Understanding Transpiration

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Transpiration is the loss of water from a plant in the form of water vapor. Water is absorbed by roots from the soil and transported as a liquid to the leaves via xylem. In the leaves, small pores allow water to escape as a vapor and CO2 to enter the leaf for photosynthesis. Of all the water absorbed by plants, less than 5% remains in the plant for growth and storage following growth. This lesson will explain why plants lose so much water, the path water takes through plants, how plants might control for too much water loss to avoid stress conditions, and how the environment plays a role in water loss from plants.

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Evaporative cooling: As water evaporates or converts from a liquid to a gas at the leaf cell and atmosphere interface, energy is released. This exothermic process uses energy to break the strong hydrogen bonds between liquid water molecules; the energy used to do so is taken from the leaf and given to the water molecules that have converted to highly energetic gas molecules. These gas molecules and their associated energy are released into the atmosphere, cooling the plant.

Accessing nutrients from the soil: The water that enters the root contains dissolved nutrients vital to plant growth. It is thought that transpiration enhances nutrient uptake into plants.

Carbon dioxide entry: When a plant is transpiring, its stomata are open, allowing gas exchange between the atmosphere and the leaf. Open stomata allow water vapor to leave the leaf but also allow carbon dioxide (CO2) to enter. Carbon dioxide is needed for photosynthesis to operate. Unfortunately, much more water leaves the leaf than CO2 enters for three reasons:
  1. H2O molecules are smaller than CO2 molecules and so they move to their destination faster.
  2. CO2 is only about 0.036% of the atmosphere (and rising!) so the gradient for its entry into the plant is much smaller than the gradient for H2O moving from a hydrated leaf into a dry atmosphere.
  3. CO2 has a much longer distance to travel to reach its destination in the chloroplast from the atmosphere compared to H2O which only has to move from the leaf cell surface to the atmosphere.
This disproportionate exchange of CO2 and H2O leads to a paradox. The larger the stomatal opening, the easier it is for carbon dioxide to enter the leaf to drive photosynthesis; however, this large opening will also allow the leaf to lose large quantities of water and face the risk of dehydration or water-deficit stress. Plants that are able to keep their stomata slightly open, will lose fewer water molecules for every CO2 molecule that enters and thus will have greater water use efficiency (water lost/CO2 gained). Plants with greater water use efficiencies are better able to withstand periods when water in the soil is low.

Water uptake: Although only less than 5% of the water taken up by roots remains in the plant, that water is vital for plant structure and function. The water is important for driving biochemical processes, but also it creates turgor so that the plant can stand without bones.

The driving force for transpiration is the difference in water potential between the soil and the atmosphere surrounding the plant. This difference creates a gradient, forcing water to move toward areas with less water. The drier the air around the plant, the greater the driving force is for water to move through the plant and the faster the transpiration rate. The following section, FACTORS AFFECTING RATES OF TRANSPIRATION, expands on how changes in the environment alter this driving force and thus transpiration.

There are three major resistances to the movement of water out of a leaf: cuticle resistance, stomata resistance and boundary layer resistance. These resistances slow water movement. The greater any individual resistance is to water movement, the slower the transpiration rate. The following section, FACTORS AFFECTING RATES OF TRANSPIRATION, expands on how changes in the plant alter these resistances and thus transpiration.

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What path does water take to reach the leaf from the root hair? Once water has entered a root hair, it must move across the cortex and endodermis before it reaches the xylem. Water will take the path of least resistance through a root to reach the xylem.

Water can move across the root via three different pathways. One path is the apoplastic path where the water molecule stays between cells in the cell wall region, never crossing membranes or entering a cell. The other two routes, called cellular pathways, require the water molecule to actually move across a membrane. The first cellular pathway is the transmembrane path where water moves from cell to cell across membranes; it will leave one cell by traversing its membrane and will re-enter another cell by crossing its membrane. The second cellular path is the symplastic path which takes the water molecule from cell to cell using the intercellular connections called the plasmodesmata which are membrane connections between adjacent cells. Regardless of the pathway, once the water molecule has traversed the cortex, it must now cross the endodermis. The endodermis is a layer of cells with a waxy inlay or mortar called the Casparian strip that stops water movement between cells. At this point, water is forced to move through the membranes of endodermal cells, creating a sieving effect. Once in the endodermal cells, the water freely enters the xylem cells where it joins the fast moving column of water or transpiration stream, headed to the leaves.

