Greenhouse tomato production is not controlled by light alone.
Tomatoes use photosynthetically active radiation (PAR) as the energy source for photosynthesis, CO₂ as the carbon source for building carbohydrates, and the surrounding temperature and humidity determine vapor pressure deficit (VPD), which influences transpiration and stomatal behavior.
These factors interact.
A tomato canopy may receive strong light but use it inefficiently if CO₂ becomes depleted. High CO₂ does not compensate for severe heat or water stress. And a VPD that is too high or persistently too low can alter transpiration and gas exchange even when light and CO₂ appear favorable.
For that reason, PAR, CO₂ and VPD are more useful when measured together than when treated as three independent target numbers.
This guide explains what published greenhouse research tells us about these variables and how their importance changes as tomato plants move from seedlings to mature fruiting crops.
Quick Reference: What Should You Measure?
| Measurement | What It Tells You | Useful Reference |
|---|---|---|
| PPFD / PAR | Photosynthetic light reaching the crop right now | Measure at canopy level and across multiple locations |
| DLI | Total PAR accumulated across the day | Approximately 20–30 mol/m²/day is a common reference for productive mature tomato crops |
| CO₂ | Carbon available for photosynthesis | Ambient air is around 400+ ppm; greenhouse enrichment studies commonly investigate approximately 600–1000 ppm |
| VPD | Atmospheric demand for water from the plant | Greenhouse tomato literature commonly reports favorable conditions roughly around 0.3–1.0 kPa, depending on conditions |
These are reference ranges, not universal setpoints.
Cultivar, crop stage, light level, temperature, irrigation, greenhouse design, ventilation strategy and production goals all affect how the crop responds.
Why PAR, CO₂ and VPD Need to Be Considered Together
Photosynthesis requires both light and CO₂.
When PAR increases, the potential rate of photosynthesis generally increases — but only if the plant has sufficient CO₂ and its stomata can continue exchanging gases effectively.
VPD influences that exchange.
When the air is very dry relative to the leaf, VPD rises and transpiration demand increases. If water loss becomes excessive, stomata may partially close, which can reduce CO₂ uptake.
At the other extreme, persistently very low VPD means the air is highly humid. Transpiration can become weak, and prolonged high humidity can create additional crop-management and disease problems.
This is why the same PPFD can produce different plant responses under different CO₂ and VPD conditions.
PAR vs. DLI: Do Not Use One Midday Reading as a Daily-Light Target
PPFD is measured in µmol/m²/s.
It answers:
How much photosynthetically active light is reaching the crop right now?
DLI is measured in mol/m²/day.
It answers:
How much photosynthetically active light did the crop receive across the entire day?
This distinction is important in a greenhouse.
Structural elements, glazing, seasonal sun angle, cloud cover and neighboring crop rows can change light dramatically throughout the day.
A greenhouse may record a strong PPFD around noon but still have a relatively low DLI during a short winter day.
Conversely, moderate PPFD maintained for many hours can produce a useful daily photon total.
For mature tomato production, published horticultural guidance commonly places tomatoes around 20–30 mol/m²/day as a general DLI reference.
Other commercial greenhouse research has reported approximately 22–25 mol/m²/day as an effective range for year-round tomato production.
These values should be treated as production references rather than universal physiological thresholds.
Seedling Stage: Establishing a Compact, Healthy Plant
Tomato seedlings should not be managed using the same environmental targets as a mature greenhouse crop carrying fruit.
Young plants have:
- smaller leaf area
- smaller root systems
- lower total carbon demand
- lower whole-plant light interception
Research on tomato seedlings has successfully produced plants under a wide range of DLIs, including treatments above 20 mol/m²/day in controlled production systems.
That does not mean seedlings require those DLIs.
It means there is no strong scientific basis for assigning one universal number such as “150–250 µmol/m²/s is optimal for all tomato seedlings.”
Instead, use measurements to evaluate plant response.
Look for:
- stem elongation
- leaf expansion
- plant compactness
- leaf temperature
- substrate drying rate
- differences between propagation locations
PAR / DLI
Measure PPFD at seedling-canopy height rather than at the fixture.
If seedlings stretch strongly, insufficient total light or uneven light distribution may be contributing.
If high light causes rapid heating or excessive water demand, increasing PPFD further may not improve seedling quality.
