It provides important information about the spectral response of leaf photosynthesis, but it is not a universal grow-light recipe.
Do plants use green light?
Yes.
Green photons contribute to photosynthesis and can penetrate deeper into leaves and plant canopies than wavelengths that are strongly absorbed near the surface.
Is red light better than green light?
There is no universal answer.
The result depends on what is being measured, light intensity, background spectrum, leaf structure, canopy structure and the plant response of interest.
Does a PAR meter weight light according to the McCree Curve?
No.
Conventional PPFD measurement approximately counts photons equally across the specified PAR range rather than weighting each wavelength according to the McCree Curve.
Can two lights have the same PPFD but different spectra?
Yes.
PPFD measures photon flux density, not the complete spectral distribution.
Does that mean PPFD is inaccurate?
No.
PPFD and spectrum are simply different measurements.
PPFD provides a standardized way to quantify photosynthetic photon intensity, while spectral measurements describe how those photons are distributed by wavelength.
The Key Principle
The McCree Curve established an important idea:
Plant photosynthesis responds to wavelength.
But the correct lesson is not:
“Use only red and blue light.”
Nor is it:
“Green photons are wasted.”
The better interpretation is:
Spectrum affects plant response, while PPFD provides a practical measurement of photon quantity.
The McCree Curve remains a foundational tool for understanding plant-light science, but modern horticultural lighting requires considering the complete system:
spectrum + PPFD + DLI + photoperiod + crop response + growing environment.
That is a much more useful way to interpret plant light than trying to turn one classic curve into a universal lighting recipe.
References and Further Reading
McCree, K. J. — The Action Spectrum, Absorptance and Quantum Yield of Photosynthesis in Crop Plants. Agricultural Meteorology, 1971–1972.
McCree, K. J. — Test of Current Definitions of Photosynthetically Active Radiation Against Leaf Photosynthesis Data. Agricultural Meteorology, 1972.
Terashima, I., Fujita, T., Inoue, T., Chow, W. S. & Oguchi, R. — Green Light Drives Leaf Photosynthesis More Efficiently than Red Light in Strong White Light: Revisiting the Enigmatic Question of Why Leaves Are Green. Plant and Cell Physiology, 2009.
Sukhova, E. et al. — Re-interpreting the Photosynthetically Active Radiation (PAR) Curve in Plants. Plant Science, 2019.