The xylem is probably the longest part of the pathway that water takes on its way to the leaves of a plant. It is also the path of least resistance, with about a billion times less resistance than cell to cell transport of water. Xylem cells are called tracheids (cells with narrower diameters) or vessels (cells with wider diameters). Their cell walls contain cellulose and ligin making them extremely rigid. Xylem cells contain no membranes and are considered dead. These cells overlap to create a series of pathways that water can take as it heads to the leaves. There is no single column of xylem cells carrying water.

How do stomata open? Stomata sense environmental cues, like light, to open. These cues start a series of reactions that cause their guard cells to fill with water.

Cavitation is the filling of a xylem vessel or tracheid with air. It is also known as an ‘embolism’ or ‘air-lock’. Remember that during transpiration, the column of water is being pulled out of the plant by evaporation at the leaf cell surface. When this ‘pulling’ of water out of the plant becomes greater than the ability of the water molecules to stay together, the column of water will break. Using sound-sensing equipment, one can actually hear a ‘click’ when the water molecules split from one another. Unique structural characteristics help the plant contain the air bubble so that it does not totally disrupt water movement up the plant.

Plants are particularly sensitive to cavitation during the hottest part of the day when there is not enough water available from the soil to keep up with the demand for water while it is evaporating off the leaf surface. Cavitation also occurs under freezing conditions. Because the solubility of gas in ice is very low, gas comes out of solution when the xylem sap freezes. Freezing of xylem sap is a problem in the spring when the ice thaws, leaving a bubble in a xylem vessel. These bubbles can block water transport and cause water deficit in leaves.

Plants avoid cavitation or minimize its damage through several mechanisms.
  1. Xylem cells possess pits or tiny holes that allow liquid water transport, but do not allow the gas bubble to escape; this structural characteristic helps keep the gas bubble in one cell, so the other xylem cells can continue to transport water up the plant.
  2. Water will detour around any xylem cell containing an air bubble through the pits as well.
  3. The gas bubble will re-dissolve into liquid water when the pulling of water through the xylem is reduced, such as during the night when water is not being pulled out of the leaf via transpiration because the stomata are closed.
  4. Xylem cells with narrower diameters (tracheids) compared to those with wider diameters (vessels) avoid cavitation because the column of water in a cell with a narrow diameter is better able to resist bubble formation or rupture.
The stomata are the primary control mechanisms that plants use to reduce water loss and they are able to do so quickly. Stomata are sensitive to the environmental cues that trigger the stomata to open or close. The major role of stomata is to allow carbon dioxide entry to drive photosynthesis and at the same time allow the exit of water as it evaporates, cooling the leaf. Two specialized cells called ‘guard cells’ make up each stoma (stoma is singular for stomata). Plants have many stomata (up to 400 per mm2) on their leaf surfaces and they are usually on the lower surface to minimize water loss.

SIDE VIEW OF STOMATA– Environmental cues that affect stomata opening and closing are light, water, temperature, and the concentration of CO2 within the leaf. Stomata will open in the light and close in the dark. However, stomata can close in the middle of the day if water is limiting, CO2 accumulates in the leaf, or the temperature is too hot. If the plant lacks water, stomata will close because there will not be enough water to create pressure in the guard cells for stomatal opening; this response helps the plant conserve water.

If the leaf’s internal concentration of CO2 increases, the stomata are signaled to close because respiration is releasing more CO2 than photosynthesis is using. There is no need to keep the stomata open and lose water if photosynthesis is not functioning. Alternatively, if the leaf’s CO2 concentration is low, the stomata will stay open to continue fueling photosynthesis. High temperatures will also signal stomata to close.

High temperatures will increase the water loss from the leaf. With less water available, guard cells can become flaccid and close. Another effect of high temperatures is that respiration rates rise above photosynthesis rates causing an increase of CO2 in the leaves; high internal CO2 will cause stomata to close as well. Remember that some plants may open their stomata under high temperatures so that transpiration will cool the leaves.

