Translocation of Organic Substances Notes

Translocation of Organic Substances

  • Definition: Translocation is the movement of organic solutes (like sugars) from source (areas of synthesis) to sink (areas of utilization or storage) in plants.

  • Importance of Translocation:

    • Supports growth and metabolism of non-photosynthetic tissues (roots, fruits, etc.).
    • Ensures supply of photoassimilates to developing parts.

  • Photoassimilates:

    • Produced during photosynthesis, temporarily stored as sucrose or starch.
    • Transported according to metabolic demands of the plant parts.
Directions of Translocation
  • Types of Translocation:
    • Upward: During seed germination and leaf development (growing shoot).
    • Downward: To non-green parts such as roots for storage and utilization.
    • Radial: Occurs in fruit and seed development.
    • Bidirectional: Movement shows polar concentration gradients from high to low.
Phloem Tissue for Translocation

  • Conducting Tissue:

    • Comprises xylem and phloem; xylem transports water/minerals, phloem transports organic solutes.

  • Ringing Experiment:

    • Demonstrates the role of phloem in translocation. Removal of bark (phloem) leads to accumulation of sugars above the ring, while the lower part dies due to lack of nutrients.
Structure of Phloem Tissue

  • Components:

    • Sieve Elements: Cells that form sieve tubes for solute transport.
    • Companion Cells: Assist sieve elements by providing metabolic support.
    • Phloem Parenchyma: Living cells that aid in lateral transport and storage.
    • Phloem Fibres: Provide structural support.

  • Sieve Plates: Porous to allow cytoplasm connection; contain P-protein bodies aiding in sealing injuries.

Theories of Translocation Mechanism
  1. Diffusion Hypothesis: Suggested simple diffusion mechanism; rejected due to insufficient rate.
  2. Protoplasmic Streaming Hypothesis: Posits that streaming of cytoplasm aids movement; lacks support based on rate differences.
  3. Electro-osmosis Hypothesis: Focuses on electric gradients in sieve tubes; not widely accepted.
  4. Interfacial Flow Theory: Proposed rapid solute movement across interfaces, but lacks robust evidence.
  5. Pressure Flow Hypothesis (Munch's Theory):
    • Most accepted theory; solutes move en masse due to hydrostatic pressure differences between source (high solute concentration) and sink (low concentration).
    • Water moves along with solutes, creating a flow dependent on turgor pressure.
Mechanism of Translocation
  1. Loading of Solutes:
    • From mesophyll cells into sieve tubes, increasing osmotic pressure.
  2. Translocation:
    • Driven by pressure gradients; water influxes to maintain turgor.
  3. Unloading:
    • Assimilates removed at sink; reduces pressure, allowing continuous flow.
Source-Sink Relationship
  • Source: Parts producing excess assimilates (typically leaves).
  • Sink: Parts consuming assimilates (roots, fruits, etc.).
  • Importance of Relationship:
    • Determines translocation direction and efficiency based on growth phase (vegetative vs. reproductive).
Factors Affecting Translocation
  1. Source-Sink Demand: Balance affects transport efficiency.
  2. Light: Influences photosynthesis and, consequently, assimilate concentration.
  3. Temperature: Optimal range affects metabolic activity.
  4. Oxygen: Essential for respiration in tissues involved in translocation.
  5. Minerals: Certain nutrients (like boron) enhance translocation.
  6. Metabolic Inhibitors: Substances that hinder respiration reduce translocation.
  7. Growth Hormones: Influence sink formation and mobilization of nutrients.
Summary of Questions to Consider
  • Loading and unloading processes of solutes.
  • Identifying sources and sinks in various plant theories.
  • The detailed mechanism of Munch's flow hypothesis.
  • Effects of environmental factors on translocation efficiency.