Translocation of Sugars

• Soil Salinization

  • Plant takes up water and some minerals and leaves behind the salt from rain
  • Salinization reduces plant growth - salt in soil reduces plant ability to access water
  • Use drop irrigation to help - less water applied, less evaporated
  • Switch fields - rejuvenation, could leave organic matter on old field to bind water, reduce evaporation, and reduce the deposit of salts
  • Change where the source of the water is
  • Add soil amendments - things that change the pH of the soil (like gypsum)
  • Add proper drainage
• Sugars

  • Translocation of sugar bi-directional - shoots to roots + roots to shoots
  • Moves where sugars are produced to where they’re needed or stored
  • Moves from high concentration (source) to low concentration (sink) - movement by bulk flow (like water in the xylem)
  • Sources and sinks may change during growing season
  • Sources supply sinks that are closer to that source (proximal)
• Translocation of Sugars

  • Pressure flow hypothesis - pressure gradient in phloem
  • Source cell moves sucrose into companion cell, then sieve-tube element (active process) - plant sends energy in the form of ATP to move sugars
  • Now there’s a higher level of concentration of sugar in the sieve-tube of the phloem, water wants to move by osmosis (from a lower to higher solute concentration)
  • When water moves in, increases turgor pressure, drives phloem sap (contain sugars, minerals, water) to move down pressure gradient
• Translocation of Sugars: Active Process

  • Protein transporters are pumps, use ATP energy to pump molecule from area of low concentration to area of high concentration - primary active transport
  • Symporter - molecules move down their concentration gradient by bringing along another molecule - secondary active transport
  • Antiporter - two molecules go in opposite directions, essentially swap - secondary active transport
  • Companion cells use proton pumps to move H+ out of cell (requires ATP)
  • H+ moves down its concentration gradient back into the cell through a symporter, bringing sucrose with it
• Evidence of Pressure Flow

  • Aphids feed on phloem sap by inserting its stylet, a syringe-like mouthpart, into sieve-tubes
  • Pressure moves the phloem sap through aphid and out its anus as droplets of “honeydew” - excess that aphid doesn’t need
  • Aphids rob plant of nourishment (sugars)
• Phloem Unloading at Sinks

  • Moving passively
  • Some use sugar as it arrives, some store sugars
  • Sugars storage in cells is within a tonoplast (many within each cell), which is a type of vacuole (a membrane bound organelle that stores something - main vacuole stores water)
  • Proton pumps on the cell membrane of the vacuole, pumps hydrogen ions into tonoplast
  • Protons then migrate down concentration gradient out of vacuole, using an antiporter, brings sugars in