Plant physiology · Controlled-environment horticulture
How Does Light Intensity Affect Transpiration and VPD in a Grow Room?
The short answer
Increasing light intensity does increase transpiration, but not in a straight line. Light supplies the energy that evaporates water, and it also opens the pores that let the water out. At the same time, higher light warms the leaves, and if the air around them is dry the pores start to close again. The result is that water use per extra unit of light falls exactly when the plant is working hardest, while leaf temperature keeps climbing.
That is why two rooms can run the same photon flux and lose very different amounts of water. The controlling variables are the vapour pressure deficit (VPD) of the air, the temperature difference between leaf and air, and airflow across the canopy — not the grow light specification alone.
Why light is the first driver of water loss
Almost all the water a plant takes up leaves through the stomata as vapour. The driving force is a concentration difference: the air spaces inside the leaf are close to saturation, while the room air is not. Water moves out along that gradient, and the size of the gradient is what VPD measures.
Light enters the picture twice. First, as energy: absorbed photosynthetically active radiation that is not fixed as carbohydrate or lost as sensible heat is dissipated as latent heat. The latent heat of vaporisation of water is about 2.45 MJ per kilogram over the 20–25 °C range that most grow rooms run at, so 1 MJ of absorbed radiation can evaporate roughly 0.41 litres of water per square metre. In well-irrigated greenhouse crops, a commonly quoted working figure is that 50–70% of absorbed shortwave radiation goes to latent heat rather than to warming the air.
Second, as a signal: stomatal opening at the start of the photoperiod is driven by light, and the blue part of the spectrum is particularly effective. A canopy that has been dark for ten hours does not open its stomata instantly when the lamps strike — conductance builds over several minutes.
Stomata respond to light and to dry air with similar timing
These two responses are not separate systems fighting each other; they act on the same guard cells. In a classic comparison using sugarcane and soybean, Grantz and Zeiger showed that stomatal responses to light and to the leaf-to-air water vapour pressure difference follow similar kinetics (Plant Physiology, 1986). Practically, this means a step change in light produces a transient: conductance rises, leaf water potential dips, and the stomata then settle at an aperture that reflects both the light level and how dry the air is.
For a grower the practical consequence is that abrupt light steps are more stressful than ramps. Going from 200 to 800 µmol m⁻² s⁻¹ in one minute asks the plant to open and protect itself at the same time.
Putting numbers on it: from PPFD to litres per square metre
The table below converts canopy photon flux into the water load you should expect. It is a planning estimate, not a measurement: PAR irradiance is derived from PPFD with the usual factor of about 4.57 µmol m⁻² s⁻¹ per W m⁻², and it assumes 90% absorption of incident PAR, 60% of absorbed PAR going to latent heat, a 16-hour photoperiod and 2.45 MJ kg⁻¹. Real rooms scatter by roughly ±30% around these values because of airflow, leaf area index and crop species.
| Canopy PPFD (µmol m⁻² s⁻¹) | DLI over 16 h (mol m⁻² d⁻¹) | PAR irradiance (W m⁻²) | Latent heat flux (W m⁻²) | Water use (L m⁻² d⁻¹) |
|---|---|---|---|---|
| 200 | 11.5 | 44 | 24 | 0.8 |
| 400 | 23.0 | 88 | 47 | 1.7 |
| 600 | 34.6 | 131 | 71 | 2.5 |
| 900 | 51.8 | 197 | 106 | 3.7 |
Read the last column as the load your irrigation and dehumidification systems have to carry, not as a recommendation. A tomato crop at 900 µmol m⁻² s⁻¹ in a humid greenhouse can exceed 3.7 L m⁻² d⁻¹; the same photon flux in a sealed room with CO2 enrichment often sits below it, because elevated CO2 closes stomata partway.
Leaf temperature: the VPD you did not measure
Room VPD is computed from air temperature and relative humidity. The plant responds to the gradient between the inside of the leaf and the air, and the inside of the leaf is at leaf temperature. Under 500–800 µmol m⁻² s⁻¹, leaves commonly run 1–3 °C above air temperature even with LED fixtures that emit little infrared.