CO₂
Under normal ventilation, ambient CO₂ may be sufficient for many propagation situations.
Research has shown that tomato seedlings can respond positively to CO₂ enrichment, particularly when light availability is also sufficient.
But this response depends strongly on DLI.
Increasing CO₂ while providing very little usable light does not create the same benefit as increasing CO₂ under a photosynthetically productive light environment.
VPD
Avoid treating one exact VPD as mandatory for seedlings.
Young plants generally benefit from avoiding both severe atmospheric dryness and persistently saturated humidity.
The objective is a stable environment that supports transpiration without excessive water stress.
Vegetative Growth: The Canopy Changes the Measurement Problem
After transplanting, tomato plants rapidly increase leaf area.
This changes how PAR should be measured.
A single top-canopy reading becomes increasingly inadequate because greenhouse structures and leaves create spatial differences in light distribution.
Measure:
- several positions along the row
- brighter and darker greenhouse zones
- the upper canopy
- representative locations inside the canopy
- different times of day
The goal is to understand not only how much light reaches the brightest leaves, but how light is distributed through the crop.
Why DLI Becomes More Useful
As canopy size increases, daily carbon demand also rises.
DLI becomes useful for comparing:
- cloudy vs. sunny days
- winter vs. summer
- different greenhouse bays
- shaded vs. unshaded rows
- supplemental-light schedules
During low-light seasons, supplemental lighting can substantially increase tomato DLI and yield.
Research in semi-closed Mediterranean greenhouses demonstrated that supplemental LEDs raised daily light received by the crop during low-natural-light periods and increased tomato yield.
Flowering and Fruit Set: More Light Does Not Work in Isolation
Flowering and fruit set are often where environmental interactions become most obvious.
The crop now needs to support:
- continued canopy growth
- flower development
- pollination
- fruit set
- developing fruit
Light therefore becomes increasingly important.
But simply maximizing PAR is not a complete strategy.
Fruit set can also be affected by:
- temperature
- CO₂ availability
- humidity
- VPD
- irrigation
- nutrient supply
- cultivar
- crop load
A high PPFD reading cannot compensate for an environmental condition that is restricting stomatal function or reproductive development.
CO₂: Why Greenhouse Concentration Can Become Limiting
Outside atmospheric CO₂ is now around the low-400-ppm range.
Inside a greenhouse, actively photosynthesizing plants can reduce CO₂ concentration if the structure is relatively closed and fresh-air exchange is limited.
Under strong light, this can restrict photosynthesis.
CO₂ enrichment is therefore widely used in commercial greenhouse production.
A recent systematic review of protected agriculture found that tomato studies commonly report positive responses within approximately 600–1000 µmol/mol (ppm) CO₂.
The review reported improved photosynthetic performance across tomato developmental stages and cited studies in which photosynthetic rate peaked around 1000 ppm under the specific experimental conditions.
Other studies have used approximately:
- 700–800 ppm
- 800 ppm
- 1000 ppm
with positive effects on photosynthesis, biomass, water-use efficiency or yield.
These results do not mean 1000 ppm is automatically the ideal tomato setpoint.
More CO₂ Is Not Automatically Better
The value of CO₂ enrichment depends on the environment.
CO₂ is most useful when the crop also has:
- adequate light
- appropriate temperature
- sufficient water
- functioning stomata
- sufficient nutrients
If PAR is very low, adding large amounts of CO₂ may provide diminishing returns.
If greenhouse ventilation is wide open, maintaining high CO₂ concentrations may be economically inefficient.
If the crop is experiencing severe heat or water stress, carbon availability may no longer be the main limitation.
That is why CO₂ should be interpreted together with PAR and climate data.
Light and CO₂ Interact
Research and crop models consistently show interaction between light availability and CO₂ response.
One greenhouse tomato simulation estimated that increasing CO₂ from approximately 400 to 800 ppm could substantially increase annual fresh-weight yield under its modeled production conditions.
Other protected-agriculture studies have found strong responses when supplemental lighting and CO₂ enrichment are used together.
The practical lesson is more important than any single percentage:
The benefit of CO₂ enrichment depends partly on how much photosynthetically useful light the crop can use.
If light is strongly limiting, carbon enrichment alone cannot fully compensate.
If light is abundant but CO₂ is depleted, additional photons may not be used as effectively as expected.
