5 Effects of Vapor Pressure Deficit That Are Observed In Plants


Any farmer worth their salt understands that it takes certain environmental factors to achieve healthy plant growth and optimal yield volume. This article looks at one such aspect known as VPD. 

Below is a guide to what VPD is and how this environmental aspect can affect your plant growth.

What Is VPD?

Vapor pressure deficit, commonly abbreviated as VPD, refers to the current amount of water vapor measured against the maximum amount that particular air can hold at full saturation.  To obtain a vapor pressure deficit, you’ll need to determine the actual vapor pressure of the air and then subtract it from the saturated vapor pressure. The formula can simply be expressed as VPsat – VPair. 

VPD is probably not the first thing that comes to mind when discussing environmental factors that affect plant growth. However, not maintaining the ideal VPD levels is one of the surest ways of setting up crops for failure. In fact, the concept of vapor pressure deficit is so significant that the agricultural industry now teems with HVACD systems manufacturers. HVACD systems are designed to help growers achieve and maintain the recommended VPD levels by controlling proper temperature, humidity level, and air ventilation. This equipment benefits greenhouses, commercial grow rooms and any other indoor farms.

Altaqua is among the most renowned brands that deal in high-end HVACD equipment. The company provides a range of HVACD systems, which can work at high efficiency of dehumidification, air cooling, and air heating

How High VPD Can Affect Your Plants

1. High VPD Reduces Stomatal Opening

An increase in vapor pressure deficit causes a proportionate decrease in the stomata size. But perhaps the biggest question is how a diminished stomata size affects plant growth. 

Like animals, plants have an elaborate gaseous exchange system that primarily involves the uptake and removal of two primary gasses – oxygen and carbon dioxide. Plants generally take in carbon dioxide and give off oxygen during the day, while the converse is true at night. 

Gaseous plant exchange occurs through porous organs in the leaves known as stomata. 

Smaller stomata size translates to a reduced gaseous exchange efficiency. It further means that your crops cannot absorb enough carbon dioxide, critical in producing carbohydrates. 

Note that plants require carbohydrates nearly the same way as animals do. A deficiency in this nutrient means that your crops will not be able to generate enough energy necessary for other physiological processes, such as the excretion of waste products. It will only be a matter of time before the plants die naturally.

2. High VPD Reduces Transpiration Rate

The decrease in stomatal size caused by an increase in vapor pressure deficit can have far-reaching implications besides lowering carbon dioxide intake. The smaller the stomata size gets in response to an increase in VPD, the lower the transpiration rate. 

Transpiration is a physiological process similar to gaseous exchange in that both rely heavily on efficient stomata opening. The difference is that transpiration primarily involves the loss of water vapor. 

A higher transpiration rate means that your crops can lose sufficient water vapor. This helps cool the plants down. Therefore, restricted water vapor loss caused by poor stomata opening can quickly heat your plants. 

Immediate signs of heat stress include wilting of the leaves. Without urgent interventions, your plants may eventually die from overheating.

Vapor Pressure Deficit

3. High VPD Increases Toxic CO2 and O2 Accumulation

Plants produce gaseous waste products, namely oxygen during photosynthesis and carbon dioxide during respiration. While these gasses may appear harmless, an over-accumulation of either can affect your crops’ growth. 

Excess oxygen concentration in a plant can be a significant inhibitor to photosynthesis. Reduced photosynthesis means your plants cannot produce enough adenosine triphosphate (ATP). It also leads to a diminished regeneration of RuBP (Jmax), a key enzyme involved in carbon dioxide assimilation. 

On the other hand, too much carbon dioxide can decrease the rate of respiration. That’s because the excess concentration of this gas would typically result in the closing of stomata. Reduced respiration directly affects energy production. This can quickly lead to cellular stress and significantly impact various physiological processes. 

Therefore, it’s clear why plants require a robust gaseous exchange system. The fact that high vapor pressure deficit reduces stomatal opening explains how dangerous your crops may accumulate carbon dioxide and oxygen.

How Low VPD Can Affect Your Plants

1. Low VPD Levels Increase Stomatal Conductance

A high vapor pressure deficit isn’t the only thing that should worry you. Low VPD can be equally harmful to your tender crops. 

While a high vapor pressure deficit reduces stomatal opening, a low vapor pressure deficit makes these pores open abnormally wider and more frequently. That’s another reason to always aim at the ideal VPD range throughout various stages of your crops’ growth cycle.

Preventing VPD levels from falling below certain thresholds can help maintain the regular opening of stomata. This will, in turn, cool your crops, enhance gaseous exchange, and prevent toxins accumulation.

Always monitor your crops to reduce susceptibility to higher stomatal conductance caused by very low VPD levels. Not only will this enhance the uptake of gasses, water, and other essential nutrients. It will also ensure that these compounds benefit your plants instead of getting quickly flushed out through the overactive stomata. 

2. Low VPD Levels Can Cause Leaf Water Damage

When the vapor pressure deficit is too low, moisture will build up on the plant leaves’ surface. This is because the moisture deposited on the leaves cannot evaporate from the leaf surface. The phenomenon can ultimately prevent a plant from transpiring. 

Remember that there’s already a high amount of water being driven from the roots up the leaves by capillary force when VPD is too low. The fact that there’s also surface water inhibiting transpiration can spell double tragedy for your crops. It means that no significant nutrient activity occurs in a plant. 

Your priority as a farmer is to see that your crops experience all normal physiological processes. The best way to make that possible is by addressing the factors that impede photosynthesis and respiration, one of which is low VPD. The goal is to ensure that the crops absorb and assimilate all the valuable nutrients available in the soil.

Vapor Pressure Deficit

Final Word

The effects of vapor pressure deficit fluctuations in plants cannot be overemphasized. That’s why experts recommend maintaining the ideal VPD levels throughout your crops’ growth cycle, which is between 0.8 kPa and 1.2 kPa, while the plants are vegetative and 1.0 kPa and 1.5 kPa during the flowering stage.