How leaves breathe through stomata

The underside of a leaf is dotted with tiny pores called stomata, each framed by two guard cells that swell to open the pore and slacken to close it. When open, carbon dioxide diffuses in for photosynthesis and water vapor diffuses out, a loss called transpiration. How hard the air pulls that vapor out depends on the vapor pressure deficit: how far the air is from being saturated at its temperature.

Controls

Labels

Animation speed

Air temperature

Relative humidity

Plant well watered

Step through it

Leaf layersSunlight passes the waxy upper skin into columns of palisade cells packed with chloroplasts.
Air spacesLoosely packed spongy cells leave air spaces that stay humid, close to 100 percent relative humidity.
Guard cellsWhen guard cells take up water they bow apart and open the pore. When the plant runs short of water they go limp and the pore closes.
Hot, dry air pulls hardAt 35 °C and 30 percent humidity the vapor pressure deficit is high, so water leaves fast and stomata start to narrow.
Drought responseTurn off watering and the stomata shut to save water, but that also blocks the CO2 the leaf needs to grow.

Vapor pressure deficit

Warm air can hold more water vapor than cool air. The Tetens equation estimates the saturation vapor pressure in kilopascals as 0.6108 x exp(17.27 x T / (T + 237.3)), with T in °C. The actual vapor pressure is that times relative humidity, and the deficit is the difference: VPD = saturation pressure x (1 minus RH / 100).

Inside the leaf the air is nearly saturated at leaf temperature, so VPD is a good measure of the pull on water leaving the stomata. This model uses air temperature for both, a common simplification; a sunlit leaf is often a few degrees warmer, which raises the real deficit. Stomatal opening here is a simple teaching curve, not a physiological model.

Try it: hold humidity at 60 percent and raise the temperature from 20 to 35 °C. The deficit more than doubles even though relative humidity never changed, and the water vapor stream speeds up.

Why growers watch VPD

Greenhouse growers often aim for roughly 0.4 to 1.6 kPa. Too low and leaves cannot move water and nutrients well, and fungal disease thrives. Too high and plants close their stomata to avoid wilting, which slows photosynthesis and growth.

Where Avid uses this

Temperature and humidity readings turned into VPD are among the signals a precision agriculture system like Avid's VerdantaIQ can use to time irrigation.

Sources

12.5: Stomatal Opening and Closure (General Botany)LibreTexts Biology, Norco College
Systemic effects of rising atmospheric vapor pressure deficit on plant physiology and productivityGlobal Change Biology, via PubMed Central

How to cite this model

Avid Solutions International. (2026, November 11). How leaves breathe through stomata [Interactive 3D model]. Avid Field Lab. https://avidsolutionsintl.com/explore/transpiration-stomata

Last updated November 11, 2026

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