45-50 minutes

Describe water uptake and water movement in plants.

One of the most vital of plant functions is uptake of water from soil. The driving mechanism for water uptake and transport within the plant is transpiration, the loss of water from plant tissues in the form of a vapor. Transpiration plays several important roles in keeping the plant alive. Foremost, the evaporation of water from leaf surfaces allows plants to cool themselves. Without this ability the plants would quickly overheat and die on bright sunny days. In fact, when moisture in the soil is not available for transpiration (as occurs during drought), plant tissues can be as many as 8 °C (15 °F) warmer thatn the air temperature. Leaf temperatures above 38 °C (100 °F) can lead to tissue damage in many summer-grown crop plants.

A second major role of transpiration is facilitation of nutrient uptake into the plant. S oil nutrients must come in contact with roots for the plant to absorb them. One way that nutrients contact roots is through nutrient flow in the soil water solution. Transpiration of water from the plant creates a gradient that drives water movement in the soil towards the roots. As the water moves it carries some nutrients with it and as the water enters the plant so do the nutrients. This is a very simple mechanism for nutrient uptake, but not all nutrients can be taken up this easily.

Much of the water transport within the plant occurs in the xylem. These elongated, pipe-like cells die shortly after they are produced and serve as a conduit for water flow (much like a straw).  Water escapes the plant through stomates, openings in the upper or lower surfaces of the leaves.  Specialized cells, called guard cells , are located on the perimeter of the somates and serve to regulate the size of the opening. This allows the plant to control the amount of water loss. The stomates also permit carbon dioxide (CO₂) gas to enter the plant. CO₂ is one of the main building blocks for production of materials during photosynthesis. The plant is functioning best when there is ample soil moisture available for transpiration, which allows adequate plant cooling and maximum uptake of nutrients and CO₂.

The following demonstration uses celery stalk to display water movement through plants. Celery works well for this demonstration because it has prominent xylem tissue and is readily purchased at a local grocery store.

• Celery stalk with leafy top still attached
• Beaker
• Red food coloring
• Water
• Razor blade

1. Make a cut across the base of the celery stalk to expose a new section of the stem. Make sure to leave the leafy top intact.
2. Fill the beaker half full with warm water and add several drops of red food coloring.
3. Place the celery in the beaker of colored water .
4. After 30 minutes the red food coloring should be seen in the xylem of the celery stalk. The xylem can be made more visible by scraping the celery stalk with the razor blade.

Over time the red food coloring will rise up through the celery stalk as the plant tissue transpires water. The red color will be found only in the xylem tissue. The xylem is what makes the celery stringy.

The celery leaves must be in good condition for this demonstration to work.