Look at a chlorophyll absorption spectrum and then look at the McCree photosynthetic action spectrum.
At first, they seem difficult to reconcile.
Chlorophyll a and chlorophyll b absorb strongly in the blue and red regions of the spectrum. Their absorption is much weaker through much of the green region.
Yet the classic McCree data show something very different:
green wavelengths can still drive photosynthesis effectively.
Why?
The key is that these two graphs measure different things.
A chlorophyll absorption spectrum asks:
How strongly does a particular pigment absorb different wavelengths?
The McCree experiments asked a much more practical question:
How effectively do photons of different wavelengths drive CO₂ assimilation in an intact leaf?
Those are not the same measurement.
Once that distinction is understood, the apparent mystery of green light largely disappears.
What Is the McCree Curve?
The term McCree curve generally refers to results from K. J. McCree’s classic research on the spectral response of photosynthesis.
McCree measured the:
- action spectrum
- leaf absorptance
- spectral quantum yield of CO₂ uptake
across wavelengths from approximately:
350 to 750 nm
The experiments included leaves from 22 crop species and considered differences such as species, growing conditions, leaf orientation and measurement conditions.
The goal was not simply to determine which wavelengths chlorophyll absorbs.
It was to understand how effectively different wavelengths actually drive photosynthesis in real leaves and to provide a scientific basis for defining photosynthetically active radiation.
That difference is fundamental.
The McCree Curve Is Not a Chlorophyll Absorption Curve
This is the most important point.
A typical chlorophyll graph shows the absorption spectrum of:
chlorophyll a
and
chlorophyll b
often after the pigments have been extracted from plant tissue and dissolved in a solvent.
These curves usually show strong absorption in:
- blue wavelengths
- red wavelengths
and much weaker absorption in the middle of the visible spectrum.
It is easy to look at this graph and conclude:
Green light must be poor for photosynthesis.
But the McCree experiments did not measure isolated chlorophyll molecules.
They measured the photosynthetic response of intact leaves.
An intact leaf contains:
- chlorophyll a
- chlorophyll b
- carotenoids
- pigment–protein complexes
- chloroplasts at different depths
- palisade and spongy mesophyll
- cell walls
- air spaces
- internal scattering surfaces
Light interacting with this structure behaves very differently from light passing through a solution of purified chlorophyll.
What Did McCree Actually Find?
McCree’s measurements showed that photosynthetic efficiency varies across the spectrum.
The average spectral quantum-yield curve contained broad maxima around:
440 nm
and
620 nm
with an additional shoulder around:
670 nm
The blue maximum was lower than the red maximum.
But the green region did not collapse to near zero.
A commonly reproduced photon-weighted version of the McCree data gives approximate relative quantum yields such as:
| Wavelength | Relative Quantum Yield |
|---|---|
| 400 nm | 0.42 |
| 425 nm | 0.68 |
| 450 nm | 0.70 |
| 475 nm | 0.63 |
| 500 nm | 0.65 |
| 525 nm | 0.72 |
| 550 nm | 0.82 |
| 575 nm | 0.91 |
| 600 nm | 0.97 |
| 625 nm | 1.00 |
| 650 nm | 0.90 |
| 675 nm | 0.90 |
| 700 nm | 0.48 |
The exact values and how a McCree curve is plotted depend on whether the graph represents action spectrum, quantum yield, normalization and other methodological choices.
The important point is the overall pattern:
green photons were capable of driving substantial photosynthesis.
In parts of the green-to-yellow region, the relative response was surprisingly high.
Green Light Is Not High on the Curve Because Sunlight Contains Lots of Green
This is a common misunderstanding.
The McCree curve does not primarily show the spectral distribution of sunlight.
It is not asking:
How many green photons are present outdoors?
Instead, it asks something closer to:
For photons at this wavelength, how effectively do they contribute to photosynthetic CO₂ assimilation?
Therefore, the amount of green light in sunlight does not explain why green wavelengths have substantial values in the McCree action spectrum.
Sunlight certainly contains green photons.
