Transport of Water and Solutes in Plants

Transport of Water and Solutes in Plants

Water Potential

• Plants transport water to great heights using water potential.
• Water potential is a measure of the potential energy in water.
• It is the difference between the potential energy of a water sample and the potential energy of pure water.
• Expressed as the Greek letter Ψ (psi).
• Measured in units of pressure - megapascals (MPa).
• The potential energy of pure water is set to zero, so other potentials are relative to it.

Factors Influencing Water Potential

• Water potential in a plant is influenced by:
  • Solute concentration
  • Pressure
  • Gravity
  • Matrix effects: ion effects within the plant

Water Movement and Equilibrium

• As each component changes, the water potential of the system raises and lowers.
• Water moves to equilibrate the system, flowing down a potential gradient from an area of higher potential to lower potential.
• For water to move from the soil into the plant and up to the atmosphere (transpiration):
  Ψ{soil} > Ψ{root} > Ψ{stem} > Ψ{leaf} > Ψ_{atmosphere}
• Water movement occurs exclusively in response to changes in water potential.

Solute Potential (Ψ_s)

• Related to solute concentration, described by the van 't Hoff equation: Ψ_s = -MiRT
  • M = molar concentration of the solute
  • i = van 't Hoff factor (ratio of particles in solution to formula units dissolved)
  • R = ideal gas constant
  • T = temperature in Kelvin degrees

Characteristics of Solute Potential

• Solute potential is negative in plant cells and zero in distilled water.
• Solutes reduce water potential, resulting in a negative water potential.
  • This is due to the bonds between solutes and hydrogen molecules; energy in hydrogen cannot do work when bonded.
• Solute potential decreases with increasing solute concentration, causing a decrease in the system's water potential.

Pressure Potential (Ψ_p)

• Pressure is an expression of energy.
  • The higher the pressure, the more potential energy in a system.
• Positive pressure potential (compression) increases the system's water potential.
• Negative pressure potential (tension) decreases the system's water potential.

Plant Manipulation of Pressure Potential

• Plants manipulate pressure potential by adjusting solute potential and osmosis.
  • ↑solute concentration = ↓solute potential = ↓total water potential = ↑water into cell = ↑pressure potential
• The opening and closing of stomata also influences pressure potential by releasing water from the plant.

Gravity Potential (Ψ_g)

• In a plant with no height, gravity potential is close to negative 0.
• Gravity always reduces the water potential of the system by pulling downward towards the soil.
• The taller the plant, the higher the water column, and the greater the influence of gravity potential.
• Plants cannot manipulate gravity potential.

Matric Potential (Ψ_m)

• Always negative - 0, and 0 in a water-saturated system.
• The binding of water to a matrix always removes potential energy from the system.
• Similar to solute potential in binding water to another component, such as insoluble, hydrophilic molecules of the cell wall.
• Very (negatively) large in dry seeds; decreases as seeds hydrate.
• Cannot be manipulated by the plant.
• Typically ignored in well-watered plant systems because it is close to zero.

Transpiration

• The loss of water through evaporation at the leaf surface.
• Main driver of water movement through xylem.
• Causes negative pressure (tension) at the leaf surface, varying due to atmospheric humidity.
• During transpiration, water is drawn from the leaf mesophyll cells, through the xylem, down the stem, through the root cells and root hairs, and into the soil.
• Stomata close at night, and water is stored in the xylem vessels and tracheids by cohesion and adhesion.
• Passive process - NO ATP is required.

Control of Transpiration

• Atmosphere drives transpiration rate and causes massive water loss (up to 90% of water taken up).
• Cuticle: a waxy covering on leaves that prevents some water loss.
• Stomata & guard cells open and close to regulate gas exchange & water loss.

Adaptations to Water Loss

• Thick waxy cuticle to prevent water loss, as seen in desert plants or xerophytes.
• Thick covering of trichomes or stomata under the leaf surface reduces water loss during transpiration.

Transportation of Photosynthates in the Phloem

• Photosynthates: the products of photosynthesis, typically in the form of simple sugars like sucrose.
• Sources: structures in the plant that produce photosynthates (leaves).
• Translocation: transporting photosynthates to the growing parts of the plant.
• Sinks: points for sugar delivery; e.g., roots, shoots, developing seeds.
• Photosynthates are sent to different parts of the plant during different stages of development.
• Sugars from leaves may be sent out to the roots for storage, or sugars in storage in the roots may be sent to leaves.

Translocation Process

• Photosynthates are produced in the mesophyll of leaves, connected by plasmodesmata.
• Sugars are transported to reach phloem sieve tube elements (STE), AGAINST its concentration gradient (ATP required).
• STE’s have reduced cytoplasmic material & are separated by porous plates, which allow for pressure-driven bulk flow.

Movement to Closest Sink

• Photosynthates are moved to the closest sink.
• Phloem sap is an aqueous solution containing ~30% sugar, minerals, amino acids, and plant growth regulators.
• High sugar content decreases solute water potential, thereby decreasing total water potential, which drives water into the phloem from the xylem, increasing pressure.
• Increased pressure drives bulk flow because sink concentration is lower than the phloem.