Why should greenhouse growers pay attention to vapor-pressure deficit and not relative humidity?

Vapor-pressure deficit is independent of temperature and is a more accurate measure to predict plant transpiration and water loss than relative humidity.

When finishing plants, MSU Extension recommends maintaining a greater vapor pressure deficit (greater than 0.5 kPa) to allow plants to effectively transpire and prevent foliar diseases. All photos by Heidi Wollaeger, MSU Extension

When finishing plants, MSU Extension recommends maintaining a greater vapor pressure deficit (greater than 0.5 kPa) to allow plants to effectively transpire and prevent foliar diseases. All photos by Heidi Wollaeger, MSU Extension

Greenhouse ornamental and vegetable growers manage many environmental factors in their greenhouses, sometimes including humidity. Air with a high relative humidity, greater than 85 percent, can provide an environment conducive for fungal pathogens because the water lost through the stomata is slowly lost to the air and therefore remains on the leaves. While relative humidity is the most common way growers express the air’s saturation with water, it is not the best measurement to accurately predict plant transpiration, or water loss. Why?

Relative humidity refers to the amount of water vapor in the air versus what it can hold. The amount of water that air can hold varies with temperature; warmer air has a greater water-holding capacity than cooler air. The water-holding capacity of air approximately doubles with every 20 degree Fahrenheit increase in temperature. Therefore, air that is at 80 F can hold twice the amount of water compared with at 60 F.

When temperature is increased by 15 F increments and relative humidity stays the same, vapor pressure deficit varies from 0.55 kPa to 1.45 kPa.

Temperature (F)

Relative humidity (%)

Vapor pressure deficit (kPa)

60

70

0.55

75

70

0.90

90

70

1.45

 

When temperature is increased by 15 F increments and vapor pressure deficit stays the same, the relative humidity varies from approximately 22 to 71 percent.

Temperature (F)

Relative humidity (%)

Vapor pressure deficit (kPa)

60

22

1.38

75

41

1.38

90

71

1.38

 Instead of relative humidity, the more accurate way to express the driving force of water loss from a leaf is vapor pressure deficit (VPD). Its value is independent of temperature. VPD is the difference between the amount of moisture in the air and how much moisture the air could potentially hold when it is saturated. It is often measured in pounds per square inch (psi) or kilopascal (kPa). A high VPD (greater than 1.0 kPa) means that the air can still hold a large amount of water. Therefore, there is a fairly large gradient between plants (nearly saturated with water) and the air, enabling the plants to transpire and over time, dry out. A low VPD indicates the air is near saturation. A VPD of zero means the air is 100 percent saturated and thus plants cannot transpire effectively.

Growers should aim to have fairly low VPD, for example 0.3 kPa, when rooting cuttings in greenhouses. This will reduce the drying of young plants, thereby reducing the frequency of misting and watering required to keep plants hydrated. However, Michigan State University Extension recommends maintaining a greater VPD (greater than 0.5 kPa) in greenhouses while finishing plants, especially when there is a dense plant canopy. Plants will be able to transpire, cool themselves and be less stressed while the environment is less conducive to disease.

Greenhouse vegetable growers harvesting fruits should be aware that one study, “Vapor Pressure Deficit (VPD) Effects on the Physiology and Yield of Greenhouse Tomato,” reported that a VPD of 0.8 kPa during the day and night increased photosynthetic rates and tomato fruit yields compared to plants grown with a VPD of 0.5 kPa. Too dry of an environment can also cause problems. For example, another study, “High Vapor Pressure Deficit Influences Growth, Transpiration and Quality of Tomato Fruits,” showed that a very high VPD of 2.2 kPa could cause plant stress and fruit cracking in tomato.

Propagation Greenhouse 
In propagation greenhouses, MSU Extension recommends maintaining a fairly low vapor pressure deficit of about 0.3 kPa to prevent desiccation of young plants, especially during rooting of cuttings.

Peppers greenhouse 
When growing greenhouse vegetables, MSU Extension recommends maintaining a greater vapor pressure deficit (greater than 0.5 kPa) to allow plants to effectively transpire and to prevent foliar diseases.

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