But their abundance is a separate question from their photosynthetic effectiveness.
This distinction matters whenever a spectral power distribution is compared with a biological action spectrum.
Why Doesn’t the Green Region Match the Chlorophyll Trough?
There are several reasons.
The first is simple:
leaves absorb more green light than isolated-chlorophyll diagrams imply.
Plants look green because they reflect and transmit proportionally more green wavelengths than blue and red wavelengths.
But “more reflection” does not mean:
100% reflection
or:
zero absorption
A significant fraction of incident green light is still absorbed by real leaves.
The remainder may be reflected or transmitted through the leaf.
So the statement:
“Chlorophyll reflects green light.”
is far too simplistic.
The Whole Leaf Is an Optical System
A leaf is not a flat layer of chlorophyll.
When light enters a leaf, photons can:
- be absorbed
- scatter
- change direction
- travel deeper into the mesophyll
- encounter additional chloroplasts
- pass through the leaf
- emerge from another surface
This repeated interaction increases the probability that photons not immediately absorbed near the surface may eventually encounter photosynthetic pigments deeper in the tissue.
Internal leaf anatomy therefore has a major effect on the relationship between pigment absorption and whole-leaf photosynthesis.
That is why an absorption curve measured from extracted pigment cannot be used as a direct map of the photosynthetic efficiency of an intact leaf.
Weak Absorption Can Sometimes Help Green Light
This sounds counterintuitive.
If red light is absorbed strongly, shouldn’t stronger absorption always be better?
Not necessarily.
Imagine intense light striking the upper surface of a leaf.
Strongly absorbed blue and red photons tend to be intercepted relatively close to that illuminated surface.
This can concentrate excitation in chloroplasts near the top of the leaf.
Green light is absorbed less strongly.
As a result, a larger fraction of green photons can penetrate farther into the leaf before being absorbed.
That allows green photons to reach chloroplasts deeper in the mesophyll.
So there is a trade-off:
strong absorption helps capture photons quickly
while
weaker absorption allows photons to be distributed more deeply
Both can be useful.
A Leaf Has Photosynthetic Machinery Below the Surface
Photosynthetic capacity is not restricted to the first chloroplasts encountered by incoming light.
Chloroplasts exist throughout photosynthetically active leaf tissues.
Under strong illumination, the chloroplasts nearest the surface can receive more excitation than they can use efficiently, while chloroplasts deeper in the leaf receive substantially less light.
Because green photons penetrate farther, they can distribute excitation more evenly through the leaf.
This helps explain an important result from later research:
under sufficiently strong background light, additional green light can sometimes increase whole-leaf photosynthesis more efficiently than additional red light.
That does not mean green is universally “better than red.”
It means wavelength efficiency depends partly on the light environment and where photons are absorbed.
The Result Depends on Light Intensity
This is another reason a single wavelength-efficiency curve should not be treated as a permanent ranking.
Under relatively weak light, nearly every absorbed photon may have a good chance of contributing to photochemistry.
Under strong light, chloroplasts near the leaf surface can approach photosynthetic saturation.
Adding more strongly absorbed red or blue photons may therefore concentrate additional excitation where the photosynthetic machinery is already highly illuminated.
Green photons can travel farther and reach chloroplasts that remain less light-saturated.
This means the relative usefulness of wavelengths can change with:
- background PPFD
- spectral composition
- leaf anatomy
- species
- leaf orientation
- canopy position
There is no universal ranking such as:
red > blue > green
that applies under every condition.
Green Light Can Reach Deeper Into a Plant Canopy Too
The same general principle extends beyond a single leaf.
Upper leaves in a dense canopy intercept much of the incoming light.
Blue and red wavelengths are often strongly absorbed by upper foliage.
Green photons tend to penetrate leaves more effectively.
Some can therefore:
- pass deeper into upper leaves
- transmit through those leaves
- reach lower leaves in the canopy
This can improve the vertical distribution of light within a crop.
A photon reaching a lower, light-limited leaf may sometimes contribute more to whole-canopy carbon gain than another photon delivered to an already highly illuminated upper leaf.
