Several environmental factors influence the healthy growth of plants.

You’re probably already familiar with common weather elements like temperature and humidity. However, there are many other lesser-known environmental forces that can make a difference between crop failure and a bumper harvest. One such aspect is VPD.

But if you’re encountering this term for the first time, you’re probably wondering what it is and how it might impact plant health. Well, you’re in luck.

This article introduces VPD and looks at how you can leverage this aspect to optimize crop production.

What Is VPD?

Vapor pressure deficit, commonly abbreviated as VPD, refers to the difference between the amount of moisture or water vapor currently present in the air and the maximum amount of moisture that particular air can hold when saturated.

VPD‘s influence on plant growth is sometimes overlooked, yet crops can suffer significantly if ideal levels are not maintained.

HVACD system manufacturers are causing a flurry in the agricultural industry, offering growers cutting-edge approaches to attain and maintain appropriate VPD levels through accurate temperature management, humidity regulation, and enhanced air ventilation. They are among the best choices for greenhouses, industrial grow rooms, and indoor farms.

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Now, any given air can theoretically hold 100% of water vapor at saturation. When that particular air currently contains 50% of water vapor, it means it’s only half saturated.

The air’s VPD would be calculated using the formula below.

VPD = VP (sat) – VP (air)

Whereby:

• VP stands for vapor pressure.
• Sat stands for saturated vapor pressure.
• Air stands for current vapor pressure.

Based on the simple illustration above, the current VPD would be calculated as 100% – 50% = 50%.

What Is the Foundation Of VPD?

The best way to understand the basics of vapor pressure deficit is to familiarize yourself with two key principles in physics – water holding capacity and diffusion.

1. Water Holding Capacity

According to science, air can hold more water vapor at higher temperatures. The concept is akin to more sugar molecules getting dissolved in hotter water.

The logic here is that matter molecules vibrate at a much higher frequency in warmer conditions than in cooler ones. As such, more molecules can survive in a hotter space than in a colder one.

2. Diffusion

The law of diffusion states that molecules generally move from regions of higher concentration to regions of lower concentration. Perhaps the most relevant illustration is when you apply a deodorant in the bedroom, and the entire house is suddenly bathed in the sweet fragrance.

Another key aspect of the law of diffusion is that the speed with which molecules move depends partly on the difference in concentration between the two regions. The bigger the difference, the faster the diffusion process will take place.

Is VPD Similar to Relative Humidity?

Vapor pressure deficit is closely related to relative humidity in that both concepts rely on water vapor. Besides, both VPD and relative humidity analyze the current amount of moisture in air based on other environmental aspects.

But that’s where the similarity ends.

Relative humidity simply measures the saturation of air with moisture at a given temperature. On the other hand, vapor pressure deficit measures the difference between saturation and the current amount of water vapor in terms of pressure.

How Does VPD Affect Plant Growth?

Vapor pressure deficit plays a key role in regulating transpiration in plants, as well as nutrient uptake from a plant’s roots to its upper parts.

The speed with which water and mineral salts move through a plant’s vascular bundles depends mainly on the rate of transpiration. A higher transpiration rate translates into a faster uptake of water and mineral salts. The converse is true when the transpiration rate slows down.

Now, plants transpire primarily through sites located on their leaves called stomata. The stomata consist of guard cells which form small pores on the leaf surfaces. It’s these cells that control the opening and closing of stomata in response to various environmental factors, such as VPD.

A low vapor pressure deficit means that the ambient air is closer to saturation.

During low VPD conditions, the moisture in the air will condense into water droplets. These droplets are deposited on plant leaves as dew, and the implications can be far-reaching.

Water droplets on plant leaves can inhibit the opening and closing of stomata. This can slow down the uptake of water and mineral salts from a plant’s roots to the upper parts. It can also reduce the efficiency of gaseous exchange, which also takes place through the stomata. Even worse is that excess moisture build-up on plant leaves can provide an excellent breeding ground for fungal diseases. One such disease is bud rot, also known as botrytis.

On the other hand, high VPD levels mean that the ambient air is less saturated. Therefore, there’s plenty of room to hold much more water vapor.

High VPD conditions can make plant stomata more overactive. That’s because they must draw up and lose more water in a bid to fix the environmental moisture deficit.

Summary

Vapor pressure deficit is a key weather element that affects normal plant growth. As indicated, both high and low VPD levels can impact crop production adversely.

Therefore, it’s crucial to maintain ideal VPD levels. These can generally range from 0.45 kPa to 1.25 kPa, with the most favorable range being 0.8 to 0.95 kPa for a broad spectrum of plants. However, keep in mind these values might vary depending on the specific type of plant, its growth stage, and the prevailing environmental conditions.