When stomata are signaled to open, potassium ions (K+) enter the guard cells. This causes water to enter down its water potential gradient, creating a hydrostatic pressure in the guard cell that changes the shape of the stoma. Guard cells expand on the outer edges of the stoma, but not on the inner side, resulting in kidney-shaped cells and an opening or pore between the two guard cells for gas exchange. Kidney-shaped guard cells are characteristic of dicots; however, many plant (e.g. grasses, other monocots) have dumbbell-shaped stomata. The shape taken by the guard cells is dependent on cellulose microfibrils that fan out radially from the pore, somewhat similar to radial tires. The cellulose microfibrils are rigid and do not stretch when water has entered the cell. The cell walls surrounding the stomatal opening are thickened, preventing that side of the guard cell from expanding. Therefore, when pressure in the cell increases due to water entry, guard cell does not widen, but rather the outer edge stretches disproportionately more than the inner edge. This unequal stretching allows the pore to form between the two guard cells.

Stomata must be open for the plant to photosynthesize; however, open stomata present a risk of losing too much water through transpiration. Stomata close when the guard cells lose water and become flaccid. This occurs because potassium ions move back out of the guard cell, followed by water that lowers the pressure.

The boundary layer is a thin layer of still air hugging the surface of the leaf. This layer of air is not moving. For transpiration to occur, water vapor leaving the stomata must diffuse through this motionless layer to reach the atmosphere where the water vapor will be removed by moving air. The larger the boundary layer, the slower the rates of transpiration.

Plants can alter the size of their boundary layers around leaves through a variety of structural features. Leaves that possess many hairs or pubescence will have larger boundary layers; the hairs serve as mini-wind breaks by increasing the layer of still air around the leaf surface and slowing transpiration rates. Some plants possess stomata that are sunken into the leaf surface, dramatically increasing the boundary layer and slowing transpiration. Boundary layers increase as leaf size increases, reducing rates of transpiration as well. For example, plants from desert climates often have small leaves so that their small boundary layers will help cool the leaf with higher rates of transpiration.

Some environmental conditions create the driving force for movement of water out of the plant. Others alter the plant’s ability to control water loss. Relative humidity – Relative humidity (RH) is the amount of water vapor in the air compared to the amount of water vapor that air could hold at a given temperature. A hydrated leaf would have a RH near 100%, just as the atmosphere on a rainy day would have. Any reduction in water in the atmosphere creates a gradient for water to move from the leaf to the atmosphere. The lower the RH, the less moist the atmosphere and thus, the greater the driving force for transpiration. When RH is high, the atmosphere contains more moisture, reducing the driving force for transpiration.

Temperature greatly influences the magnitude of the driving force for water movement out of a plant rather than having a direct effect on stomata. As temperature increases, the water holding capacity of that air increases sharply. The amount of water does not change, just the ability of that air to hold water. Because warmer air can hold more water, its relative humidity is less than the same air sample at a lower temperature, or it is ‘drier air’. Because cooler air holds less water, its relative humidity increases or it is ‘moister air’. Therefore, warmer air will increase the driving force for transpiration and cooler air will decrease the driving force for transpiration.

The source of water for transpiration out of the plant comes from the soil. Plants with adequate soil moisture will normally transpire at high rates because the soil provides the water to move through the plant. Plants cannot continue to transpire without wilting if the soil is very dry because the water in the xylem that moves out through the leaves is not being replaced by the soil water. This condition causes the leaf to lose turgor or firmness, and the stomata to close. If this loss of turgor continues throughout the plant, the plant will wilt.

Stomata are triggered to open in the light so that carbon dioxide is available for the light-dependent process of photosynthesis. Stomata are closed in the dark in most plants. Very low levels of light at dawn can cause stomata to open so they can access carbon dioxide for photosynthesis as soon as the sun hits their leaves. Stomata are most sensitive to blue light, the light predominating at sunrise.

Wind can alter rates of transpiration by removing the boundary layer, that still layer of water vapor hugging the surface of leaves. Wind increases the movement of water from the leaf surface when it reduces the boundary layer, because the path for water to reach the atmosphere is shorter.


