PAR, CO₂, and VPD Requirements for Greenhouse Tomatoes at Different Growth Stages

Greenhouse tomatoes are one of the world’s most intensively optimized crops. To achieve maximum yield, sugar production, fruit density, and consistent flavor, growers must carefully control three key variables:

• PAR (light intensity)
• CO₂ concentration
• VPD (leaf transpiration / humidity balance)

Below, we break down the ideal environmental targets for tomatoes throughout their entire growth cycle.

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1. Seedling Stage (Propagation Phase)

Plant Status:
Small leaf area, fragile roots; energy investment goes into root establishment and leaf expansion.

Parameter	Range	Notes
PAR	150–250 µmol·m⁻²·s⁻¹	Gentle light to avoid photo-stress
DLI	8–12 mol·m⁻²·d⁻¹	Slow daily light accumulation
CO₂	400–600 ppm	Elevated CO₂ not beneficial yet
VPD	0.5–0.8 kPa	Higher humidity helps rooting
Temp	22–25 °C	Warm and moist

Explanation:
Seedlings have limited photosynthetic capacity.
At this phase, root development > photosynthesis.
Too much PAR or VPD can quickly dry seedlings.

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2. Vegetative Growth Stage (Rapid Leaf & Stem Expansion)

Plant Status:
Leaves expanding rapidly, big increase in carbon demand.

Parameter	Range	Notes
PAR	350–700 µmol·m⁻²·s⁻¹	High light accelerates canopy building
DLI	18–25 mol·m⁻²·d⁻¹	Strong daily accumulation
CO₂	800–1000 ppm	Major boost to biomass
VPD	0.8–1.2 kPa	Encourages transpiration & CO₂ uptake
Temp	22–26 °C	Efficient for carbohydrate production

Explanation:
This is the stage where CO₂ enhancement matters the most.
Leaves are now large enough to absorb CO₂ efficiently, and stomatal conductance is high under proper VPD.

This is when increasing CO₂ results in:

• faster growth
• thicker stems
• larger leaves
• stronger nutrient demand

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3. Flowering / Pre-Fruit Stage

Plant Status:
Transition from vegetative to reproductive growth.

Parameter	Range	Notes
PAR	550–800 µmol·m⁻²·s⁻¹	Strong PAR supports energy storage
DLI	22–30 mol·m⁻²·d⁻¹	High daily light load
CO₂	900–1200 ppm	Supports fruit-set potential
VPD	1.0–1.3 kPa	More transpiration & CO₂ uptake
Temp	22–26 °C	Stable microclimate

Explanation:
At this stage, flower production and carbohydrate transfer intensify.
Balanced VPD ensures pollen viability and healthy flower development.

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4. Fruiting / Yield Development Stage

Plant Status:
Sugar redistribution toward fruits; vegetative expansion slows.

Parameter	Range	Notes
PAR	500–700 µmol·m⁻²·s⁻¹	Still high, but less than peak
DLI	20–25 mol·m⁻²·d⁻¹	Consistent daily light
CO₂	700–1000 ppm	Still helpful but lower demand
VPD	1.2–1.5 kPa	Avoid excess humidity to prevent disease
Temp	20–24 °C	Slightly cooler for fruit quality

Explanation:
Elevated CO₂ still helps sugar formation,
but the carbon sink now shifts to fruit, not leaf mass.

A too-humid environment leads to:

• blossom-end rot risk
• botrytis
• fungal spread

Thus, higher VPD improves transpiration and reduces disease risk.

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How PAR, CO₂, and VPD Work Together for Tomatoes

High PAR without CO₂

→ wasted sunlight

High CO₂ without PAR

→ CO₂ not utilized

Improper VPD (too humid or too dry)

→ stomata close → CO₂ absorption drops → growth declines

The ideal balance:

Condition	Outcome
High PAR + High CO₂ + Optimal VPD	Maximum growth & sugar production
High PAR + Low CO₂	Growth bottleneck
Low PAR + High CO₂	CO₂ wasted
Incorrect VPD	CO₂ uptake compromised

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Practical Grower Advice

✔ If leaves are large, green, and actively transpiring → increase CO₂
✔ If humidity is too high → lower it to reopen stomata
✔ If VPD is low → CO₂ uptake is restricted
✔ If PAR is high → ensure CO₂ is also high
✔ During fruiting → manage humidity aggressively to avoid disease

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Final Takeaway

Tomatoes are CO₂-hungry, light-driven crops — but only when their physiological capacity matches their environment.

• Seedlings: gentle environment
• Vegetative: maximum CO₂ + PAR
• Flowering: peak light & peak CO₂
• Fruiting: balanced light and moderate CO₂
• All stages: VPD must remain optimal

This is why using a multi-parameter device such as AH-200 — which simultaneously records PAR, CO₂, temperature, humidity, and VPD — allows growers to fine-tune greenhouse conditions with scientific precision.