B2. Translocation

• Translocation is the movement of assimilates from source to sink.
• Translocation 


  is the movement of assimilates (sugars and other chemicals) from leaf through the phloem to the other areas for storage, utilization and consumption by the plant
• Why we need a transport system in plants?

  so that cells deep with the plant tissues can receive the nutrients they need for cell processes

  In fact:

  • roots can obtain water, but not sugar
  • leaves can produce sugar, but can't get water from the air.

  
• Sucrose

  1. is the Principal Photosynthetic Product
  2. Important storage sugar
  3. Major form for translocation of carbon

  • from photosynthetic leaves (source leaves)
  • in germinating seedlings after starch or lipid breakdown
• Direction of Translocation

  • From Source: a part of the plant that releases sucrose to the phloem e.g. leaf

  • To Sink: a part of the plant that removes sucrose from the phloem e.g. root

  •A plant part can act as source or sink depending on its developmental stage: for example - young leaves act as sink, but later their predominant role would be as source, once they are active in photosynthesis
• Allocation


  The channeling of fixed carbon into various metabolic pathways within an organ or tissue
• In a Source Organ

  • Metabolic utilization within the chloroplast
  • Synthesis of starch within the chloroplast
  • Synthesis of sucrose for export to sink
• In a Sink Organ

  • Metabolic utilization and growth processes
  • Storage
• Partitioning

  • Distribution of Assimilates to competing sinks

  1. Lower mature leaves feed mainly the roots
  2. Higher mature leaves feed mainly the young leaves and shoot apex

  • Source Leaves

  1. Preferentially supply sink organs with which they have vascular connection

  • Flower or fruit nearest to them (directly above or below them)

  1. Basis for flower and fruit thinning
• Mechanism of Translocation of Photosynthates

  1. Mass or Bulk flow (Münch pressure flow hypothesis) - assimilates transport from source to sink
  2. Diffusion - slow
  3. Cytoplasmic streaming - within the cytoplasm through plasmodesmatal connections between cells
  4. Others

  • Facilitated diffusion
  • Active transport across membranes
• Apoplast Pathway

  1. Sucrose is loaded into the phloem with the help of active transport

  2. A sucrose transporter protein is used to co-transport H+ and sucrose across the cell membrane

  3. The apoplast path utilizes ATP to pump H+ against the concentration gradient

  4. ATP is reduced to ADP+Pi which expels energy that enable H+ to be pushed against the gradient

  5. The H+ proton that is pumped against the gradient is then used by the sucrose transporter protein to move sucrose through the membrane

  6. The sucrose accumulated in the companion cell is able to flow down its concentration gradient via the plasmodesmata and into the phloem
• Symplast Pathway

  • Sucrose travels in the plasmodesmata which are connections between cells.
  • Therefore, the sucrose is able to flow down the sucrose concentration gradient into the phloem which has a lot of concentration of sucrose.
• Sink or Source?

  1. Fully expanded mature leaf
  2. Unripe fruit
  3. Young leaf
  4. Flowers

  Answers:

  1. Source
  2. Sink
  3. Sink
  4. Sink