Plant growth is often discussed one variable at a time.
How much light does the crop need?
What CO₂ concentration is best?
What VPD should the greenhouse maintain?
Those are useful questions, but plants do not experience light, carbon dioxide and atmospheric moisture independently.
They experience them at the same time.
Light provides energy for photosynthesis. CO₂ supplies the carbon used to build carbohydrates. The moisture relationship between the leaf and surrounding air influences transpiration and stomatal behavior.
This means a plant can receive substantial light while photosynthesis remains limited by another part of the environment.
The most useful way to interpret greenhouse measurements is therefore not:
“What is the ideal PPFD?”
but:
“Is the crop able to use the PPFD it is receiving under the current CO₂, temperature, humidity and water conditions?”
That is the central relationship between PPFD, CO₂ and VPD.
First: PAR and PPFD Are Not the Same Thing
PAR stands for:
Photosynthetically Active Radiation
Traditionally, PAR refers to the wavelength range from approximately:
400 to 700 nm
A horticultural PAR meter normally reports:
PPFD — Photosynthetic Photon Flux Density
in:
µmol/m²/s
PPFD tells you how many photons within the defined photosynthetic range are reaching a square meter each second at the measurement position.
So when growers say:
“My PAR is 500.”
the more precise expression is usually:
“My PPFD is 500 µmol/m²/s.”
That distinction matters because PPFD is the quantity that can be compared with CO₂ concentration and environmental conditions at a particular moment.
What Does CO₂ Measure?
Carbon dioxide concentration is commonly expressed in:
ppm
or equivalently, in many scientific contexts:
µmol/mol
CO₂ is a raw material for photosynthesis.
Plants take CO₂ from the surrounding air and use carbon from that CO₂ to build carbohydrates.
For many C3 crops, increasing CO₂ above ambient concentration can increase photosynthetic carbon fixation when other resources are adequate.
But the response is not unlimited.
It depends on factors including:
light,
temperature,
water,
nutrient supply,
species,
growth stage,
and the plant’s ability to use the additional carbohydrates.
Large FACE experiments and subsequent meta-analyses have consistently shown that elevated CO₂ can increase photosynthesis and biomass in C3 plants, while the size of the response varies substantially with species and growing conditions.
What Is VPD?
VPD stands for:
Vapor Pressure Deficit
It describes the difference between the amount of water vapor the air could contain at saturation and the amount it actually contains.
In horticulture, VPD is commonly expressed in:
kPa
A larger VPD represents greater atmospheric demand for water.
A smaller VPD represents a more humid environment with less evaporative demand.
But there is an important technical detail.
The physiologically relevant gradient for a leaf depends on the vapor pressure inside the leaf compared with the surrounding air.
Because leaf temperature can differ from air temperature, a VPD calculated only from room temperature and relative humidity is an environmental approximation.
This is why two crops in the same greenhouse can sometimes experience somewhat different leaf-level conditions.
PPFD Provides Energy — But More Light Is Not Always More Photosynthesis
At low light intensity, increasing PPFD can strongly increase photosynthesis.
As PPFD rises, however, the response begins to flatten.
Eventually, other processes increasingly limit carbon fixation.
One possible limitation is CO₂ supply.
Another is temperature.
Another is stomatal limitation associated with water status and atmospheric demand.
Still others include nutrient availability and the plant’s capacity to use or store additional assimilated carbon.
So the relationship is not:
double PPFD = double growth.
Photosynthesis follows a nonlinear light-response curve.
The location of that curve also changes with the rest of the environment.
Why CO₂ Changes the Response to Light
Consider a leaf receiving low PPFD.
Its photosynthetic machinery has relatively little light energy available.
Adding more CO₂ may have only a limited effect because light remains strongly limiting.
Now increase PPFD.
More energy becomes available for carbon fixation.
At that point, CO₂ availability can become more important.
This is why the benefit of CO₂ enrichment is often larger under adequate or high light than under severely light-limited conditions.
Protected-agriculture research repeatedly shows that CO₂ enrichment interacts with lighting, temperature and other environmental variables rather than acting as an isolated input.
