Translocation

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

  • so that cells deep within 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
• • Sugars required for metabolism

  – all the time, in all tissues
• • Sugars produced only

  – by source tissues

  – in light period
• • Translocation occurs

  – source to sink over short term

  – from storage tissues to young tissues over long term
• Sucrose

  • is principal photosynthetic product
  • accounts for most of CO2 absorbed
  • Glucose, as initial product of photosynthesis, is converted to sucrose which is the major form for transport or translocation
  • important storage sugar
  • tap root of carrots and sugar beet (up to 20% dry weight)
  • and in leaves, eg 25% leaf dry weight in ivy
  • major form for translocation of carbon
  • from photosynthetic leaves (source leaves)
  • in germinating seedlings after starch or lipid breakdown
• Direction of translocation:

  1. From source
  2. To Sink
• Direction of translocation

  
  

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

  • 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
• Direction of translocation

  A) Sink

  B) source

  
• ALLOCATION - The channelling of fixed carbon into various metabolic pathways within an organ or tissue
• ALLOCATION

  • In a source organ:

  – Metabolic utilization within the chloroplast

  – Synthesis of starch within the chloroplasts

  – Synthesis of sucrose for export to sink
• ALLOCATION

  • In a sink organ

  – Metabolic utilization and growth processes

  – Storage
• PARTITIONING

  
  

  • DISTRIBUTION of assimilates to competing sinks
  • Source leaves
• PARTITIONING

  – Lower mature leaves feed mainly the roots

  – Higher mature leaves feed mainly the young leaves and shoot apex
• SOURCE LEAVES

  • Preferentially supply sink organs with which they have vascular connection
  • Flower or fruit nearest to them (directly above or below them
  • Basis for flower and fruit thinning
• Mechanism of translocation of photosynthates

  1. Mass or bulk flow (Münch pressure flow hypothesis)
  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.
• • Xylem: Transports water and minerals from the roots to the rest of the plant, providing support and aiding in the movement of water against gravity.
• • Phloem: Transports organic nutrients such as sugars, amino acids, and other metabolic products produced in the leaves (source) to various parts of the plant, including roots, stems, and developing tissues (sinks).