The effect of nitrogen supply on the transpiration rate and stomatal opening of potted plants was studied in a series of experiments. The transpiration rates of N-supplied plants were higher than those of N-deficient plants when soil moisture was relatively high: as soil moister approached the wilting range, the transpiration rates of N-supplied plants dropped to below those of N-deficient plants.

Calcium deficiency is a plant disorder that can be caused by insufficient calcium in the growing medium, but is more frequently a product of low transpiration of the whole plant or more commonly the affected tissue. Calcium plays a major role in cell elongation and is an important component in cell walls; structurally it acts like cement between cells. Calcium moves up the plant via transpiration; if light levels are too low or there is high relative humidity, calcium deficiency can occur in young shoot tips. Calcium is an immobile nutrient, which means that it cannot be translocated from older tissue to the shoot tip. Therefore new growth is severely reduced. Although calcium may be adequate in the lowermost leaves, levels in the growing tip region can be too low, causing poor leaf expansion followed by necrotic patches in the young leaves. Complete necrosis of the shoot is the advanced stage causing the inability of the reproductive structures to form. If flowers were present when calcium levels become devastatingly low in the substrate, bud abortion occurs.
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Calcium and boron are taken up passively by the plant. With passive uptake, nutrients only move into the plant along with the water used for transpiration. No transpiration, no uptake, regardless of the concentration of those nutrients in the soil solution.

The environment where the plants are being grown will directly affect transpiration rates, and calcium and boron uptake. The types of environments that suppress transpiration can include:

– Hot, humid conditions, especially when light levels have been reduced with excess shade.

– Cool, humid conditions, especially when no de-humidification is occurring.

– Greenhouses with little or no air movement, especially when conditions are humid.

Other management factors can also reduce transpiration rates and affect calcium and boron uptake. For example, a high salt level (high EC) in the root media will reduce transpiration by making it harder for the plant to take up water out of the soil solution. Constant overwatering or underwatering may also reduce transpiration rates.

Calcium and boron are only taken up by the plant at the root tip. Root tip damage caused by salt burn, overwatering, fungus gnats or root pathogens such as Pythium can cause calcium or boron deficiency symptoms similar to those caused by environmental conditions. In that case, careful irrigation and a fungicide are required in order to grow a healthy root system and improve nutrient uptake. When adequate calcium is being applied, but is not being taken up by the plant, then anything that increases transpiration, like lowering the humidity levels in greenhouse or increased air movement, will increase calcium uptake.

Be extremely careful applying extra boron to a crop. The problem is that the difference between boron deficiency, adequate boron levels and boron toxicity is much smaller than any other nutrient. If the applied concentration is too high or the high concentrations are applied too frequently, then it is very easy to get boron toxicity.

Trichomes (hairs) - create a more humid microenvironment to reduce evaporative water loss
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ken dog

ken dog

Good to see other people focusing more and more on vapor pressure deficit... it is so important.

I myself am guilty of sometimes diagnosing symptoms without inquiring more about the temperature and humidity levels.

On the other hand, on the many occasions I have mentioned that humidity was too low or too high, I have sometimes been met with disbelief from the people asking about it.:)


Living dead girl
On the other hand, on the many occasions I have mentioned that humidity was too low or too high, I have sometimes been met with disbelief from the people asking about it.
I've lost count of how many times I've seen it dismissed. Yet it *is* the plant's circulation system, its heart if you will. How's everything else going to work if the heart isn't working?


Here is the source from the OP for reference if anyone needs or wants to see the couple missing pics.

I have also noticed a disparity with how people place value on VPD as well. There are some good growers that don't seem to care and others that seem to live and die by it.

When looking at the whole process of transpiration, we have to take into account a large number of factors. Some of these factors can negate other 'downstream' regulatory processes involved with transpiration. For instance a soil with lower moisture content can have a larger impact on stomatal conductance than VPD. Similarly higher EC levels can have a larger impact than VPD. As well medium texture or its' matric potential can also have a greater affect on stomatal conductance than VPD. These few things make sense as there's no point in transpiring water that roots cannot acquire.
All of these factors (and more) are all additive resistances for determination of maximum transpiration rate for a given environment / scenario.

