My Favorit Subject - Plant Edema

 

A lot of this will be old stuff for many people. But I’m really interested in complex plant mechanics and plant responses to the demands of the environment. And transpiration, which is the topic of this post, is a major function that is essential to multiple aspects of plant function and health.

Plants use very little of the water they take up. 97% to 99% leaves the plant during transpiration. Plant transpiration is a significant portion of evapotranspiration, the sum of all movement of water from the surface to the atmosphere. Plant transpiration accounts for about 10% of evapotranspiration. An acre of corn transpires 3,000 to 5,000 gallons of water every day. That’s a 3-foot deep, 12 foot by 16 foot swimming pool. Every day.

Plants can evaporate water though the surfaces of leaves, flowers and stems, but most of it is from stomata on leaves. Stomata are fascinating structures. They are pores controlled by lip-like features that can swell and shrink to open and close the access to the pore. Stomata are essential to photosynthesis. They open to let in carbon dioxide for photosynthesis. And when they are open, water from leaf tissues evaporates – if air conditions allow it.

This photo shows a leaf stomata with its guard cells open and closed.

 

Leaves also have a waxy outer layer that can pass some water when stomata are closed. And lenticles, small openings in the bark of some plants, can pass small amounts of water.

Transpiration plays an important regulatory function. Transpiration drives the process of moving water with nutrients from the roots throughout the plant. The most accepted theory is that a “Cohesion-Tension” mechanism works through the property of water sticking to itself to pull water upward. The rate of transpiration, then, is closely paired with plant growth and production. Remarkably, the phenomenon of water movements in plants, as a result of transpiration producing tension, was described in Stephen Hales in 1727.

When we understand the ways in which water movement can be disrupted, many of the symptoms of plant problems are explained. Bacteria and fungi that attack roots, especially when weakened by lack of air, destroy the surface that absorbs water. When leaves are afflicted with pathogens, their evaporative surfaces may be destroyed, the function of stomata may be altered and the integrity of the waxy cuticle may be disrupted. Insect pests can also disrupt all of those. Pathogens internal to the plant and the byproducts they throw off may block pathways in the plant. Browning, dying, curling leaves can be signs of water disruption.

Transpiration maintains water balance. As we said earlier, plants take in a lot of water. Transpiration is the means of removing excess water that has done its job. A single corn plant may take in 200 liters of water over its life. It has to go somewhere.

Water also keeps plant cells full and expanded to their normal size. Plants manipulate the water pressure in cells to create needed movement. And in fact, it’s water pressure itself that operates the stomata openings that regulate water. It is water that give plants their firm structure and shape.

And transpiration also cools plants as the water passed out of stomata evaporates. Changing liquid water into vapor consumes energy, transferring heat to the vapor and cooling the plant as it is removed.

Transpiration operates because of what is called a water vapor pressure deficit in the surrounding air. Vapor pressure inside the leaf tissue is higher than vapor pressure in the air. The difference, the deficit in the vapor pressure of the air, is what makes water vapor tend to move out of the leaves. And vapor pressure then becomes lower in the leaf tissue than in the plant’s stem, which has a lower vapor pressure then the roots, so water is drawn upward, carrying nutrients with it.

But that means that there must be a proper vapor pressure deficit in the air for transpiration to occur and the proper tension to exist. Transpiration is most active when the humidity of the surrounding air is low and when high temperature and windy conditions facilitate evaporation. And because stomata open to admit carbon dioxide, CO2 levels in the air influence transpiration.

It is useful to look at why plants need such large quantities of water. To make sugars, plants require carbon dioxide for photosynthesis. But when stomata are open to receive carbon dioxide, a lot of water is lost relative to a small amount of CO2 admitted. 400 water molecules for each molecule of CO2. There is then a fine balance. If stomata stay open too long taking in CO2, the plant risks dehydration. It takes a lot of water intake to keep up.

There are three conditions that must be met for transpiration to take place. First, water must be available in the leaf. There must be energy available to convert water to water vapor, and there must be a pressure gradient to cause vapor to flow from inside the leaf to the outside. That pressure gradient must be sufficient to move vapor beyond the boundary layer that surrounds the leaf surface. The boundary layer is a thin layer of air adhering to the leaf. This boundary layer creates some resistance to vapor movement. That resistance is affected by wind speed across the leaf. The faster the wind, the thinner the boundary layer. Resistance is greater on large leaves than on small leaves. This boundary layer is very thin, on the order of 0.5mm.

There is a range of Vapor Pressure Deficits within which a plant can manage water by transpiration. If the pressure on the atmosphere side is too high, water vapor inside the leaf cannot move out. If the pressure on the atmosphere side is too low, tension builds inside the plant tissues, and a gas bubble forms in the plant’s water conduits, blocking flow. This can happen during drought when tension becomes so high it pulls water into the empty conduit, creating a void in the water column. Anything that breaks the continuity of water moving through the plant disrupts its flow until the plant can activate a mechanism that brings water into the void until it disperses the blockage.

Air temperature and humidity determine the vapor pressure in the environment. Wind speed also plays a part. And since photosynthesis and transpiration are intimately linked, transpiration increases in light as stomata open to take in CO2 for photosynthesis.

So, let’s look at the all-important Vapor Pressure Deficit. For our purposes, VPD is the difference between the actual water vapor pressure and the saturation water vapor pressure at a given temperature. To put it another way, it is the difference between the amount of water in the air and how much water is could hold if it were saturated, loaded with water until it could hold no more. The closer to saturation, the less water the air can accept from sources like leaf transpiration. We can express the amount of water in the air relative to how much it could hold at a given temperature. That’s the relative humidity. So VPD can be calculated from the factors temperature and relative humidity.

