2.1 Water potential and plant-water relations

Water potential is key to understanding plant-water relations. It measures how water moves in plants, affecting everything from cell structure to nutrient transport. This concept is crucial for grasping how plants manage water, a vital resource for their survival and growth.

Soil-water interactions and water transport mechanisms like the cohesion-tension theory are essential for plant life. These processes explain how plants absorb water from soil and move it throughout their bodies, directly impacting their ability to thrive in various environments.

Water Potential and Components

Measuring Water Potential

Top images from around the web for Measuring Water Potential

• 

  Transport of Water and Solutes in Plants | Biology for Majors II View original

  Is this image relevant?

• 

  Transport of Water and Solutes in Plants | OpenStax Biology 2e View original

  Is this image relevant?

• 

  Transport of Water and Solutes in Plants | OpenStax Biology 2e View original

  Is this image relevant?

• 

  Transport of Water and Solutes in Plants | Biology for Majors II View original

  Is this image relevant?

• 

  Transport of Water and Solutes in Plants | OpenStax Biology 2e View original

  Is this image relevant?

1 of 3

Top images from around the web for Measuring Water Potential

• 

  Transport of Water and Solutes in Plants | Biology for Majors II View original

  Is this image relevant?

• 

  Transport of Water and Solutes in Plants | OpenStax Biology 2e View original

  Is this image relevant?

• 

  Transport of Water and Solutes in Plants | OpenStax Biology 2e View original

  Is this image relevant?

• 

  Transport of Water and Solutes in Plants | Biology for Majors II View original

  Is this image relevant?

• 

  Transport of Water and Solutes in Plants | OpenStax Biology 2e View original

  Is this image relevant?

1 of 3

• Water potential ($\Psi$) measures the free energy of water in a system compared to pure water
  • Quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects
  • Pure water at atmospheric pressure has a water potential of 0 MPa
  • Water always moves from areas of high water potential to areas of low water potential
• Osmotic potential ($\Psi_s$) is the effect of dissolved solutes on water potential
  • Dissolved solutes reduce the water potential of a solution
  • Calculated using the van't Hoff equation: $\Psi_s = -iCRT$, where $i$ is the ionization constant, $C$ is the molar concentration, $R$ is the ideal gas constant, and $T$ is the absolute temperature
  • Example: Adding solutes like sugars or salts to water lowers the osmotic potential

Pressure and Turgor

• Pressure potential ($\Psi_p$) is the hydrostatic pressure of a solution
  • Positive pressure potential increases water potential, while negative pressure potential decreases it
  • Example: Root pressure in plants creates positive pressure potential, pushing water up the xylem
• Turgor pressure is the pressure exerted by the plasma membrane against the cell wall in plant cells
  • Occurs when water enters the cell due to a lower osmotic potential inside the cell than outside
  • Maintains cell shape and rigidity
  • Loss of turgor pressure leads to wilting in plants
• Plasmolysis is the shrinking of the cytoplasm away from the cell wall due to water loss
  • Happens when the osmotic potential outside the cell is lower than inside, causing water to leave the cell
  • Can cause cell death if prolonged
  • Example: Plant cells in a hypertonic solution (high solute concentration) will undergo plasmolysis

Soil Water and Plant Interactions

Soil Water Availability

• Wilting point is the soil moisture content at which plants cannot recover from wilting
  • Occurs when the water potential of the soil is too low for roots to absorb water
  • Permanent wilting point is around -1.5 MPa for most plants
• Field capacity is the amount of water held in the soil after excess water has drained away
  • Represents the maximum water content that the soil can hold against gravity
  • Varies depending on soil texture and structure
  • Example: Sandy soils have a lower field capacity than clay soils due to larger pore spaces

Transpiration and Plant Water Loss

• Transpiration is the loss of water vapor from plant leaves through stomata
  • Driven by the water potential gradient between the leaf and the atmosphere
  • Influences the rate of water uptake by roots and transport through the xylem
  • Factors affecting transpiration include temperature, humidity, wind, and light intensity
  • Example: On a hot, dry day, plants will transpire more water to cool their leaves, increasing water uptake from the soil

Water Transport in Plants

Cohesion-Tension Theory

• Cohesion-tension theory explains how water moves up through the xylem from roots to leaves
  • Water molecules exhibit strong cohesive forces due to hydrogen bonding, forming a continuous water column in the xylem
  • Transpiration creates tension (negative pressure) at the top of the water column, pulling water up from the roots
  • Adhesion between water molecules and xylem cell walls helps maintain the water column
  • Example: As water evaporates from leaf stomata, it creates tension that pulls water up the xylem like a rope

Xylem Structure and Function

• Xylem is the vascular tissue responsible for transporting water and dissolved minerals from roots to shoots
  • Composed of tracheids and vessel elements, which are long, hollow cells with lignified cell walls
  • Tracheids are found in all vascular plants, while vessel elements are more efficient and evolved later in angiosperms
  • Xylem also provides mechanical support to the plant due to the lignified cell walls
• Xylem transport is a passive process driven by the water potential gradient
  • Water moves from the soil (high water potential) to the leaves (low water potential) through the xylem
  • The rate of xylem transport is influenced by factors such as transpiration, root pressure, and xylem structure
  • Example: In a tall tree, water must overcome gravity and friction to reach the top leaves, requiring a strong cohesion-tension mechanism in the xylem