Understanding Transpiration

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ken dog

ken dog

To me, high transpiration rates simply waste water, nutrients, and money.

There are many ways to stress your plants into producing more trichomes.
First thing that comes to mind, is UVB light.


To me, high transpiration rates simply waste water, nutrients, and money.

There are many ways to stress your plants into producing more trichomes.
First thing that comes to mind, is UVB light.
I'm sorry, high transpiration grows the highest quality cannabis. You're using the same amount of nutrients and a bit more water.

It would be interesting to see if UVB actually causes additional trichome production. Everything is already pretty much at maximum frost levels. (Where would the trichs grow from?)
ken dog

ken dog

One could also use the shells of an invasive bug...Chitin

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noun, Biochemistry.


a nitrogen-containingpolysaccharide, relatedchemically to cellulose, thatforms a semitransparenthorny substance and is aprincipal constituent of theexoskeleton, or outercovering, of insects,crustaceans, and arachnids


During about week 5 in flower during night cycle, I have seen large drops in CO2 where they were almost non existent a few weeks prior. Just find it curious and contrary to what I have heard elsewhere in terms of nighttime CO2 uptake. Havent seen it enough to believe this is always the case though and not nearly enough to warrant having the gas go on during darks, just seeing what others have observed.


I think the lesson that can be learned from all this, is that some sort of stress is necessary for increased trichome production.

It doesn't matter how it is done...and so far on this thread, three different ways have been mentioned.
There are distinct advantages for using low humidity, low temps to increase trichome density.

I don't have to mess with additional lights and efficient 'coverage' of said lights. I don't have to add something I don't already have on hand. The low humidity is universal, causing an entire plant effect and is already part of my growing style.

Low humidity/low temps also seems to produce the highest terpene content. I've never seen the same genetics be anywhere near as frosty, in other gardens. I've never shared genetics with a grower that grows low temp/low humidity though.

All in all, it's a win, win, win, for me.


Keep in mind, if you're in soil or any other system where the roots are not in constant exposure to the nutrient solution, your root zone temps need to be different. 75F is great for soil/soilless or other intermittent feed hydro.


Experimenting with this right now, looking forward to the result in a few weeks.
Excellent. :)
Remember the plant needs time to respond to the change in environment. I would make sure the plant was already acclimating to a dry environment before initiating flower. Changing in mid-flower will most likely produce stress as the plant adapts.

Looking forward to your results. :)


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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.
Damn bro, most excellent post. Props ++++. Can you source a link where you came by this level of info. I would like to check more of your source out if possible. Peace


Good stuff.
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