Managing Vapor Pressure Deficit in Greenhouse Crops(book excerpts)
Vapor pressure deficit (VPD) is a more accurate way to measure greenhouse climate when determining water loss from the plant. VPD is simply the difference between the vapor pressure inside the leaf compared to the vapor pressure of the air. If the VPD is high (low humidity), meaning that the vapor pressure inside the plant is higher than the outside air, then more water vapor escapes out through the stomates (pores in the bottom of leaves). This process of water loss through the leaves is called “transpiration.” If the VPD is low (high humidity), the stomatal openings close and little water and fertilizer is taken up by the plant from the growing medium. VPD is important to know because it is used to schedule irrigations, to determine if air exchanges are needed, and if air temperature needs to be increased in order to hold more moisture. VPD is typically integrated into many greenhouse environmental control systems to manage humidity and for scheduling crop irrigation.
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