This is why plant-light efficiency cannot always be understood by studying only the top surface of a single leaf.
Why the Chlorophyll Graph Is Still Useful
None of this means chlorophyll absorption spectra are wrong.
They answer an important biochemical question.
They show that chlorophyll a and b have characteristic absorption bands and absorb blue and red wavelengths strongly.
These spectra help explain:
- pigment properties
- excitation of photosynthetic complexes
- why leaves appear green
- differences between chlorophyll a and b
The problem occurs only when we ask the graph to answer a question it was not designed to answer.
A chlorophyll absorption curve does not directly tell us:
How efficiently will an intact plant photosynthesize under this wavelength?
For that, measurements of whole-leaf or whole-plant physiology are more relevant.
Absorptance and Quantum Yield Are Different
Another useful distinction is between:
absorptance
and
quantum yield
Absorptance asks:
What fraction of the incoming photons is absorbed?
Quantum yield asks:
How much photosynthetic response occurs relative to the number of photons?
A wavelength can have lower leaf absorptance but still make a meaningful contribution to photosynthesis.
Conversely, strong absorption does not guarantee that every absorbed photon results in useful carbon fixation.
Some absorbed excitation can be:
- dissipated as heat
- re-emitted as fluorescence
- involved in photoprotective processes
Photosynthesis depends on what happens after absorption as well as on absorption itself.
Green Is Not a Single Wavelength
Even the phrase green light hides substantial variation.
The green region is commonly discussed approximately as:
500–600 nm
But plant response does not suddenly change at the artificial boundaries between blue, green and red.
McCree’s data show a continuous spectral response.
For example, the response around 500 nm can differ substantially from the response around 575 nm.
So saying:
“Green light has an efficiency of X.”
is usually an oversimplification.
The answer depends on exactly which wavelengths are included.
Why Does the McCree Curve Often Rise Through Green Toward Red?
In averaged McCree data, photosynthetic quantum yield generally rises as wavelength progresses from shorter green wavelengths toward yellow-orange and red wavelengths, reaching a broad maximum around the red region.
For example:
500 nm may show a lower relative efficiency than 575 nm,
while wavelengths around:
600–625 nm
can be particularly effective.
This is another reason the visual categories humans call “green,” “yellow,” and “red” are not ideal biological divisions.
Photosynthesis responds continuously to wavelength.
What About Blue Light?
Another surprising aspect of the McCree data is that blue light is not always as photosynthetically efficient per incident photon as simplified chlorophyll graphs might suggest.
Blue is strongly absorbed by leaves.
But blue photons are also absorbed by pigments other than the photosynthetic chlorophyll reaction system, including carotenoids and photoreceptors.
In addition, high absorption near the leaf surface can reduce penetration to deeper tissue.
Blue light remains extremely important.
It contributes to photosynthesis and also affects processes such as:
- stomatal responses
- phototropism
- leaf morphology
- pigmentation
- photomorphogenesis
But its strong chlorophyll absorption peak should not be interpreted as proof that it must have the highest photosynthetic quantum yield.
Action Spectrum vs Absorption Spectrum
The distinction can be summarized simply:
| Spectrum | What It Measures |
|---|---|
| Chlorophyll absorption spectrum | How strongly isolated pigment absorbs different wavelengths |
| Leaf absorptance spectrum | How much incoming light an intact leaf absorbs |
| Photosynthetic action spectrum | How effectively incident wavelengths drive a photosynthetic response |
| Quantum-yield spectrum | Photosynthetic output relative to photons supplied or absorbed |
| Lamp spectrum | How many photons or how much radiant power the light source emits at each wavelength |
These graphs may have similar features.
But they are not interchangeable.
A grow-light spectrum cannot be evaluated properly by laying it on top of an isolated chlorophyll absorption curve and assuming that matching the absorption peaks produces the best light.
Does the McCree Curve Mean We Should Weight Every PPFD Photon Differently?
For most routine horticultural measurement:
no.
