Vapor Pressure Deficit - An Important Factor in Plant Health

 

Why am I writing this one? Well, for some of you, it will be old news. But there are clearly a lot of growers who don’t have this knowledge. As always, one reason I write is that it forces me into a more solid understanding of issues. In this case, it’s Vapor Pressure Deficit (VPD), and in it you will recognize some things we talk about regularly without being very specific about the mechanics and about all the manifestations.

What’s deficient in the deficit? It is the difference in the amount of moisture in the air and the amount of moisture that particular temperature air can hold when it is saturated. I know. That doesn’t make the concept much more clear. Think of it this way. When the difference is low, meaning the air has nearly a much water as it can hold, it is difficult for the plant to add more by transpiration. When the difference is high, the air is very dry, and the normal process of transpiration is disrupted. Between those two conditions, the plant can function normally. The remainder of this paper will discuss how all that works.

CONCEPT #1 SVP – Saturation Vapor Pressure

What we are interested in first is the saturation vapor pressure (SVP). That’s the maximum amount of water air can hold at a given temperature. If it actually contains that maximum amount of water, it is saturated. SVP goes up and down with temperature. Hot air can hold more water, cooler air less. When temperatures drops in the morning so that the SVP drops and the air is over-saturated, dew forms.

CONCEPT #2 – AVP – Actual Vapor Pressure

At any given moment the actual amount of water in the air is the actual vapor pressure (AVP).

You are already familiar with this principle. Relative humidity is the percent of the maximum capacity, the SVP, the air is now holding. At 100% relative humidity, the AVP and SVP are equal, and the air is saturated and can hold no more water. So AVP can never exceed SVP, the potential capacity.

CONCEPT #3 – VPD – Vapor Pressure Deficit

Which brings us to the term we want to know about, VPD, vapor pressure deficit.

Now, I am not going to take you through the derivation of the calculation of VPD. But I am going to cure you of wanting to know.

610.78 x e^(T / (T +238.3) x 17.2694)) x (1 – RH/100) = VPD in kPa, which is KiloPascals – or thousands of Newtons per Square Metre.

610.78 is a constant. T is the temperature in Celsius. E is Euler’s number, approximately 2.71828. Euler’s number is a sort of magical number that appears throughout mathematics and the universe. It is as important as 0, 1, π or ί. RH is relative humidity.

Isn’t this fun?

Fortunately for us, there are published VPD charts covering all the combinations of temperature and humidity we would ever encounter in growing operations. We don’t even need to know about the numbers, because the charts are color-coded. After we talk about why VPD matters, we will look at one. All you need to know for now is that for our common Earth environments, VPD will range from zero to about 10. And we are most interested in VPD values near 1.0 .

CONCEPT #4 - Transpiration

Let’s shift gears here and talk about plants. We’ve all heard of transpiration, the process of moving water through a plant and evaporating it from upper plant parts. Water carries nutrients and is necessary to plants, but it only actually uses a small portion of the water it takes up. The rest, almost all it takes up, is processed out by transpiration. (Kind of like us humans who excrete one way or another nearly all of the water we take in. If we can’t, we become very ill indeed.)

 

 CONCEPT #5 – Leaf Somata

Leaf surfaces have pores called stomata. Stomata do a lot. They open to admit carbon dioxide for photosynthesis. And they transpire water away. Transpiration cools the plant and by removing water changes the osmotic pressure of cells that enables movement of nutrients and water from the roots. That’s all we need to know about transpiration right now. But it should be obvious that plants have to continuously take up water to get and move nutrients, and the excess has to go somewhere or there’s going to be trouble, so transpiration has to work efficiently.

These are microscopic images of a leaf stoma open and closed. The opening is controlled by the guard cells. The guard cells work by swelling with water.

 

Plant transpiration mechanisms are some of the most highly evolved features of plants. Quite miraculous, when you consider that openings have to open and close, coordinated with transport of water. But they naturally wouldn't evolve to handle every condition on Earth. Most plants of interest to use are evolved for fairly comfortable conditions, neither extremely humid all the time nor extremely dry all the time. (Although specialized plants evolved in those extreme environments can handle them.)

These are stomata visible on a 100,000,000 million year old fossilized leaf.

 

If you think about it, humans began to cultivate plants they found growing in temperate zones, which happened to be the zones favored by most sedentary humans who would in time stay in pleasant places long enough to learn about growing plants. And those zones are most congenial to more varieties of plants. So, they grew what they found where they liked to be and improved them where they grew well and where the farmer was comfortable. The plants mostly stayed adjusted to a sort of Earth average.

The mechanism of plant transpiration works efficiently within the zone of vapor pressure balance it was evolved to function within. The balance is between the vapor pressure outside the plant and the vapor pressure inside. The plant is evolved to have a limited range of tolerable difference. They needed to have no more elaborate ability.

But one of the serious concerns in climate change is how plants will respond to increasing VPD as temperatures rise. And the responses of stomata to changes in VPD vary with plant species. Remember that when you look at the following chart and understand why the zones are gradated and not a narrow range.

 

Now let’s look at a VPD chart for vegetative growth.

 

Pulse, the ag data platform company, kindly gives us charts for Fahrenheit and Celsius and for different growth stages here:

https://pulsegrow.com/blogs/learn/vpd-charts-in-fahrenheit-and-celsius-with-leaf-temperature

But we can use the above chart to explore their use. There are not huge differences among growth stages anyway.

The favorable plot, the green areas, run roughly from warm and moderately humid to dry and rather cool. Cold and wet is orange, meaning not good. Hot and dry, the dessert, is also orange on the other side of the green zones and also not good. The plot of the best range happens to run through a combination of temperature and humidity that we humans instantly recognize as reasonably good growing conditions.