The practical principle is:
Do not evaluate CO₂ independently from light.
Why More CO₂ Does Not Make Light Unlimited
The reverse is also true.
If CO₂ concentration increases, the crop may be able to maintain a higher photosynthetic rate at a given PPFD.
But that does not mean:
more CO₂ makes unlimited PPFD useful.
At some point, other constraints become important.
Photosynthesis may become limited by:
electron transport,
temperature,
nutrient availability,
sink capacity,
or other metabolic processes.
Long-term exposure to elevated CO₂ can also produce acclimation, meaning the initial increase in photosynthesis is not always maintained at its original magnitude.
So CO₂ enrichment should not be treated as permission to simply maximize lighting.
C3 and C4 Plants Do Not Respond Identically
Crop type matters.
Most greenhouse vegetables such as:
tomato,
cucumber,
lettuce,
pepper,
and many herbs
use C3 photosynthesis.
C3 plants generally show a substantial photosynthetic response to elevated CO₂ when other resources are available.
C4 plants already possess a mechanism that concentrates CO₂ around Rubisco.
As a result, their direct photosynthetic response to elevated atmospheric CO₂ is often smaller under well-watered conditions.
Large field studies have repeatedly observed stronger direct CO₂ responses in C3 than C4 crops.
This is another reason a universal CO₂ recommendation for “plants” is not scientifically useful.
VPD Connects the Air Environment With Plant Water Loss
Water moves from roots through the plant and eventually exits primarily through stomata.
When atmospheric VPD rises, the driving force for water loss increases.
If water supply is adequate and VPD rises moderately, transpiration may increase.
But when evaporative demand becomes high enough, plants often reduce stomatal conductance to limit water loss.
That response can reduce CO₂ diffusion into the leaf.
Research across many species has linked increasing VPD with reductions in stomatal conductance and, under sufficiently high VPD, reductions in photosynthesis and productivity.
This creates an important interaction:
High PPFD does not guarantee high carbon fixation if atmospheric demand causes stomata to restrict gas exchange.
High VPD Does Not Simply Mean “More Transpiration Is Better”
It is tempting to think:
higher VPD → more transpiration → more nutrient movement → better growth.
That is too simple.
When VPD becomes high, plants may experience:
greater water loss,
higher irrigation demand,
reduced leaf water potential,
stomatal closure,
and reduced CO₂ uptake.
If root-zone water supply cannot keep pace with atmospheric demand, photosynthesis can decline.
Purdue Extension describes this relationship clearly: larger VPD creates greater water demand, while large VPD combined with dry root conditions can produce water deficits that slow photosynthesis and plant growth.
Very Low VPD Is Not Automatically Ideal Either
At the other extreme, very low VPD means highly humid air and weak evaporative demand.
That can reduce transpiration.
In some horticultural crops, persistently low transpiration can contribute to localized calcium-distribution problems in rapidly expanding tissues.
High humidity can also increase condensation and disease-management challenges in greenhouse environments.
But it would be incorrect to write:
“Low VPD prevents nutrient uptake.”
Mineral nutrition is much more complex than that.
A more accurate statement is:
VPD influences transpiration and therefore can affect the movement and distribution of some nutrients, especially calcium, while root-zone nutrient concentration, irrigation, airflow and crop physiology also matter.
Extension guidance on greenhouse crops specifically links humidity, transpiration and localized calcium disorders such as lettuce tipburn and tomato blossom-end rot.
There Is No Universal “Perfect VPD”
One of the most common greenhouse mistakes is publishing a single value such as:
VPD = 1.2 kPa
and presenting it as ideal for all crops and all stages.
The appropriate range depends on:
species,
leaf temperature,
growth stage,
water availability,
light intensity,
root-zone conditions,
and production objective.
Even tomato and cucumber research produces ranges that vary by season and environmental context.
VPD is therefore best treated as an environmental variable to interpret, not a universal score that should always be driven toward one number.
The Same PPFD Can Produce Different Photosynthesis
Imagine two crop environments with the same:
500 µmol/m²/s PPFD
Environment A
CO₂ is readily available.