Traditional PPFD counts photons in the defined PAR range approximately equally rather than weighting every wavelength according to the McCree curve.
There are good practical reasons for this.
Plant response varies with:
- species
- intensity
- spectrum
- acclimation
- canopy structure
- environmental conditions
A fixed biological weighting function could imply more precision than actually exists.
Equal photon counting provides a standardized and highly useful measure of incident photosynthetic photon flux.
That is why PPFD remains useful even though quantum yield is not perfectly identical at every wavelength.
PPFD and the McCree Curve Answer Different Questions
A PAR meter measuring traditional PPFD asks:
How many photons between approximately 400 and 700 nm reach this surface every second?
The McCree action spectrum asks:
How effectively can different wavelengths drive photosynthesis under the experimental conditions?
Those are complementary measurements.
PPFD provides the photon quantity.
Spectrum provides the wavelength distribution.
Plant physiology determines how those photons are absorbed and used.
None of these quantities alone provides a complete description of plant growth.
Should Grow Lights Include Green Light?
The McCree curve is one piece of evidence showing why green light should not automatically be treated as wasted radiation.
Green photons can:
- drive photosynthesis
- penetrate deeper into leaves
- penetrate farther through some canopies
- contribute to carbon gain in lower tissue
- participate in plant signaling
Broad-spectrum white LEDs naturally contain substantial green light and can support efficient plant growth.
However, this does not mean every grow light requires a fixed percentage of green photons.
The appropriate spectrum depends on factors such as:
- crop
- canopy architecture
- growth stage
- desired morphology
- lighting intensity
- electrical efficiency
- human visibility requirements
The McCree curve is not a recipe for building a grow light.
Can You Multiply a Lamp Spectrum by the McCree Curve?
You can mathematically apply an action-spectrum weighting to a spectral distribution.
Historically, analyses like this have been useful for studying different definitions of photosynthetically active radiation.
But the resulting number should not be interpreted as a perfect predictor of plant performance.
The McCree data represent measurements obtained under defined experimental conditions and averaged across crop species.
Real plants may respond differently as:
- light intensity changes
- wavelengths interact
- leaves acclimate
- canopy structure changes
So a McCree-weighted number can be informative, but it should not replace direct measurements of PPFD, DLI, spectrum and plant response.
What Later Research Added
Research after McCree has helped explain why green photons can be valuable.
Experiments examining the distribution of light within leaves have shown that red and blue wavelengths tend to be absorbed relatively strongly near illuminated surfaces.
Green light penetrates more deeply.
In 2009, Terashima and colleagues examined this effect under background white light.
They found that under moderate to strong white illumination, additional green light could drive sunflower leaf photosynthesis more effectively than additional red light.
The reason was not that green suddenly became more strongly absorbed than red.
Instead, green light could reach deeper chloroplasts that were less light-saturated.
This result illustrates a broader principle:
The value of a photon depends not only on whether it is absorbed, but also on where and under what conditions it is absorbed.
Does Green Light Matter More in Dense Canopies?
It can.
In a dense canopy, the upper leaves can receive high PPFD while lower leaves remain light-limited.
A spectrum that distributes photons more deeply through the canopy may improve whole-canopy light utilization.
Because green wavelengths generally penetrate foliage more effectively than strongly absorbed blue and red wavelengths, they can make a meaningful contribution to the lower canopy.
However, the actual effect depends strongly on:
- leaf area index
- leaf angle
- species
- pigment concentration
- canopy architecture
- incident spectrum
So “green penetrates deeper” is a useful principle, not a guarantee of a specific yield increase.
Why Plants Are Green Makes More Sense in This Context
At first, a green leaf appears inefficient.
Why would evolution produce leaves that reflect part of the solar spectrum?
But complete absorption at the surface would create another problem.
If all useful photons were intercepted by the first chloroplast layer, deeper photosynthetic tissue would receive little light while upper chloroplasts could receive more excitation than they can efficiently process.
Partial penetration helps distribute light.
Seen this way, the relatively weaker absorption of green wavelengths is not necessarily a flaw.