And one would instinctively not try to grow in constant cool, wet conditions or in hot, dry conditions, nor in seasons that presented untoward VPD numbers. We would not encounter many edible plants growing then and there. So, the favorable VPD range, which seems rather odd at first glance really makes sense. To most of us as growers, it seems a bit odd to have bad zones that are both high and low humidity, but I think that's because we most of us don't have to cope with very dry conditions. We think of transpiration problems to involve high humidity, etc. But that is not always the case.

The plant mechanisms are evolved to operate within a balance of vapor pressures determined by the combination of temperature and humidity. It is less a matter of the plant trying to manage pressures as it is a plant working best within environmental "specifications." It uses vapor pressure difference to drive transpiration, but it can only work within a range of pressure differences that its mechanisms were designed to work within. A "superplant" might have super mechanisms that could handle anything. But we don't have many superplants, or if we do, they're probably not good to eat.

So, the “deficit” in VPD can be thought of as a deficit in the plant’s ability to transpire. Plants just weren’t made to work over the full VPD range of all Earth conditions.

CONCEPT #6 - Edema

High or low VPD hinders transpiration by driving the balance of internal and external vapor pressures beyond the plant’s working limits. The consequences of transpiration being hindered by either low or high VPD are pretty much the same. Transpiration fails to one degree of another. Fluid backs up in the leaf tissues, eventually rupturing them and leaking water and nutrient. Water can evaporate, given time, in any environment, but nutrient minerals can't, so they are often seen as crystals on the leaves. How large the crystals are is probably determined by the strange rules of crystal lattice formation. We call that condition edema.

This a leaf of a plant afflicted with edema. Note the swollen tissue between the leaves, the areas of ruptured tissue and the crystals left behind in the rupture areas. This plant was actually in an environment where room temperature and humidity were comfortably within a favorable VPD zone. But the close quarters among lower leaves seems to have created a micro environment of unfavorable VPD close to the leaves where transpiration contributed water to the air, and moving air did not penetrate sufficiently to disperse it. In this case, the VPD was low, being higher humidity and just moderate temperature.

 

Almost always, when we are presented with edema, it is on account of high humidity. Look at the chart for VPD’s associated with high humidity. When humidity gets around 80%, VPD is well below 1.0 at every temperature we would likely ever inflict on our plants. And most of our growing environments have lots of water around and leafy plants that stifle air flow anyway. Humidity is often high. It can be high near the surface of plants emitting water when it’s tolerably low a foot away.  

But some environments present the much less often seen low humidity scenarios. Look at the chart for the conditions in the range of human comfort temperatures. At low humidity, 30% and down, at those temperatures, VPD was, at best, marginal on the high side.

There is a different chart for flowering stage. The numbers are naturally the same, because VPD is a specific calculation. But the favorable zone on the flowering chart shifts downward, and the curve becomes a little flatter. You can easily look that chart up. But right now, we are more interested in the concept of VPD, because we now see how moving away from the best VPD zone for the plants causes trouble.

Why don’t plants evolve mechanisms that can function in those poor conditions? After all, there are some other odd conditions plant can adapt to. In nature, plants have to cope with seasonal floods, where roiling, splashing , raining water, fairly well aerated, floods them. But that water eventually gradually stops moving and becomes hypoxic over time. To survive the flood and reproduce, plants evolved the ability, when they sense hypoxia approaching, to create internal channels to bring oxygen to the roots from the upper parts of the plant.

But a plant’s flood response doesn't help it survive an air pump failure in DWC, because the nutrient becomes hypoxic far too quickly for the plant to carry out the adaptation, which takes over a week to become effective. They didn’t anticipate that, because that rarely happens in nature. Certainly, it doesn’t happen often enough for evolution to respond with adaptations.

But what about VPD conditions? Plants don’t get lifted up and dropped into different VPD zones very often in nature. On their home ground, they always get seasonal growing seasons with conditions within the favorable VPD zones. There was no need to adapt to anything but small vapor pressure imbalances. And a growing season is all a plant asks for.

So, there was no sufficient evolutionary drive to conquer extremes of VPD. People live in some bad VPD environments, but people in hot, wet climates simply used different plants or differently adapted versions of plants that had evolved to work with those VPD’s outside of our temperate plants’ working ranges.

But still, our plants can work within a reasonably wide VPD range, about 0.40 to 1.25. Of course, the closer to the ideal, the best match of vapor pressures, the more efficient the transpiration and the smoother all the plant functions that depend on water movement. And there are different good working VPD ranges for different plants. Leafy greens and herbs favor lower VPD, from 0.65 to 0.9, a little wetter relative to temperature. They don’t want to lose too much water from their large water-filled leave. Tomatoes and peppers prefer it drier, 0.9 to 1.2 . Like most things, we can rarely present our plants with a constant ideal environment. But we can usually prevent VPD persisting outside any reasonable working range.

TAKE-AWAYS

What you should take away from this is that plants are evolved to work within reasonable ranges of vapor imbalances, meaning within specific VPD ranges. In an environment that persists in conditions outside that range, we may see one or many symptoms of edema, swollen leaves, swollen stomata, patches of burst leaf tissue and crystals of dried nutrient.

A VPD chart gives us a range of conditions to shoot for. Happily, we can move VPD numbers by changing either temperature or humidity or both. And it gives us an idea of where we can move conditions to remove hinderances to transpiration that are producing the edema symptoms. And when VPD is marginal, all it may take to restore efficient transpiration is more moving air to bring less water-loaded air more quickly to the leaves. Once again, the more you know, the more you grow.