VPD: Why Relative Humidity Alone Is Not Enough
Relative humidity changes with temperature.
That means 70% RH at one temperature does not impose the same atmospheric demand on a leaf as 70% RH at another temperature.
VPD combines temperature and humidity into a value that more directly describes the vapor-pressure difference driving transpiration.
For greenhouse tomatoes, published reviews commonly cite a favorable VPD range around:
0.3–1.0 kPa
under many greenhouse conditions.
Other commercial tomato research has described approximately 0.2–1.3 kPa as a broader range associated with little or no major physiological limitation.
These should again be treated as reference ranges rather than rigid stage-specific targets.
What Happens When VPD Is Too High?
High VPD means the atmosphere is placing strong evaporative demand on the crop.
As VPD rises:
- transpiration demand increases
- water loss can increase
- stomata may partially close
- CO₂ uptake can decline
- photosynthesis can become constrained
Greenhouse tomato research has reported higher transpiration but lower photosynthesis under high VPD.
One study cited in greenhouse reviews found that reducing midday VPD from approximately 1.4 to 0.8 kPa increased tomato biomass and yield under its winter experimental conditions.
This demonstrates why high light plus high CO₂ does not automatically mean maximum photosynthesis.
The crop must still be able to maintain gas exchange.
What Happens When VPD Is Too Low?
Very low VPD usually means high humidity.
That is not automatically ideal either.
Persistently low VPD can reduce transpiration.
It can also contribute to conditions associated with:
- weak nutrient transport
- canopy condensation
- leaf wetness
- fungal disease risk
Greenhouse reviews have also associated continuously low VPD combined with inconsistent root-zone moisture with calcium-related physiological problems in tomato.
The objective is therefore not to drive VPD as low as possible.
It is to avoid prolonged extremes.
Fruiting Stage: The Whole Crop Environment Matters
A mature high-wire tomato crop represents a very different biological system from a seedling.
It has:
- a large photosynthetic canopy
- a substantial fruit load
- high daily water use
- high carbon demand
- significant self-shading
Commercial greenhouse tomato research commonly operates at DLIs around the 20–30 mol/m²/day range, although the economically appropriate amount of supplemental light varies by region and season.
CO₂ enrichment may improve photosynthesis and yield when other conditions support active carbon assimilation.
VPD influences whether the crop can maintain effective transpiration and stomatal gas exchange.
The three measurements should therefore be interpreted as a system.
Why a “Perfect PAR Number” Is Misleading
Suppose two greenhouse tomato canopies both measure:
600 µmol/m²/s PPFD
at noon.
That does not mean their photosynthetic environments are equivalent.
Greenhouse A
- PPFD: 600 µmol/m²/s
- CO₂: 330 ppm
- high VPD
- plants experiencing water stress
Greenhouse B
- PPFD: 600 µmol/m²/s
- CO₂: 700 ppm
- moderate VPD
- adequate root-zone water
The light reading is identical.
The crop response may not be.
This is why PAR should not be interpreted independently from the surrounding greenhouse environment.
A Better Measurement Workflow
For practical greenhouse monitoring, start by asking a question rather than searching for one “perfect” number.
1. Measure PAR at Crop Level
Do not measure beside the fixture or only in the brightest greenhouse location.
Place the sensor:
- around canopy height
- in representative crop locations
- away from your own shadow
- at consistent positions when comparing measurements
2. Measure Across Space
Check more than one location.
Greenhouse beams, screens, neighboring rows and canopy structure can produce significant variation.
3. Look at DLI, Not Only Noon PPFD
If the question concerns total crop light, log PAR across the entire photoperiod.
This is particularly important in winter, during variable cloud cover or when evaluating supplemental lighting.
4. Record CO₂ During the Light Period
Do not assume greenhouse CO₂ equals outdoor concentration.
Watch what happens after photosynthesis becomes active.
If CO₂ falls while PAR is high, carbon availability may be becoming a limiting variable.
5. Track Temperature, Humidity and VPD Together
A humidity reading alone can be misleading because temperature changes the atmospheric vapor-pressure relationship.
Watch how VPD changes during:
- sunrise
- midday
- ventilation
- heating
- irrigation
- supplemental lighting
6. Look for Patterns, Not Single Readings
One measurement is a snapshot.
Greenhouse management decisions are stronger when based on trends across hours and days.
