Vapor pressure deficit (VPD) is an environmental factor that significantly influences plant health and growth. It describes the “thirst” of the air for water, indicating how much more moisture the air can hold before reaching saturation. Understanding and managing VPD is important in plant cultivation, as it directly impacts how plants interact with their surrounding atmosphere.
What is Vapor Pressure Deficit?
Vapor pressure deficit (VPD) quantifies the difference between the air’s current moisture content and the maximum it could hold if fully saturated at the same temperature. It measures the air’s “drying power,” indicating its capacity to absorb more water vapor. A higher VPD means the air is drier and can hold more moisture, while a lower VPD indicates more humid air with less capacity for additional water vapor.
Temperature and humidity combine to create VPD. Warmer air holds more water vapor than cooler air; for instance, a 20°F (11°C) temperature increase doubles the air’s water-holding capacity. This means that even if relative humidity remains constant, a rise in temperature will increase VPD. VPD is measured in kilopascals (kPa).
Unlike relative humidity, which is a percentage, VPD provides a more direct measure of the evaporative demand on plants. Relative humidity indicates the percentage of water vapor in the air compared to its saturation point at a given temperature, making it temperature-dependent. VPD offers a more precise understanding of the driving force behind water movement from plants into the atmosphere.
How VPD Influences Plant Processes
VPD impacts several physiological processes within plants, beginning with transpiration. Transpiration is the process where plants release water vapor through tiny pores on their leaves called stomata. VPD acts as the driving force for this water movement, pulling water from the roots, through the stem, and out of the leaves.
When VPD is high, dry air creates a strong pull, increasing transpiration. While increased transpiration can enhance nutrient uptake, excessively high VPD can lead to water stress, causing stomata to close to conserve water. This closure reduces carbon dioxide (CO2) intake, directly impacting photosynthesis. Conversely, very low VPD, indicating high humidity, slows transpiration. This reduced water movement can hinder the uptake of dissolved nutrients from the soil, potentially leading to deficiencies like calcium deficiency.
Photosynthesis is also significantly affected by VPD. Stomata must be open for CO2 intake, but also release water vapor. Extreme VPD levels, either too high or too low, can compromise photosynthetic efficiency by forcing stomata to close or by reducing the necessary water flow for nutrient transport. An imbalanced VPD can result in stunted growth, reduced yields, and increased susceptibility to various stresses or diseases.
Optimizing VPD for Plant Growth
Achieving optimal VPD is a dynamic process, as ideal ranges vary depending on the plant species and its growth stage. For instance, during the early vegetative stage, a VPD between 0.4 and 0.8 kPa is recommended to reduce excessive transpiration and encourage root development. As plants transition to the vegetative growth phase, a VPD of 0.8 to 1.2 kPa supports active growth and nutrient uptake. During the flowering stage, a slightly higher VPD, between 1.2 and 1.6 kPa, is beneficial to prevent fungal diseases like bud rot.
Growers monitor VPD by measuring air temperature and relative humidity using sensors. While charts provide VPD values based on air temperature and humidity, it is important to consider that leaf surface temperature can differ from air temperature, influencing the actual VPD experienced by the plant. Infrared thermometers can measure leaf temperature directly for more precise VPD calculations.
In controlled environments like greenhouses or indoor grow tents, growers employ various strategies to adjust VPD. To increase VPD (make the air drier), they can raise air temperature or decrease humidity using dehumidifiers or by increasing ventilation to remove moist air. To lower VPD (make the air more humid), they can decrease temperature or add moisture to the air using humidifiers or misting systems. Proper air circulation also plays a role by dispersing humid air around the plant canopy, which helps maintain a consistent VPD around the leaves.