Vapor Pressure Deficit

The air can hold only so much water vapour at a given temperature — the saturation point — and warmer air can hold more than cooler air. VPD is the difference between that saturation capacity and the amount of vapour actually present. It is closely related to relative humidity but is generally more informative for plants, because it accounts for how the air’s capacity changes with temperature.

When VPD is high, the steep gradient between the moist interior of a leaf and the drier surrounding air pulls water out of the plant through its stomata; when VPD is low, that gradient is weak and transpiration slows.

VPD is a key driver of transpiration, the loss of water vapour from plants. Under high VPD, plants transpire faster and may close their stomata to conserve water, which also slows the uptake of carbon dioxide and can limit photosynthesis; under very low VPD, transpiration and the flow of water and nutrients it drives may be too slow, and prolonged high humidity can favour some diseases. There is generally a comfortable middle range that varies by crop.

• High VPD increases transpiration and can trigger stomatal closure and water stress
• Very low VPD can slow transpiration and nutrient movement and encourage some diseases
• VPD interacts with soil water supply: high demand is more damaging when soil water is limited
• Managing VPD is central to controlled-environment and greenhouse production

In greenhouses and other controlled environments, growers actively manage VPD through heating, ventilation, humidification, and shading to keep transpiration and growth in a favourable range. In the field, VPD helps explain patterns of water demand and stress and contributes to estimates of evapotranspiration used in irrigation planning, alongside radiation, wind, and temperature.

This entry explains VPD as a concept and its role in transpiration and management. The favourable VPD range and the responses to it are crop- and situation-specific, and VPD is one of several interacting factors — including radiation, wind, temperature, and soil water — that together determine crop water use. Specific target values are not given here.