Vapor pressure deficit can impact the rate of photosynthesis affecting plant growth, flowering and fruiting of controlled environment crops.
Regardless of the type of controlled environment structure you grow in, maintaining the proper temperature and relative humidity are critical to producing a quality crop. Relative humidity refers to the amount of water vapor in the air compared to the maximum amount that air can hold at the same temperature.
Relative humidity affects the water relations of plants. Relative humidity fluctuates with changes in air temperature, and is not an ideal indicator for assessing plant transpiration potential.
Another important parameter of growing controlled environment crops is vapor pressure deficit (VPD). VPD is important for all types of crops and can impact different stages of growth. VPD can affect flowering causing the abortion of flower buds. VPD can also lead to nutrient deficiencies that cause issues with fruiting.
“Vapor pressure in greenhouse applications is usually related to the difference in the vapor pressure inside the leaf surface and in the air,” said Ying Zhang, assistant professor and controlled environment specialist at the University of Florida. “Vapor pressure deficit is the difference between these two vapor pressures. VPD is important because plants respond to changes in the vapor pressure directly. Vapor pressure is the key driver for transpiration which can indirectly influence the photosynthetic rate and nutrient uptake of plants.”
Controlling VPD with ventilation alone can be challenging unless the weather conditions are ideal, meaning it’s not too hot or cold or too dry or humid. Ventilation is primarily responsible for bringing in dryer air to replace the moist air in semi-closed greenhouses. VPD is commonly used as a reference for temperature and humidity control.
“If the outside climate conditions are ideal for growth, then VPD is going to be much easier to control in a greenhouse,” Zhang said. “In Florida, the weather in the summer is typically hot and humid. Ventilation may help to reduce the air temperature and remove some of the moisture in the greenhouse. But it is difficult to bring the temperature and humidity to the desired levels to maintain ideal VPDs.”
In hot, dry climates like Arizona during spring and winter, the outdoor climate may be well suited for maintaining VPD through ventilation.
“During the summer in Arizona the climate can be very hot and dry,” Zhang said. “Evaporative cooling works better in dry environments. However, due to the intensive radiation received in the greenhouse and the impact of high outdoor temperatures, it is still challenging to reduce the air temperature to ideal conditions in the summer.
“In Florida during the summer, evaporative cooling won’t be able to help lower the greenhouse temperature to ideal levels because the outdoor humidity is high so it is very challenging to reduce the temperature and humidity levels. In Florida, some growers shut down their greenhouses during the summer to avoid trying to grow under these conditions because it is difficult to maintain a suitable environment for plant production for many crops.”
Impact of high, low VPD
A high VPD can result in an accelerated transpiration rate in plants and a slowing down or stopping of photosynthesis.
“Plants have the mechanism to control water loss through transpiration,” Zhang said. “When plants lose too much water, their stomates close to prevent excess water loss. Through this process plants reduce the cooling effects from transpiration causing the plants to heat up. The closing of the stomates also reduces or stops photosynthesis because the stomates are responsible for the intake of carbon dioxide and release of oxygen and water vapor.”
Even if enough water is supplied to the plants the closure of the stomates can lead to high surface temperatures which can result in damage or death of plants. Too much water loss can impact the growth and health of plants.
VPD is the driving force of plant transpiration. Low VPD with high humidity conditions can slow down the transpiration rate.
“Low transpiration can lead to less water uptake, which can impact nutrient uptake,” Zhang said. “Insufficient uptake of nutrients can result in deficiencies, including calcium. Calcium deficiency can be linked to the symptom of tipburn in lettuce and other leafy greens. Calcium deficiency is also associated with blossom end rot of some fruiting crops including tomatoes, cucumbers and peppers.”
Challenge of controlling VPD
How VPD is controlled depends on the type of structure plants are grown in.
“VPD control is challenging,” Zhang said. “The challenge for greenhouse production is growers usually don’t operate a dehumidifier for precise VPD control. Most greenhouses are equipped with ventilating, cooling and heating equipment to control the temperature and humidity to maintain VPD in acceptable ranges.
“Even if a greenhouse is equipped with dehumidifiers, this is very energy intensive. For greenhouse operations it would be very challenging to control VPD at a specific level as temperature and humidity could fluctuate throughout the day.”
Crops grown in indoor farms or vertical farms are also affected by VPD.
“In indoor farms there are usually air conditioning systems,” Zhang said. “Ideally these systems can be designed considering the transpiration rate of the crops. This will enable VPD to be more precisely controlled.”
Another challenge for controlled environment growers is trying to maintain a uniform VPD across the plant canopy.
“Growers want to control the microclimate at the plant canopy instead of across the whole production system,” Zhang said. “Air movement or air flow is important for VPD control. Air movement helps accelerate the water and gas exchange of plants. If there isn’t good air circulation the humidity levels will be higher around the plant canopy. Air circulation can help mix the water vapor in the air to improve environmental uniformity.”
Adequate air movement around the plant canopy ensures sufficient water movement and gas exchange.
“As a general rule, the air speed for crop production should be at least 0.3 to 1 meter per second,” Zhang said. “Studies showed that if the air speed is below 0.3 meter per second there are low transpiration and photosynthetic rates. With an increase in air speed there is an increase in transpiration and photosynthesis.”
For more: Ying Zhang, University of Florida, Department of Agricultural and Biological Engineering; email@example.com; https://abe.ufl.edu/people/faculty/ying-zhang/.
Editor’s note: Want to learn more about VPD, its effect on plants and how to manipulate it? Ying Zhang discussed “Humidity and Vapor Pressure Deficit (VPD) Control in Greenhouse Production” during Module 3: Humidity Control Systems and VPD of the GLASE Climate Control Short Course.
David Kuack is a freelance technical writer in Fort Worth, Texas; firstname.lastname@example.org.