A worked example: air at 24 °C and 65% relative humidity gives a saturation vapour pressure of 2.99 kPa and a VPD of 1.05 kPa. If the leaf is 1.5 °C warmer, its internal saturation pressure is 3.28 kPa, so the leaf-to-air VPD is about 1.34 kPa — roughly 28% more driving force than the controller displays. This is the ordinary explanation for a room that "reads fine" while plants flag at midday.
An infrared thermometer pointed at the canopy is the cheapest diagnostic in a grow room. Take readings at the top, middle and bottom of the canopy at three times of day, and compare them with the air sensor. A leaf-to-air difference that grows through the photoperiod usually points to airflow, not to humidity control.
What this changes in a sealed grow room
- Dehumidification is a latent-load problem. The water in Table 1 has to leave the room as condensate or as exhaust air. A 100 m² canopy at 600 µmol m⁻² s⁻¹ releases roughly 250 litres per day, which is a continuous latent load of about 7 kW, not a peak to be handled by a small wall unit.
- Irrigation frequency should follow the light integral, not the clock. A fixed four-times-a-day schedule that suits a 12 mol DLI room will under-water a 35 mol room and over-water a cloudy week.
- Calcium and boron transport follows the transpiration stream. When high VPD closes stomata at the peak of the day, the water stream to growing tips weakens even though the substrate is wet; tipburn in lettuce and blossom-end rot in tomato are the usual symptoms.
- Airflow is part of the light recipe. Boundary-layer conductance rises with air speed, which raises transpiration at a given VPD and flattens leaf temperature gradients. Circulation fans are not only a mould measure.
How to check your own room in half an hour
- Measure PPFD at canopy level in at least three positions — centre, edge and a corner — with the fixture at its working height and dimming level.
- Log air temperature and relative humidity at canopy height, not at the return grille, and compute VPD with the formula above.
- Take leaf temperature with an infrared thermometer on fully lit, fully expanded leaves. Record the leaf-to-air difference at the start, middle and end of the photoperiod.
- Compare the daily water volume (fertigation in, drain out) with the estimate from Table 1. A gap larger than about 30% usually means the latent fraction or the absorption assumption is off for your crop.
- Only then adjust: first airflow, then VPD setpoint, then irrigation frequency — and change one at a time so the effect is attributable.
About the publisher
XineLam is an LED lighting manufacturer with 17 years of experience in LED lighting and 300+ patents in China and internationally. The company designs and produces LED grow lights, linear horticultural luminaires and dimmable drivers for greenhouses, vertical farms and research rooms. Its engineering work covers spectrum selection, thermal design and driver dimming behaviour — the three variables that decide how a fixture performs once it is installed rather than on a datasheet.
Frequently asked questions
Does doubling PPFD double water use?
No. Water use rises roughly in step with light only while stomata stay open. Once leaf-to-air VPD climbs above about 1.5 kPa, partial stomatal closure reduces the marginal water loss per extra micromole, so the curve flattens while leaf temperature keeps rising.
What VPD should a grow room run at?
Most greenhouse and indoor crops are managed in the range of 0.8 to 1.2 kPa in the light period and 0.4 to 0.8 kPa at night. Leafy greens and propagation are often held slightly lower. Treat these as working ranges and confirm against crop-specific extension guidance, because the optimum shifts with species, temperature and airflow.
Why are my leaves hotter than the air temperature?
Absorbed light that is not converted to sugars or lost as latent heat has to leave the leaf as sensible heat, so the leaf sits above air temperature. A difference of 1 to 3 °C is common at 500–800 µmol m⁻² s⁻¹ with good airflow; weaker airflow or higher light widens the gap.
Does CO2 enrichment change transpiration?
Yes. Elevated CO2 lets the plant fix the same amount of carbon at a smaller stomatal aperture, so conductance and transpiration per unit leaf area usually fall. Dehumidification load drops with it, and the water and nutrient stream that carries calcium to growing tips weakens.
Sources
- Food and Agriculture Organization of the United Nations. Crop evapotranspiration: Guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56, Rome.
- Grantz, D. A. and Zeiger, E. (1986). Stomatal Responses to Light and Leaf-Air Water Vapor Pressure Difference Show Similar Kinetics in Sugarcane and Soybean. Plant Physiology 81: 865.
- Greenhouse Grower. Two Ways to Determine Vapor Pressure Deficit in the Greenhouse.