phloem

• Translocation


  The transport of assimilates in the phloem tissue
• Translocation within phloem tissue


  1. Transport of assimilates from source to sink

  2. Requires the input of metabolic energy (ATP)
• Phloem sap


  The liquid being transported within phloem sieve tubes
• Composition of phloem sap


  • Sugars (mainly sucrose)
  • Water
  • Amino acids
  • Hormones
  • Minerals
• Sources of assimilates


  • Green leaves and green stem (photosynthesis produces glucose which is transported as sucrose)
  • Storage organs e.g. tubers and tap roots (unloading their stored substances at the beginning of a growth period)
  • Food stores in seeds (which are germinating)
• Sinks (where assimilates are required)


  • Meristems (apical or lateral) that are actively dividing
  • Roots that are growing and / or actively absorbing mineral ions
  • Any part of the plant where the assimilates are being stored (e.g. developing seeds, fruits or storage organs)
• Loading and unloading of sucrose

  1. From source to phloem

  2. From phloem to sink
• Loading and unloading of sucrose


  Active process
• High temperatures or respiratory inhibitors


  Can slow down or stop the translocation of assimilates
• Translocation of assimilates is not fully understood yet by scientists
• Studies that have improved understanding of translocation


  • On plants whose sap 'clots', so that it is still possible to collect and study the sap (e.g. castor oil plants)
  • Using aphids to collect the sap – after the aphid inserts its stylet (tubular mouthpart) scientists remove the aphids head and collect the sap that continues to flow
  • Using radioactively labelled metabolites (e.g. Carbon-14 labelled sugars) which can be traced during translocation
  • Advances in microscopes enabling the adaptations of companion cells to be seen
  • Observations about the importance of mitochondria to the process of translocation