Chapter 35 - Plant Water Balance

Water Potential (Ψ): Determining Water Movement

• Measure of the potential energy of water in a system relative to pure water under standard conditions

• Movement: HIGH (less negative) → LOW (more negative)

  • Occurs via osmosis across membranes and bulk flow thru vascular tissues

• A continuous gradient exists from soil to atmosphere in plants to drive water transport

  • From soil (highest Ψ) to atmosphere (lowest Ψ)

Water Potential as Potential Energy

• Water potential represents stored energy that determines the ability of water to move within a system.

• Water with higher potential energy has a greater capacity to perform work by moving to lower energy regions.

• The addition of solutes decreases the free energy of water, making its potential more negative.

• This energy gradient is what drives all passive water movement in plants.

Components of Water Potential

• Calculation: Ψ = Ψs + Ψp.

• Solute potential (Ψs) reflects the effect of dissolved solutes and is always negative relative to pure water.

• Pressure potential (Ψp) reflects physical forces acting on water and can be either positive or negative.

• The combined effects of solute and pressure potential determine the overall direction of water movement

Solute Potential and Pressure Potential: Influence on Water Movement

• HIGH solute concentration = LOW solute potential

  • Causes water to move TOWARDS low solute potential because that high concentration of solutes will lower the free water availability

• Pressure potential causes water to move from areas of higher physical pressure to areas of lower pressure.

• Positive pressure (i.e. turgor pressure in cells) resists additional water entry

• Negative pressure (i.e. tension in xylem) actively pulls water upward through the plant.

Moving from Low → High Solute Concentration

• Water moves across a selectively permeable membrane by osmosis from regions of low solute concentration to regions of high solute concentration.

• This movement occurs because regions with higher solute concentration have lower water potential.

• The process continues until equilibrium is reached or opposing pressure balances the gradient.

• Solutes effectively create a “pull” that attracts water into the region

Water Movement in Response to Pressure Differences

• Water moves from high → low pressure thru bulk flow

• Xylem: water is pulled upward due to negative pressure (tension) generated by transpiration

• Phloem: water is pushed along due to positive pressure created by sugar loading

• These pressure-driven movements allow long-distance transport of water and solutes.

Calculating Water Potential

• Adding solute potential and pressure potential

• Water always moves from the region with the higher (less negative) to the region with the lower (more negative)

• Cell has Ψ = -1.5 MPa and the surrounding solution has Ψ = -2.0 MPa

  • So, water will move out of the cell.

Water Obtaining in Salty Soil

• Plants growing in salty soils accumulate solutes in their root cells to lower their internal water potential.

• This allows the plant’s water potential to remain lower than the surrounding soil, enabling water uptake.

• Specialized enzymes increase the concentration of organic molecules inside cells to maintain this gradient.

• This adaptation prevents dehydration in environments where external water potential is very low

Plant Survival in Dry Habitats

• Reduce their internal water potential by accumulating solutes as the amount of water available in the soil decreases

• Allows them to maintain a gradient that continues to draw water into the plant

• If the plant cannot maintain this gradient, turgor pressure drops and wilting occurs.

• Extended loss of turgor pressure can lead to tissue damage and plant death.

Water Entering Plant Roots

Establishes the initial step in water movement through the plant.

• Root cells actively transport ions into their cytoplasm, which lowering their internal water potential

• Water is then pulled from the soil (where water potential is higher) into the root cells via osmosis

• Root hairs significantly increase the surface area available for water absorption.

Three Water Pathways Through Roots

• Symplastic route: water moves through the cytoplasm of cells connected by plasmodesmata.

• Transmembrane route: water crosses cell membranes repeatedly, often using aquaporin channels.

• Apoplastic route: water travels through cell walls and intercellular spaces without crossing membranes.

• These pathways allow flexibility in how water reaches the vascular tissue

Functions of Casparian Strip

A waterproof barrier located in the endodermis of roots

• Blocks the apoplastic pathway which prevents water from bypassing cell membranes.

• This forces water and solutes to pass through selectively permeable membranes.

• As a result, the plant can regulate which ions enter the xylem

Root Pressure Generated in Xylem

• Root cells actively transport ions into the xylem, lowering its water potential

• Water enters the xylem from surrounding cells by osmosis

• The accumulation of water creates positive pressure within the xylem

• This pressure can push water upward, especially when transpiration is low.

Guttation

• The appearance of water droplets at the edges of leaves.

• It occurs when root pressure forces water out of special openings called hydathodes.