Leaf temperature is appropriate.
Water supply is adequate.
Atmospheric demand is moderate.
Environment B
CO₂ has been drawn down inside a poorly ventilated growing space.
Air is hot and dry.
VPD is high.
The root zone cannot fully replace transpired water.
The PPFD reading is identical.
The plant response may not be.
This is why PPFD should be interpreted as:
available photon flux
rather than:
predicted growth rate.
The Same CO₂ Concentration Can Also Produce Different Results
Now reverse the example.
Two growing environments both measure the same CO₂ concentration.
But one crop receives substantial PPFD.
The other receives weak light.
CO₂ is not equally valuable under both conditions.
If photon supply is strongly limiting, increasing CO₂ cannot fully compensate for the lack of light energy.
This is why CO₂ measurements should always be interpreted in the context of the lighting environment.
The Same VPD Can Represent Different Leaf Conditions
VPD has another complication.
Many environmental controllers calculate VPD using:
air temperature
and:
relative humidity.
But the actual leaf surface can be warmer or cooler than the surrounding air.
Under strong radiation, a leaf may warm above air temperature.
Under active transpiration, evaporative cooling may lower leaf temperature.
Therefore, the leaf-to-air vapor pressure gradient can differ from the value calculated using air temperature alone.
For routine greenhouse monitoring, air-based VPD remains useful.
For high-precision physiological interpretation, leaf temperature provides additional context.
Elevated CO₂ Can Also Change Stomatal Behavior
The relationship becomes even more interesting because CO₂ itself affects stomata.
Elevated CO₂ commonly reduces stomatal conductance while simultaneously increasing photosynthesis in many C3 plants.
A major meta-analysis of FACE studies reported an average reduction in stomatal conductance alongside increased light-saturated photosynthesis under elevated CO₂.
So it would be incorrect to say:
“More open stomata always mean better CO₂ conditions.”
Plants can sometimes assimilate more carbon while maintaining lower stomatal conductance when external CO₂ concentration is greater.
This is one reason water-use efficiency can improve under elevated CO₂.
PPFD, CO₂ and VPD Affect Water-Use Efficiency Together
Water-use efficiency broadly describes how much carbon a plant gains relative to water loss.
Higher CO₂ can allow greater carbon fixation while stomatal conductance and transpiration decline.
High VPD, on the other hand, increases atmospheric water demand and can reduce stomatal conductance when conditions become stressful.
Light provides the energy that drives photosynthesis but can also increase leaf temperature and crop water demand.
These interactions explain why greenhouse climate management cannot be reduced to one sensor reading.
A Better Way to Think About Environmental Limitation
Instead of asking:
“Which variable is most important?”
ask:
“Which variable is limiting useful photosynthesis under the current conditions?”
Under one condition, light may be limiting.
Under another, CO₂ may be limiting.
Later in the same day, high VPD and water stress may become the dominant constraint.
This limiting factor can change hour by hour.
That is why environmental trends can be more informative than a single snapshot.
Morning Conditions Can Differ From Afternoon Conditions
A greenhouse illustrates this well.
In the morning, PPFD rises as sunlight increases.
Photosynthesis begins increasing and the canopy consumes CO₂.
If ventilation is limited, CO₂ concentration may decline.
Later, solar radiation and air temperature may increase further.
VPD may rise.
Even if PPFD remains high, stomatal behavior can change as atmospheric demand increases.
The limiting factor at:
09:00
may therefore differ from the limiting factor at:
14:00.
A single daily average can hide this sequence.
Why Time-Series Data Can Be Useful
If PPFD, CO₂, temperature and humidity are logged together, growers can see relationships that are difficult to identify from separate spot checks.
For example, a data record might show:
PPFD increasing,
CO₂ decreasing,
temperature rising,
humidity changing,
and calculated VPD increasing.
That does not automatically prove one variable caused a crop response.
But it creates a much stronger environmental record for understanding what conditions occurred at the same time.
