VPD = VPleaf - VPair
VPleaf = 610.7 * 107.5*Tleaf/(237.3+Tleaf) / 1000
VPair = 610.7 * 107.5*Tair/(237.3+Tair)/1000 * RH/100
VPleaf - vapor pressure of the leaf (canopy) in kPa
VPair - vapor pressure of the air in kPa
Tleaf - leaf temperature in Celsius
Tair - air temperature in Celsius
RH - air relative humidity
VPD - vapor pressure deficit in kPa
The ideal VPD range varies with the crop species and the stage of growth.
What if VPD is too low ?
Humidity is high and plants are unable to evapotranspirate enough water to enable the transport of nutrients to cells, even though the stomata may be fully open.
Some plants may even exude water through special leaf tip or edge structures (hydathodes or water glands), forming drops, in a process called guttation.
In cases where the VPD is extremely low, water may condense onto leaves and other plant parts. This can provide a medium for fungal/mold growth and disease.
Plants are unable to evaporate water and turgor pressure within the cells can cause splitting and cracking of fruits.
To increase the VPD, moisture must be removed from the air, or the moisture holding capacity of the air must be increased by raising the temperature. Moisture removal can be accomplished by using dehumidifiers (expensive), or by replacing moist air with drier air (typically through ventilation). This is the standard practice for avoiding direct condensation onto crop or greenhouse surfaces.