Transport: Transportation of Water and Nutrient in Plants

• Water


  Very polar, a good solvent for charged or polar solutes, has high cohesive strength
• Water movement


  Water moves from high water potential to low water potential
• Water potential


  Measure of the potential energy in water, influenced by solute concentration, pressure, gravity, and matrix effects, denoted by the Greek letter ψ (psi) and expressed in units of pressure (megapascals, MPa)
• For water to move through the plant from the soil to the air


  Ψsoil must be > Ψroot > Ψstem > Ψleaf > Ψatmosphere

  
• Xylem


  • Transport water and minerals from the roots up the plant stem and into the leaves, a vascular tissue

  
• Xylem cell types


  • Xylem parenchyma
  • Tracheids
  • Vessel elements

  
• Transpiration


  Loss of water from the plant through evaporation at the leaf surface, the main driver of water movement in the xylem
• Cohesion-tension theory of sap ascent

  Explains transpiration
• Regulation of transpiration


  Achieved primarily through the opening and closing of stomata on the leaf surface

  
• Guard cells


  • Two specialized cells that surround the stomata which open and close in response to environmental cues
• Photosynthates


  Products of photosynthesis, usually in the form of simple sugars such as sucrose
• Sources


  Structures that produce photosynthates
• Sinks


  Points of sugar delivery, such as roots, young shoots, fruits and developing seeds
• Translocation


  The process of delivering sugars produced in sources to growing parts of the plant via the phloem
• Phloem


  • Conducts foods made in the leaves during photosynthesis to all other parts of the plant, a vascular tissue

  
• Phloem cell types


  • Sieve-tube cells/elements
  • Companion cells
  • Phloem parenchyma
  • Phloem fibers

  
• How translocation takes place

  1. Photosynthates synthesized in the leaves move through plasmodesmata to phloem sieve-tube elements

  2. Sucrose is actively transported from source cells into companion cells and then into the sieve-tube elements, reducing the water potential and causing water to enter the phloem from the xylem, resulting in positive pressure that forces the sucrose-water mixture down toward the roots