This is particularly useful for:
greenhouses,
grow tents,
hydroponic systems,
plant factories,
and other controlled environments.
Correlation Is Not Proof of Cause
This distinction is important for both growers and technical writing.
Suppose yield declines during a week when VPD was higher.
That does not prove high VPD caused the yield change.
Other conditions might also have changed:
temperature,
DLI,
irrigation,
root-zone EC,
disease pressure,
crop load,
or nutrition.
Environmental logging helps identify patterns.
Controlled experiments are required to isolate causation.
AquaHorti articles should therefore separate:
what was measured
from:
what biological conclusion can legitimately be drawn.
Should You Increase CO₂ When PPFD Is High?
Sometimes, particularly in controlled production of responsive C3 crops.
But the correct answer depends on the crop and system.
CO₂ enrichment can improve photosynthesis and productivity, especially when light, temperature, nutrition and crop sink capacity are suitable.
Recent systematic reviews of protected agriculture continue to find substantial benefits from CO₂ enrichment, but they also emphasize that crop response depends strongly on environmental management.
So the correct rule is not:
high PPFD → always add CO₂.
It is:
evaluate whether CO₂ is becoming limiting and whether the crop and production system can economically use enrichment.
Closed Growing Spaces Can Experience CO₂ Depletion
In enclosed environments, active photosynthesis can reduce CO₂ concentration if fresh air or supplemental CO₂ does not replace what plants consume.
This can happen in:
grow rooms,
plant factories,
tightly controlled greenhouses,
and other low-ventilation spaces.
The risk becomes greater when:
plant density is high
and:
photon supply is strong.
In these environments, measuring CO₂ can reveal whether the assumed carbon supply is actually present during the light period.
Ventilation Solves More Than One Problem — But Creates Trade-Offs
Ventilation can introduce outside CO₂.
It can also remove:
heat
and:
humidity.
But ventilation can simultaneously release supplemented CO₂ and conditioned air.
This creates a common greenhouse-management trade-off.
A grower may want:
high CO₂,
controlled temperature,
and controlled humidity
at the same time.
Those objectives can conflict.
This is why environmental control strategies frequently change with:
weather,
time of day,
crop stage,
and energy cost.
High PPFD + High CO₂ Is Still Not Enough if the Crop Is Water Stressed
Suppose the grower supplies:
strong light
and:
elevated CO₂.
If VPD becomes very high and the root system cannot supply enough water, stomatal restriction and other stress responses can still reduce plant performance.
Additional CO₂ does not eliminate the need for adequate water relations.
Likewise, lowering VPD cannot replace insufficient photon supply.
Each variable contributes to a different part of the crop environment.
Low PPFD + High CO₂ Has Diminishing Value
If light is severely limiting, large CO₂ enrichment may provide limited additional benefit because the crop lacks enough photon energy to exploit the extra carbon efficiently.
This is one reason greenhouse CO₂ strategies often consider available solar radiation.
During dark periods or extremely low-light conditions, enrichment can have a very different economic and physiological value from enrichment during strong photosynthetic activity.
Crop Stage Matters
Seedlings, vegetative plants and heavily fruiting crops do not necessarily respond identically.
As crop leaf area increases:
total photon interception increases,
CO₂ consumption can increase,
transpiration changes,
and canopy microclimate becomes more complex.
A dense mature canopy can also create large differences between:
the greenhouse air above the crop
and:
the air inside the canopy.
This means environmental sensor placement becomes increasingly important.
Sensor Placement Matters
A CO₂ sensor near an open vent may not represent CO₂ concentration inside a dense crop canopy.
A temperature and humidity sensor mounted near the greenhouse roof may not represent conditions at leaf level.
A PPFD sensor far above the crop may not represent the light reaching the active canopy.
For meaningful comparisons:
measure as close as practical to the environment you are trying to understand.
Consistency is also important.
If sensor position changes, the trend may reflect the new position rather than a true environmental change.
PPFD Should Be Measured at the Crop
For grow-light measurement, place the quantum sensor around the crop reference plane or canopy level.