Practical Reference by Growth Stage
Instead of assigning unsupported “ideal” PPFD, CO₂ and VPD numbers to every stage, use the following framework.
Seedlings
Primary questions:
- Is daily light sufficient for compact growth?
- Is the propagation area uniform?
- Is the atmosphere too dry or persistently saturated?
Do not apply mature fruiting-crop targets automatically.
Vegetative Growth
Primary questions:
- Is canopy light distribution becoming uneven?
- Is DLI sufficient as the crop gets larger?
- Does CO₂ fall during periods of strong photosynthesis?
- Does VPD rise excessively during the middle of the day?
Flowering and Fruit Set
Primary questions:
- Is usable light sufficient across several days?
- Are temperature and VPD supporting reproductive development?
- Is CO₂ becoming depleted during high-light periods?
At this stage, environmental interactions often matter more than maximizing any one variable.
Mature Fruiting Crop
Primary questions:
- Is total DLI appropriate for the production system?
- Is supplemental lighting filling a real seasonal deficit?
- Is CO₂ enrichment economically useful under the available light?
- Is VPD allowing effective transpiration and gas exchange?
- Is light reaching enough of the canopy rather than only the upper leaves?
Practical Research-Based References
For mature greenhouse tomatoes, a useful starting framework from published literature is:
DLI: approximately 20–30 mol/m²/day
CO₂: ambient conditions may be adequate in ventilated systems, while enrichment studies commonly investigate approximately 600–1000 ppm
VPD: approximately 0.3–1.0 kPa is frequently cited as a favorable greenhouse reference range, with some literature describing a wider approximately 0.2–1.3 kPa physiological range
These are not specifications that every grower should program into a controller.
They are evidence-based reference points for interpreting measurements.
Final Takeaway
Greenhouse tomatoes do not have one universal PAR, CO₂ or VPD requirement for every stage of growth.
What changes through the crop cycle is not simply a set of three target numbers.
The entire plant changes:
- leaf area increases
- canopy structure becomes more complex
- fruit creates additional carbon demand
- water use increases
- self-shading increases
- environmental interactions become more important
For mature tomato production, approximately 20–30 mol/m²/day DLI is a useful literature-based reference.
CO₂ enrichment in the 600–1000 ppm region has produced positive responses in many protected-agriculture studies, but its benefit depends strongly on light and climate.
Greenhouse tomato literature commonly places favorable VPD conditions around 0.3–1.0 kPa, while emphasizing the risks of both excessively high and persistently low VPD.
The most useful strategy is therefore not:
Find one perfect number.
It is:
Measure PAR, CO₂, temperature, humidity and VPD together, follow how they change through the day, and interpret those measurements in the context of crop stage and plant response.
That creates a stronger basis for greenhouse decisions than light intensity alone.
References
Virginia Tech Cooperative Extension. Calculating and Using Daily Light Integral (DLI): An Introductory Guide.
Virginia Tech Cooperative Extension. Basic Tomato Physiology and Morphology.
Appolloni et al. Supplemental LED Increases Tomato Yield in Mediterranean Semi-Closed Greenhouse. Agronomy.
Shamshiri et al. Advances in Greenhouse Automation and Controlled Environment Agriculture: A Transition to Plant Factories and Urban Agriculture.
Energy and Water Related Parameters in Tomato and Cucumber Greenhouse Crops in Semiarid Mediterranean Regions — A Review. Horticulturae.
Microclimate Monitoring in Commercial Tomato Greenhouse Production and Its Effect on Plant Growth, Yield and Fruit Quality. Frontiers in Horticulture.
CO₂ Enrichment in Protected Agriculture: A Systematic Review of Greenhouses, Controlled Environment Systems, and Vertical Farms — Part 2. Sustainability.
Simulating the Photosynthetic and Annual-Yield Enhancement of a Row-Planted Greenhouse Tomato Canopy Through Diffuse Covering, CO₂ Enrichment, and High-Wire Techniques. Horticulturae.
Related AquaHorti Tools
For handheld PAR / PPFD measurements, see AquaHorti AH-Quantuv.
For all-day PAR logging and DLI measurement, see AquaHorti AH-PARDLI.
For greenhouse monitoring where PAR needs to be viewed together with CO₂, temperature, humidity and VPD, see AquaHorti AH-200.