And that’s what we see in a VPD chart.  This is a VPD chart showing VPD for combinations of temperature and relative humidity.

For each stage of plant growth, clone, vegetative and flower, we can define the VPD values that will facilitate the most efficient transpiration.

Here’s is the chart tinted to show the optimum VPD values for vegetative stage. 

 

 

Values in the deepest green area are considered to provide the VPD environment for the most efficient transpiration. The orange regions contain VPD values too low or two high for efficient transpiration.

Note two important things. For a given relative humidity, a higher temperature results in a greater Vapor Pressure Deficit, a greater difference between how much how much water the air contains as expressed by the relative humidity and how much water the air can hold. And for a given temperature, lower relative humidity creates a greater VPD value, again, a greater difference between how much how much water the air contains as expressed by the relative humidity and how much water the air can hold.

Note that, as expected, when relative humidity is high and rising toward 100%, toward saturation, the Vapor Pressure Deficit becomes most unfavorable to transpiration. When relative humidity is 90% or more, VPD is unfavorable at all temperatures. Of course, we know all transpiration doesn’t stop at those high temperature, because we have plants growing in hot seasons.

We should note, too, that VPD can be too high as well as too low. Generally, only those of us growing in warm and very dry conditions will experience that risk.

You can readily see that, for the vegetative growth stage, the most favorable region of VPD values are in the deep green area where those values are close to 1.0, best growth being with values from 0.8 to 1.2 kPa (kiloPascals – you don’t need to know that). But it’s not all or nothing. As the green fades toward orange, VPD values move away from the optimum.

Air movement is not used in this chart, except for a small general factor, Wind, as from a fan playing on the plants, will improve the situation.

Another important point is that the optimum VPD depends also on leaf temperature relative to the surroundings. The above chart was generated for leaf temperature 2 degrees F below the environment. The color bands shift left for leaf temperatures closer to room temperature and shift right for temperatures as much as 5 degrees cooler than the room.

The consequence of out of range VPD is often edema. It was once more commonly called intumescence. Edema implies swelling, congestion with water, although that sign is not always present. Swelling is most often in leaf tissues between the veins. Another common sign is the appearance of lesions that begin to go brown in time. They do not resemble most pest damage. Nor do they look very much like most nutrient deficiencies.  They may appear initially as if the top layer of tissue were skinned off. Sometimes, they appear as small brown points. They are created when stomata cannot open, and water backs up in the tissues, eventually breaking through the surface. Another common sign is tiny white or clear crystals of nutrient mineral left behind when leaked solution evaporates.

 

These leaves show the small browning lesions where tissues have ruptured.

 

This next leaf shows larger lesions. Note that it is an understory leaf and therefore more likely to have a higher humidity region just above the surface of the leaf.

 

 

This next leaf displays very distinct swellings between the leaf veins.

 

The following basil leaves show both tissues swelling and small points of rupture.

 

This leaf shows broad lesions filled with  nutrient crystal left behind with the water from the leaked nutrient solution evaporated.

 

 

This Romaine lettuce displays gross swellings. Romaine naturally has apparent swollen area that help give its prized crispness, but they should not be this prominent.

 

These leaves show very dramatic swelling that appear ready to burst.

 

Because VPD problems can disorder transpiration that facilitates movement of water carrying nutrient within the plant, signs of apparent nutrient deficiency may appear, not because there is any problem with the nutrient in the reservoir but because nutrient cannot reach the areas of the plant where it is needed. When edema goes unchecked, it can prevent the proper movement of water, resulting in browning and drying of leaves as if the plant were deprived of water.

These signs often distract from the more specific signs of edema, prompting guesses at what nutrients may be deficient when the true need is simply to correct the edema. That is too bad, because it’s often that case that the signs of edema become easily observed as soon as edema is considered to be a possibility.

While noting temperature and humidity and referring to a VPD chart may make diagnosis easy, it is possible to have VPD disordered at the surface of the leaf, even though sensors just outside the growing surface show favorable conditions. Often, the out of range VPD exists very locally around tightly packed understory leaves that get little air movement.

When edema is even barely suspected, it is an easy rule-out. Reducing high humidity and/or temperature and playing a small fan on plants can do nothing but help the plant and is an immediate therapy that takes no time to take effect. When a proper complete hydroponic nutrient set is being used as recommended by its manufacturer and its maintenance is current and E.C. and pH within range, nutrient deficiencies are unlikely. And edema produces signs similar to those of nutrient deficiencies, but deficiencies will require days to confirm during which possible edema can already be being addressed.

 

The following are the other VPD charts for cloning and flowering growth stages.

Clone Stage VPD Chart - Leaf Temp 2F below Air

 

Flower Stage VPD Chart - Leaf Temp 2F Below Air

 

You can also download VPD charts is a slightly different format and for different variations in leaf temperature from DimLux North America here:

https://www.dimluxlighting.com/knowledge/blog/vapor-pressure-deficit-the-ultimate-guide-to-vpd/

 

Note that VPD can be a clue to cause of edema, but it's other effects vary with species. And an ideal VPD for growth may not be ideal for flowering. 

Remember that edema can present in different ways and remember to consider it when any such signs are present.

In addition to fixing temperature and/or humidity, I can't emphasize enough the value of maintaining air movement, especially in tight spaces and dense foliage. A small fan can often clear up edema. And air movement has other real benefits to plants.