Do not measure:
directly against the fixture
and assume:
the crop receives the same PPFD.
For tall crops, multiple canopy positions may be useful because photon flux can decrease substantially from upper to lower leaves.
CO₂ Can Vary Spatially
CO₂ concentration is not always perfectly mixed.
Possible sources of spatial variation include:
airflow,
ventilation,
crop density,
CO₂ injection position,
and canopy structure.
In tightly controlled systems, air circulation helps create a more representative and uniform environment.
If measurements differ strongly between locations, the issue may be distribution rather than total CO₂ supply alone.
VPD Depends on Temperature and Humidity Together
Relative humidity alone can be misleading.
For example, the same:
70% RH
at two different temperatures does not produce the same vapor pressure deficit.
That is why VPD can provide more physiological context than relative humidity alone.
However, VPD should still be interpreted with:
leaf temperature,
water availability,
crop type,
and growth stage
when those details matter.
Do Not Use One Universal PPFD + CO₂ + VPD Recipe
A table such as:
PPFD = 700
CO₂ = 1,000 ppm
VPD = 1.2 kPa
may look precise.
But without crop, cultivar, temperature, photoperiod, DLI, growth stage and production-system context, it has limited biological meaning.
The correct combination for lettuce may differ from tomato.
A seedling may differ from a mature fruiting plant.
A greenhouse using sunlight may differ from a windowless plant factory.
The goal should be:
contextual measurement
rather than:
false precision.
What Should Growers Monitor?
A practical greenhouse measurement system can consider four related variables:
| Measurement | Main Question |
|---|---|
| PPFD | How much photosynthetic photon flux is reaching the crop now? |
| DLI | How much photosynthetic light accumulated during the day? |
| CO₂ | How much carbon dioxide is available in the crop environment? |
| Temperature + humidity / VPD | What atmospheric moisture demand is the crop experiencing? |
These measurements do not tell you everything about plant health.
But together they describe much more of the photosynthetic environment than PPFD alone.
Why DLI Still Matters in a PPFD + CO₂ + VPD Discussion
PPFD is instantaneous.
A plant does not experience only one instant.
The daily photon total can differ greatly even when peak PPFD is similar.
That is why DLI should be considered when evaluating whether the crop experienced:
a genuinely high-light day
or:
only a brief high-light period.
CO₂ and VPD trends can then be aligned with the PPFD curve to understand how atmospheric conditions changed during the same photoperiod.
A Practical Monitoring Workflow
Start with the crop question.
Measure PPFD at canopy level to determine the instantaneous light environment.
Track DLI if sunlight or lighting intensity changes throughout the day.
Monitor CO₂ during the active light period rather than assuming outside-air concentration represents the crop environment.
Track temperature and humidity so that VPD trends can be interpreted.
Then compare the variables over time.
Do not immediately assume that the highest value of any one measurement is desirable.
Instead, look for periods when one environmental variable may be constraining the crop’s ability to use the others.
When More Light May Not Help
Additional PPFD may produce limited benefit when:
CO₂ is strongly depleted,
the crop is water stressed,
VPD is excessively high,
temperature is outside a useful range,
nutrition is limiting,
or the crop has insufficient sink capacity.
This is why increasing fixture output should not be the automatic response whenever crop growth slows.
Measure first.
When More CO₂ May Not Help Much
Additional CO₂ may produce limited value when:
photon supply is very low,
temperature is unsuitable,
nutrition is limiting,
the crop is unhealthy,
or enrichment rapidly escapes through ventilation.
CO₂ supplementation should therefore be treated as a production strategy, not a universal plant-care rule.
When Adjusting Humidity May Not Solve the Problem
A VPD reading can indicate atmospheric demand.
But adjusting humidity alone will not fix:
poor irrigation,
damaged roots,
excessive temperature,
low PPFD,
or inadequate CO₂.
VPD is one diagnostic dimension of the crop environment.
It should not become a substitute for evaluating the whole system.
Frequently Asked Questions
Does high PPFD require high CO₂?
Not automatically.
