Authors: Robbie Batts, InSpire Transpiration Solutions and Samuel Burgner, PhD Candidate, McGill University
Vapor pressure deficit is a critical measurement of air conditions that impact the health and vitality of your plants. You’ve likely heard about this concept before, but it can be a difficult value to measure accurately – especially when most available information is oversimplified and provides misleading information by extrapolating from estimations.
Understanding vapor pressure
Imagine a pot of water put on a stove to boil with a heavy lid that has a few pinholes in it. Crank up the heat and eventually you will start to see thin streams of steam escaping through the holes. As pressure builds up in the pot, the rate at which steam escapes will continue to increase. Why doesn’t the steam just accumulate in the pot and rest on top of the water? The highly energetic state of water vapor (which is why it is hot, as discussed in our first article) needs to move from the highly concentrated vapor within the pot to the low concentration of vapor in the outside environment. This is an intrinsic property of water – it wants to disperse.
Steam is an extremely energetic state of water vapor, but liquid water does not have to boil in order to evaporate. When water absorbs enough energy from a conductive heat surface (stove), an intense light source (sun), or even sound (ultrasonic fog), it will eventually evaporate as individual molecules gain enough energy to be liberated from the pack. When water absorbs energy, it is taking energy from the environment and holding it within its molecule structure, resulting in a net cooling effect to the surroundings (or at least an inhibition of warming). On a sunny day, which would evaporate faster: a bucket full of water or a thin puddle of water? The high surface area of the puddle allows more molecules to be exposed to the sun’s energy, and therefore the puddle will evaporate faster.
This concept is exactly what happens inside of a plant that allows the leaf to stay turgid, or swollen, and able to keep itself cool.
How plants use water
Before diving deeper into how vapor pressure is relevant to plants, we need to understand how water gets into a plant to begin with.
- From the seed stage, the root is the first thing to emerge because it can take up water on its own without requiring transpiration – the movement of water through a plant and out of the leaves.
- The seed takes up water passively by osmosis as water moves from the wet environment (high concentration) into the dry seed (low concentration). This passive movement of water builds up pressure inside the root (radicle), cracking the seed open and forcing the seed and initial leaves (cotyledons) to stand up straight and emerge into the light.
- Once the leaf is exposed to light, the process of transpiration begins and the root now has some help taking up water.
If the leaf emerged first without an already established stream of water, the vascular tissue would be full of air and would need a degassing valve just like in plumbing systems. This clearly isn’t an option, but transpiration through the leaves helps the root take up water. Plants are filled with a stream of water established by the initial root and if it is broken (drought), the plant will wilt or eventually die. For clones, this stream of water was established by the mother plant in the vascular tissue of the cutting. Think of water as separate from the plant and instead as a stream of fluid holding up the plant. It animates your plants, stands them upright, moves nutrients from the soil into respective plant organs, and allows them to take in light and CO2 by holding them in an upright position.
Roots maintain consistent water uptake by spending energy generated by root respiration to actively intake nutrients (nutrient concentration in the soil can amplify or weaken this effect). This process causes the solute concentration in the root cell to become higher than the rootzone, making water uptake happen naturally through osmosis (water follows solutes like nutrient salts or sugars). The cells surrounding vascular tissue in a plant are constantly spending high amounts of energy to pump into and remove solutes from the transpiration stream, forcing nutrients to enter the stream and deposit into various tissues throughout the plant.
The same thing happens in most developing leaf cells. After new cells are formed, plants spend energy to pump the new cell full of potassium, forcing water to enter by osmosis and stretch the cell walls. It takes a lot of pressure to make this happen – hydrostatic pressure in some plant cells has been measured as high as 300 psi. This pressure induces enzymes to loosen the cell walls, allowing them to slowly expand or grow. Roots alone can build up pressure around 30 psi, nearly as high as the tires on a car.
When the lights are on, transpiration drives most water uptake. In a developing plant, root pressure is generated primarily during dark periods when the transpiration rate is very low. The transpiration rate needs to be balanced with this pressure so that the plant can fill evenly with water and deposit nutrients consistently in both mature and developing tissues. This is important because mature leaves conduct far more water from the vascular system (xylem) since their pores (stomata) are typically larger and more mature.
Introducing vapor pressure deficit
In order to control plant transpiration, we must introduce the term vapor pressure deficit (VPD). This physical concept estimates the negative force pulling water vapor out of a leaf, allowing a stream of water to move up the plant stems from the roots. VPD is typically measured as the difference between the vapor pressure measurement in the substomatal pocket (the microscopically small pocket of air inside the leaf) and the vapor pressure measurement of the air surrounding the leaf.
