Vapour Pressure Deficit (VPD) is a metric used extensively in environmental science and agriculture to quantify the drying power of the air surrounding a plant. It acts as a single measure that integrates the effects of both temperature and humidity on the atmosphere’s ability to draw moisture away from a surface. Understanding this deficit is fundamental for optimizing plant growth, as it dictates the rate at which plants lose water to their environment.
The Core Concept of Vapour Pressure Deficit
Vapour Pressure Deficit is the difference between the maximum amount of moisture the air can hold when fully saturated and the amount of moisture it is currently holding. Air at a specific temperature has a maximum capacity for water vapor, corresponding to 100% relative humidity. The deficit is the gap between this maximum capacity and the actual vapor pressure present.
Temperature plays a significant role because warmer air holds substantially more water vapor than cooler air. For example, a plant growing in air at 50% relative humidity experiences a higher VPD if the temperature is 80°F than if it is 70°F. This occurs because the air’s potential to hold water increases exponentially with temperature, widening the gap between the actual and saturated state. Therefore, VPD provides a more accurate picture of the plant’s moisture stress than relative humidity alone.
How VPD is Measured and Calculated
Calculating VPD requires determining two components: the Saturated Vapor Pressure (SVP) and the Actual Vapor Pressure (AVP). SVP represents the maximum amount of water vapor the air can hold at a given temperature, making it a function of temperature alone. This value is determined from psychrometric tables or equations.
AVP is the measured amount of water vapor currently in the air, derived using the air temperature and the relative humidity reading. The calculation is straightforward: VPD equals the Saturated Vapor Pressure minus the Actual Vapor Pressure (VPD = SVP – AVP). The resulting figure is expressed in kilopascals (kPa), the standard unit of measurement for this atmospheric pressure differential.
VPD’s Role in Plant Health and Transpiration
Transpiration, the process by which plants move water from the roots through the stem and out through the leaves, is directly governed by the Vapour Pressure Deficit. The mechanism for this movement is the difference in vapor pressure between the inside of the leaf and the surrounding air. This difference creates a pressure gradient that pulls water upwards, much like sipping water through a straw.
When the VPD is high, the air is very dry and has a strong pulling force, causing the plant to transpire rapidly. While a high transpiration rate is beneficial for delivering nutrients, excessively high VPD forces the plant to close its stomata (the tiny pores on the leaf surface) to conserve water. This closure limits carbon dioxide intake for photosynthesis and can lead to heat stress and slowed growth.
Conversely, a very low VPD indicates that the air is near saturation, and the pressure gradient pulling water out of the leaves is weak. This condition significantly slows the transpiration rate, restricting the plant’s ability to draw water and dissolved nutrients from the roots. Stagnant water movement can result in deficiencies because the plant cannot transport sufficient calcium and other immobile nutrients to new growth.
Low VPD environments (below 0.4 kPa) increase the risk of fungal and mold issues due to persistently high humidity. The lack of evaporative cooling from slowed transpiration also makes the plant more susceptible to high leaf temperatures. Maintaining VPD within a comfort zone, often between 0.8 and 1.2 kPa for many common crops, ensures robust water movement without inducing water-conservation stress.
Managing VPD for Optimal Growing Environments
In controlled environment agriculture, such as greenhouses and indoor grow rooms, VPD is actively managed to sustain optimal rates of transpiration. Controlling VPD involves the simultaneous manipulation of both air temperature and humidity, as these two variables determine the deficit. Growers use VPD charts, which correlate temperature and relative humidity, to visualize and target desired atmospheric conditions.
To lower the VPD (reducing the air’s drying power), a grower can decrease the air temperature or increase the relative humidity, often by misting or activating humidifiers. This action is taken when plants are young or under stress to encourage open stomata and high rates of photosynthesis. Conversely, to raise the VPD, growers increase the air temperature or decrease the humidity using dehumidifiers or ventilation.
This targeted environmental control allows growers to fine-tune the plant’s physiological processes to match developmental stages. For instance, a higher VPD might be set during latter growth stages to encourage stronger cell wall development and increase nutrient uptake by promoting faster water movement. Using VPD as a guide, environmental control systems maintain a precise equilibrium that maximizes both plant health and resource efficiency.