Most generally speaking;
-If the plant is in a scenario with a full healthy root system, low EC, with a medium of proper texture AND continual moisture at or near field capacity - then VPD will most likely be the dominate "segment" of total resistances that affect transpiration rate.
-If the plant is in a scenario with smaller root development, medium to high ec, mediums with poor matric potential AND/ OR variable moisture levels - then VPD is not likely to be the dominate segment of total resistances that affect transpiration rate.

IMHO I'll bet dollars to donuts that those who have not noticed huge benefits from VPD are probably running systems with wet/dry cycles. I would presume that those with active hydro systems and healthy conditions may be more likely to have seen greater value in VPD and benefits.

Some papers that may go along here or validate statements.
SPAC ohm's.jpg

"The movement of water through plants obeys an Ohm’s law analogy, i.e., current equals driving force (the electrical potential gradient) divided by electrical resistance. Thus, water flux is more clearly understood if it is regarded as being driven by a difference in water potential, against a resistance. Under steady-state conditions, flow through each segment of the SPAC is described as follows:"
SPAC ohm's formula.jpg

"Where, rm is the resistance due to the soil matrix, rr is the root resistance, rx is the resistance through the xylem in plant stems, rs is the stomatal resistance, and ra is the aerial resistance. Ψs, Ψr, Ψl and Ψa are the water potential of the soil, root, leaf and air, respectively. Resistances are additive in a series. Figure 2.2 provides a graphical schematic representation of SPAC."

http://classes.uleth.ca/201401/biol3460a/Lecture Notes/Bio3460-4 WaterRelations.pdf
Variation of stomatal conductance relative to VPD, Light or soil moisture.
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Variation in stomata aperture size and transpiration vs air movement (boundary layer influence)
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Effect of extractable soil moisture on stomatal conductance
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"Soil texture strongly mediates plant water availability through its control on soil hydraulic characteristics, because the saturated hydraulic conductivity of soil is a function of pore size; coarser textured soils have larger pores and higher saturated conductivity than finer textured soils. Coarse-textured soils, however, lose more water and have lower conductivity at higher soil water potentials (Ψs) than fine-textured soils, to the extent that plants growing in coarse soils exhaust their water supply at higher water potentials than plants growing in fine-textured soils. Plants may overcome the effects of steeply declining soil hydraulic conductivity at high Ψs by developing higher root to leaf area ratios (Ar:Al), or by lowering the transpiration rate (E). Both features will reduce the rate of water uptake per root surface area (rhizosphere flux density), thereby minimizing the loss of hydraulic contact between the root system and the soil."
"Therefore, plants occurring on coarse-textured soils typically must operate at relatively high Ψs and may be more vulnerable to cavitation than plants occurring on fine-textured soils. This trend may be particularly apparent in roots, which are generally more vulnerable to cavitation and demonstrate a greater plasticity across moisture gradients than stems."

LINK (pg. 209)
"Stomatal conductance strongly depends on soil water status, which can be characterized by plant available water. As a general rule, plant available water is considered to be 50% of the water holding capacity of the soil, which refers to the amount of water held between field capacity and wilting point. The water-holding capacity of the soil is highest in silt soil and lowest in heavy clay soil. The value of Gs has a strong effect on the main carbon and water fluxes: GPP, total ecosystem respiration (Reco), and latent heat flux (LE)."

"Furthermore, salt stress can increase leaf instantaneous water use efficiency by reducing stomatal conductance to a greater extent than photosynthesis, thereby allowing plants under salt stress to produce more dry matter than plants in nonsaline soil on the same quantity of water. Finally, salt stress can precondition plants to low soil water potential by allowing them to osmotically adjust, enhancing their ability to survive as the soil dries. These generalizations appear to hold true for both C3 and C4 crop plants and halophytes."


Can you all make some recommendations on ideal humidity and temperatures? I read one of Ed Rosenthal's books, and if IIRC, he recommends temps for flowering plants of 75-80deg, with humidity around 50%.