Higher PPFD can increase the crop’s potential rate of photosynthesis, and responsive C3 crops can benefit from elevated CO₂ when light and other conditions are suitable.
But there is no universal PPFD threshold at which CO₂ enrichment suddenly becomes required.
Does higher CO₂ always increase plant growth?
No.
The response depends on plant type and environment.
C3 crops often respond more strongly than C4 crops, and the response can be limited by light, temperature, water, nutrition and sink capacity.
Does high VPD close stomata?
High VPD often reduces stomatal conductance as plants limit water loss, particularly when atmospheric demand becomes large relative to water supply.
The magnitude of the response varies by species and growing conditions.
Is low VPD always better?
No.
Very low VPD reduces evaporative demand and transpiration.
Persistently high humidity can also contribute to disease-management challenges and localized calcium-distribution disorders in some crops.
What is the ideal VPD?
There is no universal value for all plants.
Useful ranges depend on crop, growth stage, leaf temperature, irrigation and environmental conditions.
Can a plant have enough PPFD but still photosynthesize poorly?
Yes.
CO₂ depletion, high VPD, water stress, unsuitable temperature and other limitations can reduce carbon fixation even when photon supply is adequate.
Can CO₂ compensate for low light?
Only partially.
CO₂ cannot replace photons as the energy source for photosynthesis.
When light is strongly limiting, increasing CO₂ has progressively less value.
Why does CO₂ sometimes fall during the day in enclosed grow spaces?
Active photosynthesis consumes CO₂.
If ventilation or supplementation does not replace it quickly enough, concentration can decline during the light period.
Does elevated CO₂ always mean stomata open more?
No.
Elevated CO₂ often reduces stomatal conductance while increasing photosynthetic carbon fixation in C3 plants.
Is VPD calculated only from humidity?
No.
VPD depends on both temperature and water-vapor concentration, usually represented through relative humidity.
Leaf-level interpretation can also benefit from knowing leaf temperature.
Should I monitor PPFD or DLI?
Use PPFD to understand instantaneous intensity.
Use DLI to understand total daily photon exposure.
In greenhouses and variable sunlight, both can be useful.
Where should environmental sensors be placed?
Place them where they represent the crop environment as closely as practical.
Avoid assuming that measurements near a roof, vent or fixture represent conditions inside the canopy.
The Key Principle
PPFD, CO₂ and VPD should not be treated as three independent targets.
They describe different parts of the same photosynthetic environment.
PPFD provides photon energy.
CO₂ provides carbon.
VPD describes atmospheric water demand and influences gas exchange and transpiration.
Increasing one variable does not guarantee improved growth if another becomes limiting.
That is why the most useful greenhouse question is not:
“How high can I make PPFD, CO₂ or VPD?”
It is:
“Can the crop effectively use the light and carbon available under the current atmospheric and root-zone conditions?”
Measure the variables together.
Interpret them in crop context.
And avoid replacing a complex biological system with one universal environmental recipe.
References and Further Reading
Ainsworth, E. A. & Long, S. P. — What Have We Learned From 15 Years of Free-Air CO₂ Enrichment (FACE)? A Meta-Analytic Review of the Responses of Photosynthesis, Canopy Properties and Plant Production to Rising CO₂.
Ainsworth, E. A. & Rogers, A. — The Response of Photosynthesis and Stomatal Conductance to Rising CO₂: Mechanisms and Environmental Interactions.
Grossiord, C. et al. — Plant Responses to Rising Vapor Pressure Deficit. New Phytologist.
Poorter, H. et al. — A Meta-Analysis of Responses of C3 Plants to Atmospheric CO₂: Dose-Response Curves for 85 Traits From the Molecular to the Whole-Plant Level.
Kimball, B. A. — Crop Responses to Elevated CO₂ and Interactions With H₂O, N, and Temperature.
Purdue University Extension — Managing the Environment in High Tunnels for Cool Season Vegetable Production.
Oklahoma State University Extension — Greenhouse Carbon Dioxide Supplementation.
AquaHorti — Horticulture Measurement Guide: PAR, DLI, CO₂ & VPD.