• This process is most common at night when stomata are closed and transpiration is minimal.

• It demonstrates the presence of positive pressure in the xylem.

Capillary Action

• Movement of water through narrow spaces due to intermolecular forces.

• Adhesion causes water molecules to stick to the walls of xylem vessels.

• Cohesion causes water molecules to stick to each other via hydrogen bonding.

• Surface tension at the air-water interface contributes to upward movement.

Cohesion-Tension Theory

• Explains how water is pulled upward through xylem from roots to leaves along a water-potential gradient via forces generation by transpiration

• Cohesion between water molecules transmits this tension down the continuous water column

• Allows water to move long distances without the use of metabolic energy

Evaporation: Creating a Pulling Force on Water

• Water evaporates from mesophyll cell surfaces into the air spaces of the leaf.

• This evaporation creates a curved meniscus that increases surface tension.

• The resulting tension lowers the water potential in the leaf.

• This tension pulls water upward through the xylem from the roots.

How Xylem Can Withstand Negative Pressures

• Thick secondary cell walls reinforced with lignin that prevent vessels from collapsing under tension

• Structural strength of xylem → important adaptation for the evolution of tall vascular plants

  • Allows plants to transport water to great heights

Structural Adaptations for Reducing Water Loss

• A thick waxy cuticle reduces evaporation from leaf surfaces.

• A multilayered epidermis provides additional protection

• Stomatal crypts reduce airflow around stomata, slowing evaporation.

• Trichomes help trap moisture and reduce water loss.

• Needle-like leaves reduce surface area exposed to the environment.

Translocation

• Movement of sugars through the phloem from sources to sinks.

• Occurs via bulk flow driven by pressure differences within the phloem

• Sugars produced in photosynthetic tissues are transported to areas of use or storage.

• This process ensures distribution of energy throughout the plant.

Sources and Sinks

• Sources = tissues where sugars are produced

  • I.e. mature leaves.

• Sinks = tissues where sugars are used or stored,

  • I.e. roots or developing fruits

• The direction of sugar movement depends on the plant’s developmental stage and needs, ensuring efficient allocation of resources

Main Cell Types of Phloem

• Sieve-tube elements are elongated cells that transport sugars and lack a nucleus at maturity.

• Companion cells are metabolically active and support sieve-tube elements.

  • Load sugars into the phloem and maintain cellular function.

• These two cell types work together to enable efficient sugar transport.

Pressure Gradient: Est. in Phloem

• At the source, sugars are actively loaded into sieve-tube elements, lowering water potential.

• Water enters the phloem from the xylem, increasing turgor pressure.

• At the sink, sugars are removed, increasing water potential.

• Water exits, lowering pressure and creating a pressure gradient.

Pressure-Flow Hypothesis and Sugar Movement

• States that sugars move from sources to sinks via bulk flow.

• High pressure at the source pushes phloem sap toward areas of lower pressure at the sink.

• This movement is driven by differences in turgor pressure created by sugar loading and unloading.

• The process allows efficient long-distance transport of nutrients.

Sugars: Active Loading at the Source

• Proton pumps use ATP to create a gradient of hydrogen ions across the membrane.

• This gradient drives a sucrose-H⁺ symporter that transports sucrose into companion cells.

• Sucrose then moves into sieve-tube elements through plasmodesmata.

• This process lowers water potential and initiates pressure-driven flow.

Sugars: Unloaded at the Sink

• Removed from phloem by either active or passive transport

  • Increases the water potential

  • So water enters back into the xylem/reduces pressure at the sink

• Once inside sink cells, sugars are used for metabolism or stored

Water Movement Supporting Phloem Transport

• Water enters phloem at the source b/c of low water potential created by sugar loading

• Increases pressure and drives bulk flow of sap to the sink

• At the sink, water exits the phloem as sugars are removed.

• Maintains the pressure gradient necessary for continuous flow.

Xylem and Phloem: Functional Connections

• Water moves from xylem into phloem at sources to generate pressure for sugar transport.

• At sinks, water returns from phloem to xylem after sugars are unloaded.

• This cycling of water helps maintain both transport systems.

• The interaction ensures efficient distribution of both water and nutrients.

Big Picture: Driving Force of Water Movement Thru A Plant

• Water moves along a continuous gradient from soil to atmosphere

• Transpiration = primary driving force

  • Creates negative pressure in leaves

• Cohesion and adhesion allow this force to pull water upward through xylem

• Root pressure and capillary action contribute slightly but are not the main drivers