I just completed my first indoor organic soil grow, and the leaves on my plants all started looking pretty sad when they got near full ripeness. Really, they never looked 100% happy. They have always had a reverse-canoe effect going on - edges curling down with end tips showing slight burning, and also claw effects. I figured it was Nitrogen excess.

Just last night, I had an "ah-ha!" moment when I realized that the fan I had blowing on my canopy was fixed (not oscillating), and may have been drying the leaves. Now I know that it may have been dramatically increasing transpiration. I live in a very dry climate. I've been running the house fan 24/7 to keep the room from getting too humid, and now I'm wondering if it's been too dry. RH stays around 50%. Dark cycle temps for grow room were dropping in to the low 60's, because it I run the dark cycle from 8:00am - 9:00pm, and I run A/C in the house during day. Makes basement cold.

Great thread! Thank you!
Herb Forester

Herb Forester

People must stop running around the forums giving out advice regarding deficiencies. Many, many issues start and finish with your environment regarding transpiration and not with the deficiencies in your feeding program / medium.
Definitely the winning answer for at least 90% of 'deficiency' discussions. When spending so much on pre-mixed fertilizers, it's probably somewhat natural to mistakenly think from that angle first.


Living dead girl
Supporting photo for my post above. Leaves look sad. This was right before chopping.
I'd be calling that N+.
The rosenthal book is a good starting point, IMO, but 50% isn't doing you any favors. My best rounds have been when rh was 70%, temps around 80 and then the temp and rh tapered back to low to mid 70s and 50% by the end.
Same here! Even though, I've gotta be honest, I really detest indoor growing. Moreso the older I get. :/


Can you all make some recommendations on ideal humidity and temperatures? I read one of Ed Rosenthal's books, and if IIRC, he recommends temps for flowering plants of 75-80deg, with humidity around 50%.
The "VPD" information everyone is using says I flower in 18+.

Like SpiderK says "Trichomes (hairs) - create a more humid microenvironment to reduce evaporative water loss"

What trigger causes more trichomes to be produced, by trichome bearing plants? Low humidity. I flower with a canopy temp of 72F max (cannabis will try to cool itself using transpiration in higher temps), and a relative humidity between 20-25%, I prefer 20%. This low temp, low humidity sets up some rather high transpiration rates, making for much lower nutrient strengths being necessary. Yes, I grow hydro, no 'lack of moisture' in my soil. :)

What do I get? Massive trichome density and outrageous terpene/cannabinoid expression. Every time I hear a grower complain about how hot it is in their flower room I cringe. When I sample their cannabis I just shake my head sadly. So much effort, for such a lower quality product.

Herb Forester

Herb Forester

People must stop running around the forums giving out advice regarding deficiencies. Many, many issues start and finish with your environment regarding transpiration and not with the deficiencies in your feeding program / medium.
Funny, a thread about transpiration with great info, another new farmer quickly realizes something simple and related that should help, yet the first feedback given is for a nutrient problem.


This was my first grow, so gotta learn somehow. I was just wondering if increased transpiration due to low RH could increase nutrient uptake, possibly causing over-nutrification effects in the plant, even in a growing medium that has correctly balanced nutrients. .

On that note, if your growing medium has an excess of N, would increasing the RH of the room lessen the uptake of the N, and lessen the detrimental effects of the hot soil on the plant?


This was my first grow, so gotta learn somehow. I was just wondering if increased transpiration due to low RH could increase nutrient uptake, possibly causing over-nutrification effects in the plant, even in a growing medium that has correctly balanced nutrients. .

On that note, if your growing medium has an excess of N, would increasing the RH of the room lessen the uptake of the N, and lessen the detrimental effects of the hot soil on the plant?
Plants use transpiration for two main environmental controls, cooling and hydration. Cannabis can only handle so much transpiration, so it's best to choose one environment or the other.

High Humidity, High Heat OR Low Humidity, Low Heat
Both will give high transpiration rates when properly set up. One will also grow significantly more trichomes than the other. Ed doesn't give advice for maximum trichomes.

Yes, when your transpiration goes up, the nutrient strength needs to be lower. I rarely feed higher than 750ppm in flower.

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