An Investigation into Water Loss from Plants.

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Katie Lindridge L6GG

An Investigation into Water Loss from Plants


Preliminary Experiment



One dicotelydonous leaf was taken and approximately 1cm² of clear nail varnish was painted both on the adaxil (top) and abaxil (bottom) surfaces of the leaf. It was then left for approximately 10 minutes to dry and gently peeled off using a scalpel after the 10 minutes. The peeled nail varnish was then examined first at low power (x40 magnification) and then on high power. It was noticeable that the number of stomata on the top of the leaf was significantly less than the number of stomata on the bottom of the leaf. This provided basic information upon which the plan could be based. It gave foundation for the plan which will be an extension of these findings.

Sketch of Observation

        Top Surface                                                  Bottom Surface


It can be concluded that there appears to be more stomata on the bottom of the leaf indicating that there may be more chance for transpiration from this area.

Scientific Knowledge

Diagram of Leaf


Each leaf on a plant is comprised of layers as shown above which are not tightly compact and instead have many air spaces necessary for gaseous exchange (to allow exchange of carbon dioxide and oxygen for photosynthesis and respiration). The air in the internal spaces of the leaf has direct contact with the outside air through the stomata. If there is a higher concentration of water vapour within the leaf than there is in the outside air a diffusion gradient will be created and water vapour can diffuse out of the leaf through the stomata, this process is transpiration.

The rate of transpiration will vary in different climatic conditions as this will influence the concentration of water vapour in the air. The higher the humidity of the air the higher the water concentration in it so the concentration gradient will be lower leading to a low rate of transpiration. If the wind speed is high saturated air around the leaf is constantly being replaced by dry air leading to a higher rate of transpiration. If the temperature is high the air has a higher capacity for water vapour and is dryer so the concentration gradient will be steeper leading to more transpiration, also more water in the plant cell walls will be heated turning to water vapour and this will be lost by evaporation. The light intensity will also vary the rate of transpiration as the lighter it is the wider the stoma open to allow more gaseous exchange for photosynthesis. This leads to more water vapour being lost by transpiration.

The opening and closing of stomata varies the contact between the outside air and inner cells of the leaf which determines the rates of transpiration. For photosynthesis to occur the stomata must open to allow carbon dioxide to enter, which will happen during sunny periods. The guard cells are forced to become turgid and open when the light intensity is high as this is when photosynthesis starts. This means that Carbon Dioxide is used up during photosynthesis increasing the pH of the cells which activates an enzyme which converts starch to glucose. As starch is converted ATP energy is released giving cells energy to activate proton pumps and the cell pumps out protons. Therefore K+ ions diffuse in and make the solute potential more negative so more water moves in and the guard cells become turgid. The guard cells are semi-circular with a thicker cellulose wall on the inside so therefore the inside wall has less elasticity. As they are forced to become turgid the pressure builds up forcing unequal curvature to occur and the outer walls stretch while the inner walls remain relatively unstretched.  Therefore the guard cells open creating stoma which inevitably increases the rate of transpiration.

As water leaves the leaf by transpiration it creates a transpiration stream which pulls up other water molecules so water is constantly moving up the xylem. The theory of Dixon Jolly is that due to water molecules being cohesive the water molecules are pulled along each route by the previous one and so columns of water are pulled along.

Water moves through the leaves taking one of three different routes. The apoplast route is the movement of the water (by diffusion) from one cell wall with a high concentration of water through to the next cell wall with a lower concentration of water (down the concentration gradient) eventually leaving by evaporation when a cell wall is in contact with the air at the stomata and the cell wall has a higher concentration of water than the air. Water vapour will leave by diffusion (in the form of evaporation) as the cell wall has no membrane that the water must pass through.

The symplast route is the diffusion of water from the cytoplasm of one cell through the plasmodesmata to the cytoplasm of a neighbouring cell with a lower concentration of water. It will eventually leave when the final cell neighbours an air space (stomata)  creating a concentration gradient is created. Water will leave via osmosis as it must travel through the cell membrane to escape into the air.

Water is said to take the vacular route when it moves from the vacuole of one cell, via osmosis through the tonoplast membrane, into the cytoplasm through the plasma membrane, and through the cell wall into the next cell and into the vacuole through the tonoplast membrane of the neighbouring cell travelling down the concentration gradient. It will eventually leave the cell neighbouring an air space as the cell will have a higher concentration than the air causing water to leave through the tonoplast and cell membrane by osmosis.

All these routes will bring water from the cells to the air spaces in the leaf and only if there is a concentration gradient between the water vapour in the leaf and water vapour in the air, water will leave by transpiration.

There are more stomata on the lower surface or the leaf so this is where most water is lost from as transpiration occurs. More are located on the bottom of the leaf as it is necessary to have them and the bottom surface is the most practical place. This is because if they were located on the top, this is the surface where the sun will beat down upon, heating the water molecules even more causing more evaporation and a steeper concentration gradient so more transpiration. Having the stomata underneath the leaf ensures that they are in the shade therefore while air can still reach it for gaseous exchange the leaf is slightly cooler and so less water will be lost. This reduces the risk of the plant from losing too much water and becoming dehydrated.

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Based on the preliminary experiment and scientific knowledge above I hypothesise that there will be more transpiration therefore loss of water from the abaxil surface of the leaf than from the adaxil. This is due to the larger number of stomata on the bottom of the leaf therefore a larger surface area of cells is in contact with the outside air and so more water can escape.

Rack1 will have the maximum amount of water loss as no stomata are blocked so water will be free to escape through these via transpiration.

Rack 2 will ...

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