If we looked inside a leaf pore (stomata), we would see large porous blobs (mesophyll cells) in the open air space where water vapor is being released and CO2 is entering from the space outside the leaf. We assume this “intercellular air space” is at near saturation of water vapor. Water makes up a small percentage of moist air and air has a maximum capacity for holding water (the capacity to hold water changes based on temperature as vapor molecules move faster or slower). These mesophyll, or “middle leaf” cells, are constantly releasing liquid water out of pores on the outer surface, which then spreads across the cell exterior like a puddle, thinning the layer of water and making it more likely to evaporate. This wet surface is also where CO2 dissolves and is then able to enter these photosynthetic cells and move into the chloroplast, where carbon capture occurs.
Because the intercellular air space in the leaf is nearly saturated with water vapor (meaning it cannot hold anymore) due to evaporation of water off the mesophyll cells, the air needs to move to an area with less water. The air outside of the leaf is usually below saturation, however there is a fascinating circumstance called “guttation” in which the leaf will exude droplets of water when there is low movement of vapor from the water surface coating the mesophyll cells due to a high vapor pressure outside of the leaf and/or low transpiration (almost exclusively during dark hours when root pressure is high).
As mentioned before, water moves from high concentration to low and its rate of movement is slowed when the vapor pressures inside and outside the leaf are close together. To determine VPD, we first estimate the vapor pressure within the leaf by using the saturation assumption paired with the leaf temperature. In other words, we quantify the internal vapor pressure as the maximum amount of water air can hold at the leaf temperature. We then subtract that value from the vapor pressure of the outside air at the measured water content (absolute humidity) and temperature, and then adjust for the atmospheric pressure where the plants are growing.
Plants can adapt to shifting vapor pressures
If plants were inanimate objects, the concept of VPD would be fairly straightforward. However, plants have mechanisms to adapt to shifting vapor pressures in the atmosphere. Stomatal regulation is a key method in maintaining proper pressure within the plant to allow for growth. By modulating their aperture, plants can adapt fairly quickly to shifting conditions.
When a leaf is forming, the quantity and size of stomata on the leaves will reflect the environmental conditions present at development. If a species spends a long time in the same seasonal conditions, its genetic expression will be predicated on typical environmental conditions in addition to the current conditions, but when plants are grown indoors this concept does not help. We must determine what the conditions are that a varietal is prepared to handle, find the sweet spot and hold onto it. If you raise a plant in one set of conditions and then shift it into another, it will absolutely undergo stress trying to acclimate, slowing growth and reducing overall yield. If this is something that happens consistently, there can be tremendous growth inhibition and increased likelihood of pests (we will cover environmental uniformity and pest pressure further in a later article).
The term stomatal conductance refers to the amount of vapor or gas a plant’s pores can release and accept (opposite of stomatal resistance). Its value is measured in moles and is typically far higher than the actual transpiration rate because water composes such a small fraction of air. There are even more specific terms such as “gas phase conductance to CO2,” which is specific to the leaf’s ability to allow in CO2, or “mesophyll conductance,” which is specific to the ability of the internal cells to take in CO2. The concentration of CO2 in the atmosphere directly impacts the concentration of CO2 within the leaf and the stomatal aperture. When CO2 levels rise, the stomatal conductance drops, which does not change the VPD, but does reduce transpiration. VPD is a purely physical concept used as a method to anticipate a plant’s response to the environment, whereas stomatal conductance and transpiration measure biological functions of the plant.
Find the right balance for optimal yield
Roots rely on a balance of soluble nutrients between the cells and the rootzone, and too high of a nutrient concentration (which can be tracked with electrical conductivity or EC) can osmotically restrict the ability of a root to draw in water (solute potential). When this happens, the water supply to the leaf cell can be weakened, which will limit the amount of vapor that can be released into the leaf intercellular air space. This causes a slight wilting effect or even brown leaf tips, which will inhibit the ability of the plant to absorb light and overtime will be reflected in the amount of carbon captured and subsequent growth. Stomata will compensate for this stress to a degree, but that ability has limitations. This wilting effect also happens when plants are overwatered and the ability to access oxygen is restricted, which is necessary to maintain osmotic flow.
No matter what, if the plant is coping with stress, yield will be reduced. Think of a human sprinting or running – the lungs and body will alter their function to make that task possible, but once the extra energy runs out, the body must slow down and recharge. In the case of plants, they can’t slow down because they are controlled by their environment, so instead they will die.
The next article in this series will continue the discussion of VPD in the context of how light spectrum impacts transpiration.