Define matric potential, osmotic potential and turgor potential, and explain their interrelationships with water potential.

Defining the Key Potentials

Water potential (Ψ) is the measure of the free energy of water per unit volume and is a key determinant of water movement. It’s a composite of three main components:

Matric Potential (Ψ_m)

Matric potential is the potential energy of water associated with its attraction to surfaces, like soil particles or cell walls. It’s always a negative value because water molecules are held back from free movement. The more extensive the surface area, the greater the adhesion and the more negative the matric potential. This is crucial for water retention in soils and plant tissues.

Osmotic Potential (Ψ_s)

Osmotic potential is the potential energy of water due to differences in solute concentration. It's always negative. The higher the solute concentration, the lower the water potential, and the more negative the osmotic potential. It’s directly related to the water’s tendency to move from a region of high water potential to a region of low water potential across a selectively permeable membrane. A plant cell with a high solute concentration will have a more negative osmotic potential than the surrounding water.

Turgor Potential (Ψ_p)

Turgor potential is the pressure potential exerted by the cell contents against the cell wall. It is positive when the cell is turgid (swollen) and negative when the cell is plasmolysed (shrunken). It represents the force resisting further water entry into the cell. Turgor pressure is vital for maintaining plant rigidity and driving cell expansion.

Interrelationships and Water Potential

The relationship between these potentials is defined by the following equation:

Ψ = Ψ_m + Ψ_s + Ψ_p

Let's break down the implications:

• Water Absorption: At the soil-root interface, water moves from the soil into the root due to a difference in water potential. The soil’s water potential is typically higher (less negative) than the root’s water potential. This is driven by a combination of matric potential (water held in soil pores), osmotic potential (solute concentration in the root cells), and, to a lesser extent, turgor potential.
• Movement within the Plant: Water moves through the xylem due to a water potential gradient between the roots and the leaves. Transpiration in leaves creates a negative pressure (tension) that pulls water upwards.
• Cellular Processes: Turgor pressure is critical for cell expansion and growth. It also provides mechanical support to non-woody plants. The balance between osmotic and turgor potential determines the cell's water status.

Potential	Definition	Sign	Effect on Water Movement
Matric Potential	Water’s attraction to surfaces	Negative	Retards water movement
Osmotic Potential	Water’s potential due to solute concentration	Negative	Draws water into the region of lower potential
Turgor Potential	Pressure exerted by cell contents against the cell wall	Positive/Negative	Resists water entry; positive when turgid

Example: Root Hair Cell

Consider a root hair cell. It has a negative osmotic potential due to dissolved solutes. Water moves into the cell from the soil, initially driven by the osmotic potential difference. As water enters, turgor pressure increases, making the water potential less negative. Eventually, the influx of water stops when the water potential inside the cell equals that of the soil. Matric potential in the soil also plays a role in retaining water near the root hairs.