2.1 Water potential and plant-water relations

Water Potential and Components

Water potential is the central concept for understanding how water moves through plants. It quantifies the tendency of water to move from one location to another, and that movement governs everything from cell turgor to long-distance transport in the xylem. Once you understand water potential and its components, the rest of plant-water relations falls into place.

Measuring Water Potential

Water potential ($\Psi$) measures the free energy of water in a system relative to pure water at atmospheric pressure. Water always moves from regions of higher water potential to regions of lower water potential.

• Pure water at atmospheric pressure and no elevation difference defines $\Psi = 0$ MPa. Any factor that reduces water's free energy makes $\Psi$ negative.
• The full equation for a plant cell is: $\Psi = \Psi_s + \Psi_p$, where $\Psi_s$ is osmotic (solute) potential and $\Psi_p$ is pressure potential. (Matric and gravitational components matter in soil and tall trees but are often simplified out at the cell level.)

Osmotic potential ($\Psi_s$) captures the effect of dissolved solutes. Solutes lower the free energy of water, so $\Psi_s$ is always zero or negative.

• Calculated with the van't Hoff equation: $\Psi_s = -iCRT$
  • $i$ = ionization constant (number of particles the solute dissociates into; for sucrose, $i = 1$; for NaCl, $i = 2$)
  • $C$ = molar concentration of the solute
  • $R$ = ideal gas constant (0.00831 L·MPa·mol⁻¹·K⁻¹)
  • $T$ = absolute temperature in Kelvin
• A 0.1 M sucrose solution at 25 °C (298 K) gives $\Psi_s = -(1)(0.1)(0.00831)(298) \approx -0.25$ MPa. Adding more solute makes $\Psi_s$ more negative.

Pressure and Turgor

Pressure potential ($\Psi_p$) is the hydrostatic pressure exerted on or by water in a system.

• In a turgid plant cell, the rigid cell wall pushes back against the expanding protoplast, creating a positive $\Psi_p$ that raises the cell's overall water potential.
• In the xylem during transpiration, tension (negative hydrostatic pressure) develops, so $\Psi_p$ can be strongly negative there.
• Root pressure is an example of positive $\Psi_p$ generated in root xylem, sometimes visible as guttation droplets on leaf margins at night.

Turgor pressure is the specific case of $\Psi_p$ inside a living plant cell. When water enters a cell by osmosis (because $\Psi_s$ inside is more negative than outside), the cell contents press outward against the cell wall. This pressure keeps cells firm and drives cell expansion during growth. When turgor drops to zero, the cell is flaccid and the plant wilts.

Plasmolysis occurs when a cell is placed in a hypertonic solution (external $\Psi_s$ more negative than internal). Water leaves the cell, the plasma membrane pulls away from the cell wall, and the cytoplasm shrinks. Incipient plasmolysis, the point where the membrane just begins to detach, is when $\Psi_p = 0$ and $\Psi_{cell} = \Psi_s$. Prolonged plasmolysis can kill the cell because membrane integrity is lost.

Soil Water and Plant Interactions

Soil Water Availability

Not all water in soil is available to plants. Two benchmarks define the usable range:

• Field capacity is the water content remaining after a saturated soil has drained freely under gravity (typically around $-0.033$ MPa). This is the upper limit of water available to roots. Soil texture matters: clay soils hold more water at field capacity than sandy soils because their smaller pore spaces retain water more tightly.
• Permanent wilting point is the soil water potential at which roots can no longer extract water, roughly $-1.5$ MPa for most crop species. Below this threshold, the remaining water is held too tightly by soil particles for root uptake, and the plant wilts irreversibly.

The difference between field capacity and permanent wilting point is called plant-available water. Loam soils tend to have the most plant-available water because they balance drainage with water retention.

Transpiration and Plant Water Loss

Transpiration is the evaporation of water from leaf surfaces, primarily through open stomata. It is the main driver of long-distance water movement in plants.

The water potential gradient that powers transpiration is steep. Leaf air spaces may have a water potential of $-100$ MPa or lower on a dry day, while soil water sits near $-0.01$ to $-0.5$ MPa. Water flows down this gradient: soil → root → xylem → leaf → atmosphere.

Factors that increase transpiration rate:

• Higher temperature increases the vapor pressure inside the leaf relative to outside air.
• Lower humidity steepens the vapor pressure gradient between leaf and atmosphere.
• Wind removes the boundary layer of humid air around the leaf surface.
• Higher light intensity triggers stomatal opening (stomata open in response to blue light and low internal $CO_2$).

On a hot, dry, windy day all four factors combine, and transpiration rates can be very high. Plants regulate this by closing stomata, but that also limits $CO_2$ uptake for photosynthesis, creating a fundamental trade-off.

Water Transport in Plants

Cohesion-Tension Theory

The cohesion-tension theory is the accepted explanation for how water ascends the xylem, even in trees over 100 m tall. It relies on three physical properties:

1. Transpiration creates tension. As water evaporates from mesophyll cell walls into leaf air spaces, the menisci in the tiny cell-wall pores generate very negative pressures (tension) due to surface tension.
2. Cohesion maintains a continuous water column. Water molecules are linked by hydrogen bonds, giving liquid water high tensile strength. This means the column can be pulled from above without breaking easily.
3. Adhesion assists the column. Water molecules also adhere to the hydrophilic, lignified walls of xylem conduits, helping counteract gravity and preventing the column from pulling away from the walls.

The result is a continuous "chain" of water from root to leaf. Tension generated at the top is transmitted all the way down to the roots, lowering water potential in root xylem and drawing water in from the soil. No metabolic energy is spent on this transport; it is entirely driven by the evaporative demand of the atmosphere.

Xylem Structure and Function

Xylem is the vascular tissue that transports water and dissolved minerals from roots to shoots. It also provides structural support because of its lignified cell walls.

Two main conducting cell types make up xylem:

• Tracheids are elongated cells with tapered, overlapping ends. Water passes between tracheids through pit pairs (thin regions in the wall). Tracheids are found in all vascular plants, including ferns and gymnosperms. Their narrow diameter and pit membranes make them more resistant to embolism spread but also slower conductors.
• Vessel elements are shorter, wider cells stacked end-to-end with perforated end walls (perforation plates), forming continuous tubes called vessels. They conduct water much more efficiently than tracheids due to their larger diameter and open connections. Vessel elements are characteristic of angiosperms, though a few gymnosperm lineages also have them.

Xylem transport is a passive process. No ATP is required. The driving force is the water potential gradient from soil to atmosphere:

$\Psi_{soil} > \Psi_{root} > \Psi_{xylem} > \Psi_{leaf} > \Psi_{atmosphere}$

In a tall tree, water must be pulled upward against gravity (gravity reduces $\Psi$ by about $0.01$ MPa per meter of height) and must overcome frictional resistance inside the conduits. The cohesion-tension mechanism generates enough force to accomplish this because the atmospheric demand can produce tensions of $-2$ MPa or more in the xylem, far exceeding what gravity and friction impose.