Water Transport and Transpiration

Plants need water for a number of reasons - for photosynthesis, hydrolysis reactions and to keep their cells turgid. Water moves from the soil into the roots then up the xylem to the rest of the plant against the force of gravity. In the leaves, it evaporates out of the stomata in a process called transpiration.

Movement of water into roots

Water moves from the soil into root hair cells by osmosis. There will always be a higher concentration (or water potential) in the soil compared to the plant because water is constantly being lost through the leaves in transpiration. Water moves down its concentration gradient into root hair cells then travels through the root cortex and the endodermis before reaching the xylem.

Water can get into xylem vessels by two routes - the symplast pathway and the apoplast pathway. If water travels via the symplast pathway then it travels from cell to cell through the cytoplasm. Neighbouring cells are connected by small channels in the cell walls called plasmodesmata. However, if water is travelling via the apoplast pathway then it passes from cell to cell through the cell walls. Plant cell walls are very absorbent and water can travel through them easily, making this the main pathway that water uses to move from the roots to the xylem.

The problem with the apoplast pathway, however, is that the water (and the substances dissolved in it) is bypassing the cell membrane, who’s job it is to control what is going into the cell and ensure harmful substances such as toxins do not make their way in. The root, therefore, has something called the Casparian strip which is a waxy strip within the cell walls which is impermeable to water. This forces the water to go through a cell membrane which can then control which substances are allowed to enter the plant. Once water has passed through the Casparian strip, it can then reach the xylem.

Movement through xylem

Once water is in the xylem, it travels upwards - against the force of gravity - towards the rest of the plant. Water is able to move against gravity due to two forces: tension and cohesion. Tension is a ‘sucking force’ which is created when water evaporates from leaves (transpiration), pulling more water into the leaf. You can think of this in the same way as drinking water through the straw - any water which is lost from the top of the straw (as you drink) is immediately replaced with water molecules below it. Cohesion describes how water molecules are attracted towards each other. The strong hydrogen bonds between water molecules causes them to ‘stick’ together, creating a column of water. This means that when tension pulls water up the xylem, the whole column of water moves upwards.

Another force which facilitates the movement of water up the xylem is adhesion. Adhesion describes the attraction of water to non-water molecules (such as the molecules which make up the xylem walls). The attraction of water to the walls of the xylem help water to rise up through the vessel.

Transpiration

Transpiration is the loss of water vapour through evaporation from a plant’s surface. It mainly happens through gaps in the leaf called the stomata, which need to open during the daytime to allow gas exchange. Plants need to take in carbon dioxide for photosynthesis and get rid of oxygen, which happens through the stomata. A side-effect of this is that water vapour can also diffuse out of the leaf through the stomata - this is known as transpiration. Plants will close their stomata at night (because they are not photosynthesising so gas exchange does not need to take place) which minimises transpiration.

The following factors affect the rate of transpiration:

• Light intensity - increasing light intensity increases the rate of transpiration. This is because the plants are photosynthesising more and so more gas exchange needs to take place. Plants will have their stomata open for a much longer period on summer days with lots of sunlight, compared to the short winter days.

• Temperature - increasing temperature increases the rate of transpiration because more heat energy means that the water molecules have more kinetic energy and will diffuse faster out of the stomata. This increases the water potential gradient between the inside and outside of the leaf.

• Wind - windier conditions increases the rate of transpiration. This is because the wind will immediately blow away any water molecules that have just diffuses out of the leaf and increase the water potential gradient between the inside of the leaf and the outside.

• Humidity - more humid conditions decreases the rate of transpiration. Humidity is a measure of the level of moisture in the air. The more humid the air surrounding a leaf, the lower the water potential gradient between the inside of the leaf and the outside.

Measuring transpiration rate using a potometer

You can investigate the rate of transpiration using a piece of apparatus called a potometer. A potometer actually measures the amount of water uptake by a plant. The assumption is that the amount of water uptake by a plant is equivalent to the amount of water lost through transpiration - this is not completely true since some water will be used to keep cells turgid, for photosynthesis and other chemical reactions and some water will be created from aerobic respiration. However, it is a good way of estimating the amount of transpiration taking place. You can carry out the experiment using the following method:

1. Take a plant and cut off a shoot. You need to do this underwater to prevent air from entering the xylem. You should also cut the shoot at a slant to increase the amount of surface area for water uptake.

2. Keeping the shoot underwater, insert it into the potometer.

3. Take the potometer out of the water, but ensure that the capillary tube is submerged in a beaker of water.

4. Dry the leaves and leave the apparatus for 30 mins to enable the shoot to acclimatise.

5. Ensure that all other variables are controlled - e.g. light intensity and humidity.

6. Shut the tap so that the capillary tube system is closed them remove the end of the capillary tube from the beaker of water until one air bubble has formed. Place the capillary tube back into the beaker of water.

7. Record the starting position of the air bubble then time how long it takes for the air bubble to move along the capillary tube. Record the distance moved by the bubble and calculate the speed (distance moved per unit time).

8. The speed of the bubble can be used to estimate the rate of transpiration of the plant shoot.

Xerophytes

Xerophytes are plants which are adapted to living in regions where water is scarce. Examples include cacti, pineapple and marram grass. They have a number of adaptations which enables them to survive in such harsh conditions:

• Waxy layer on the epidermis - this waterproof outer layer reduces evaporation from the surface because water cannot easily pass through (it is impermeable).

• Sucken stomata - xerophytes have stomata which are sunken in pits. The pits shelter the stomata from the wind, reducing the water potential gradient between the inside of the leaf and the outside.

• Hairs on epidermis - hairs on the epidermis trap water vapour, reducing the water potential gradient between the inside and outside of the leaf.

• Spines - many xerophytes have spines instead of leaves which reduces the surface area for water loss.

• Rolled leaves - curled leaves traps water vapour, reducing the water potential gradient for transpiration. It also reduces the surface area of the leaf for water loss.

• Closure of stomata - xerophytes can close their stomata during conditions of particularly high temperature or light intensity. This reduces transpiration at times when the rate of transpiration would be very high.

• Stomata on the upper surface - usually stomata are found on the underside of plant leaves but for hydrophytes, this side will be submerged in water. Instead, hydrophytes have their stomata on the upper surface of their floating leaves to maximise gas exchange.

• Air spaces - pockets of air in the plant tissue helps the plant to float and can be used to store oxygen for aerobic respiration.

• Flexible leaves and stems - the flexibility of the leaves and stems helps to prevent damage from water currents. Unlike land plants, which need a sturdy stem to keep upright, hydrophytes are supported by the water around them.

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