2.4.9 Water Potential Calibration Curves

Key Concepts:

1. Water Potential (Ψ):

• Water potential is the measure of the tendency of water to move, measured in kilopascals (kPa).
• Pure water has the highest water potential of 0 kPa.
• Adding solutes lowers water potential, making it negative.

2. Isotonic Point:

• The point at which the water potential of the plant tissue is equal to the water potential of the surrounding solution.
• At this point, there is no net movement of water, and the tissue's mass or length remains unchanged.

Steps to Determine Water Potential Using a Calibration Curve:

1. Prepare Solutions:

• Create a series of solutions with known water potentials (e.g., sucrose solutions of varying concentrations).

2. Prepare Plant Tissue:

• Cut equal-sized pieces of plant tissue (e.g., potato or beetroot).
• Ensure consistency in size and surface area to maintain accurate results.

3. Immerse Tissue in Solutions:

• Place the tissue samples into each solution and leave for a set time (e.g., 20–30 minutes).
• Ensure all samples are treated identically.

4. Measure Changes:

• Remove the tissue, blot dry to remove excess solution, and record the change in mass or length.

5. Plot a Calibration Curve:

• Plot the percentage change in mass or length against the water potential of the solutions.
• The point where the curve crosses the x-axis (no change in mass or length) represents the water potential of the plant tissue.

Analysis:

1. Hypotonic Solutions (High Water Potential):

• Water enters the tissue by osmosis.
• Tissue gains mass or length as it becomes turgid.

2. Hypertonic Solutions (Low Water Potential):

• Water leaves the tissue by osmosis.
• Tissue loses mass or length as it becomes plasmolysed.

3. Isotonic Solution:

• No net movement of water.
• Tissue mass or length remains constant.

Practical Considerations:

• Ensure solutions cover a wide range of water potentials to identify the isotonic point accurately.
• Blot tissue carefully to avoid measuring external water.
• Use replicates to improve reliability.