Transport of Water and Minerals to Leaves

Optional Videos

Learn how water is transported from the soil to the leaves of a plant.

Learn how transpiration helps with the transport of water to the leaves.

We have dealt with the transport of water in plants from the soil into the root xylem. Now we need to discuss how the water is transported against gravity from the roots to the leaves where it is needed for the process of photosynthesis.

Water travels to the leaves via the stem. Recall, that three processes are necessary for the transport of water in plants, namely; transpiration, capillarity and root pressure. All three of these processes are passive and do not require an input of energy.

Transpiration: constant water loss via transpiration from the leaves causes a negative water pressure in the leaves. The negative pressure in the leaves works like a 'suction' force, pulling the water up the stem.

Capillary Action: water moves up the stem in response to the 'suction' caused by transpiration because of two forces: adhesion and cohesion. Cohesion is the tendency for water molecules to stick together and adhesion is the tendency for water molecules to stick to other surfaces, such as the inside of the xylem vessels. Stem xylem is structurally adapted to take advantage of capillarity, because they are very long with a narrow diameter.

Root Pressure: water can also be moved up the stem via a 'push' force from the roots. Water is constantly being absorbed by the roots due to the negative water potential in the root cells. This movement of water into the roots can cause the water pressure inside the roots to become high, resulting in a force that 'pushes' water up the stem xylem.

Fact:

Capillarity: refers to the ability of a liquid to flow through narrow spaces (capillary pressure).

Investigation: Examining water uptake by the stem

Aim

To examine water uptake by the stem.

Apparatus

water
food colouring dye (available at supermarket)
white flower on a stem, e.g. Impatiens, carnation or chrysanthemum
scissors
two jars, cups or measuring cylinders
plastic tray
sticky tape

Method

Fill one jar with plain water, and one with water containing several drops of food colouring dye.
Take the flower and carefully cut the stem lengthwise, either part way up the stem or right up to the base of the flower (try both, the results will be different!)
Put one half of the stem into the jar containing plain water and one half of the stem into the jar containing food colouring dye. To make it easier to insert the stalks without breaking them, it helps to wedge paper underneath the jars so that you can tilt them towards each other. Tape the jars or cylinders down onto a tray so that they do not fall over.
Observe the flowers after a few hours and the next day, and note where the dye ends up in the flower head. You can leave the flowers up to a week but be sure to make sure that they have enough water.

Variation: Instead of using one cylinder with water and one with food dye, use two different colour food dyes (e.g. blue and red). At first the flower will show two separate colours, but as time goes by the whole flower will show both dyes. This is because water can move sideways between xylem vessels through openings along their length. The ability of water to move laterally between vessels is useful for when air becomes trapped in a vessel, causing a blockage. If you cut the stem right up to the base of the flower, this will limit movement between the xylem vessels.

Variation: Try using celery stalks with leaves. Cut open the celery stalk (cross-section) and you will see darker-coloured little holes/ spots. These are the vessels.

Results

Record your observations and results

Conclusions

What did you conclude from this experiment?

Transport of Water and Minerals to Leaves