It can help distribute photon energy through photosynthetic tissue.
That does not fully explain why plants are green, but it shows why weak green absorption should not automatically be interpreted as biological inefficiency.
Frequently Asked Questions
Is the McCree curve a chlorophyll absorption curve?
No.
The McCree work measured photosynthetic responses of intact crop leaves across wavelengths, including action spectra, absorptance and quantum yield.
A chlorophyll absorption curve measures the spectral absorption of particular pigment molecules.
Why is green light effective if leaves look green?
Leaves reflect and transmit a larger fraction of green light than blue or red light, but they still absorb a significant amount of it.
Absorbed green photons can drive photosynthesis.
Is green light as useful as red light?
There is no single answer for every condition.
In McCree’s classic measurements, red wavelengths generally showed high quantum efficiency.
Later work demonstrated that under moderate or strong background illumination, additional green photons can sometimes be more effective than additional red photons because green reaches deeper, less-saturated chloroplasts.
Does sunlight abundance explain the McCree green response?
No.
The McCree curve represents wavelength-dependent photosynthetic effectiveness, not the spectral photon distribution of sunlight.
The two should not be confused.
Do plants reflect all green light?
No.
Green leaves reflect and transmit some green light but also absorb a substantial fraction.
The exact proportions depend on wavelength, species, pigment content and leaf structure.
Does the McCree curve prove that all wavelengths are equal?
No.
Photosynthetic quantum efficiency varies with wavelength.
The curve also does not mean that a wavelength has one fixed efficiency under every possible growing condition.
Why does PPFD count green photons equally with red photons?
PPFD is designed as a standardized photon-flux measurement.
It counts photons within its defined photosynthetic wavelength range without applying a fixed plant-response weighting curve.
This makes it practical and comparable across many crops and growing conditions.
Should I judge a grow light by how closely its spectrum matches the McCree curve?
No.
The McCree curve is valuable for understanding photosynthetic wavelength response, but it is not an ideal grow-light recipe.
A useful lighting evaluation should also consider:
- PPFD
- DLI
- spectrum
- uniformity
- canopy penetration
- electrical efficacy
- crop response
- photoperiod
The Bottom Line
Green light appears strongly in the McCree photosynthetic response data for a simple reason:
green photons really can drive photosynthesis.
The apparent contradiction comes from comparing two different kinds of measurements.
A chlorophyll absorption spectrum describes how isolated pigments absorb light.
The McCree work measured how intact leaves actually use photons to assimilate CO₂.
Real leaves are complex optical and biological systems.
Green light is absorbed less strongly than blue or red light in many leaves, but that weaker absorption allows some green photons to penetrate more deeply into photosynthetic tissue.
At the canopy level, green photons can also reach lower leaves that receive less blue and red light.
So the lesson from the McCree curve is not:
“Green light is better than red and blue.”
And it is not:
“Plants use every color equally.”
The better conclusion is:
Chlorophyll absorption peaks alone do not tell us how effectively a whole leaf or canopy can use light.
That is why green light can look relatively weak on a chlorophyll absorption graph while remaining an important part of the photosynthetic spectrum.
References
McCree, K. J. (1972). The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agricultural Meteorology, 9, 191–216.
McCree, K. J. (1972). Significance of enhancement for calculations based on the action spectrum for photosynthesis. Plant Physiology, 49, 704–706.
McCree, K. J. (1972). Test of current definitions of photosynthetically active radiation against leaf photosynthesis data. Agricultural Meteorology, 10, 443–453.
Terashima, I., Fujita, T., Inoue, T., Chow, W. S., & Oguchi, R. (2009). 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, 50, 684–697.
Smith, H. L., McAusland, L., & Murchie, E. H. (2017). Don’t ignore the green light: exploring diverse roles in plant processes. Journal of Experimental Botany, 68, 2099–2110.
Liu, J. & van Iersel, M. W. (2021). Photosynthetic physiology of blue, green, and red light: light intensity effects and underlying mechanisms. Frontiers in Plant Science.