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7.11 Transport of Assimilates in Plants

Posted10/11/2024
Updated23/12/2024
ByBMF
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1. Introduction to Assimilates and Assimilation in Plants

Assimilates are organic compounds produced in plants through the process of assimilation, primarily during photosynthesis and nitrogen metabolism. Key examples include:

  • Sugars (e.g., sucrose): Formed from the photosynthetic reduction of CO₂ and water in the leaves.
  • Amino acids: Synthesized by incorporating nitrogen (from nitrates or ammonia) into carbon skeletons derived from photosynthesis.
  • Why sucrose?
    • Sucrose is the predominant sugar transported in most plants because it is both highly soluble and metabolically less reactive than glucose.
    • This ensures that it remains intact during long-distance transport.

Sugar Production in Mesophyll Cells:

  • This can occur via two main pathways:
    • Symplastic Pathway: Sucrose moves cell-to-cell through plasmodesmata (cytoplasmic connections) until it reaches specialized transfer cells or companion cells associated with sieve elements.
    • Apoplastic Pathway: Sucrose is released into the cell walls (apoplast) and then actively taken up into companion cells by sucrose-H⁺ cotransporters.
  • Photosynthesis:
    • Occurs in leaf mesophyll cells within chloroplasts.
    • Light energy is used to fix CO₂ into triose phosphates.
    • These triose phosphates are exported to the cytosol, where they can be converted into glucose and fructose.
  • Sucrose Synthesis in the Cytosol:
    • Glucose and fructose are combined by the enzyme sucrose-phosphate synthase to form sucrose.
    • Sucrose accumulates in the mesophyll cell cytosol.
  • Movement of Sugars to the Phloem (Phloem Loading):
    • Once formed, sucrose must move from mesophyll cells to the phloem sieve tubes.

2. The Source-Sink Relationship

Source: Any region of a plant where assimilates are produced or mobilized into the phloem. Examples:

  • Photosynthetic leaves (mature leaves producing more sugars than they consume)
  • Storage organs under mobilization (e.g., a tuber or corm that is sprouting)

Sink: Any region of a plant where assimilates are required for growth, storage, or metabolic activity. Examples:

  • Developing fruits and seeds
  • Growing roots and shoot tips
  • Developing storage organs (e.g., a forming potato tuber)

Transport in the phloem generally moves from source to sink. For instance, fully expanded leaves (sources) export sucrose to roots or fruits (sinks).

3. Phloem Structure and Organization

The phloem is the vascular tissue responsible for translocating assimilates. Its key components are:

Sieve Tube Elements (STEs):

  • Elongated, tube-like cells arranged end-to-end.
  • Contain minimal organelles: no nucleus, few organelles, and a thin layer of cytoplasm to maximize space for sap flow.
  • Sieve Plates: Porous end walls that facilitate the flow of phloem sap between consecutive sieve tube elements.

Companion Cells (CCs):

  • Parenchyma cells closely associated with sieve tube elements.
  • Have a dense cytoplasm, a nucleus, numerous mitochondria, and are metabolically active.
  • Connected to sieve tube elements via plasmodesmata, providing metabolic support, assisting in the loading and unloading of sucrose.

Phloem Sap Composition:

  • Mainly sucrose (up to ~20% dry matter), along with amino acids, ions (K⁺, Cl⁻), and signaling molecules like hormones (auxins, cytokinins).

4. Mechanism of Phloem Transport: The Pressure-Flow / Mass-Flow Hypothesis

Phloem transport is explained by the pressure-flow AKA mass flow hypothesis:

  1. At the Source (Loading):
    • Sucrose is actively loaded into sieve tubes by companion cells.
    • This lowers the water potential inside the sieve tube, causing water to enter from the xylem by osmosis.
    • The influx of water increases the turgor (hydrostatic) pressure in the sieve tube at the source end.
  2. Along the Phloem:
    • The high pressure at the source pushes the phloem sap along the sieve tube toward the sink, where pressure is comparatively lower.
  3. At the Sink (Unloading):
    • Sucrose is actively or passively removed from the phloem at the sink.
    • Removal of sucrose raises the water potential within the sieve tube at the sink end.
    • Water moves out into the surrounding cells or back into the xylem, decreasing pressure at the sink.

The result is a bulk flow of phloem sap from regions of high pressure (sources) to regions of lower pressure (sinks) at approximately 1 meter per hour.

5. Sucrose Loading in the Phloem

Apoplastic Loading:

  • Involves the movement of sucrose from the cell walls (apoplast) into companion cells via a sucrose-H⁺ cotransporter.
  • A proton pump (H⁺-ATPase) creates a proton gradient, enabling sucrose to enter the companion cell against its concentration gradient.

Symplastic Loading:

  • Sucrose moves from cell to cell through plasmodesmata within the cytoplasm (symplast) without crossing cell membranes.
  • Common in some plant species.

Once inside the companion cell, sucrose diffuses into the sieve tube element through plasmodesmata.

Phloem Loading of Solutes

  • Primary Solutes: Sucrose is the most common carbohydrate transported, but amino acids also move through the phloem.
  • Active Loading:
    • Both sucrose and amino acids are often actively transported into companion cells and then into the sieve tube elements.
    • This process typically involves membrane transporters and may require energy (ATP) to move solutes against their concentration gradient.

Impact on Water Potential and Osmosis

  • Lowered Water Potential:
    • Loading solutes (sucrose, amino acids) into sieve elements reduces their water potential.
  • Water Influx:
    • Water moves into the sieve tube from the adjacent xylem by osmosis, following the lowered water potential.
  • Increased Turgor Pressure:
    • The influx of water raises the hydrostatic (turgor) pressure within the sieve tube at the source end.

Pressure-Flow Mechanism

Source to Sink Gradient:

  • High pressure at the source end (where solutes are loaded) pushes the phloem sap toward sinks (growing roots, fruits, storage organs) where solutes are unloaded, lowering pressure.

Similar Effect for All Solutes:

  • While sucrose is the main solute, amino acids and other assimilates contribute to the overall osmotic balance and help maintain the pressure gradient driving translocation.

6. Calculations and Practical Considerations

A) Estimating Sieve Tube Element Length

Scenario:

  • The image of a sieve tube element is magnified by a factor of ×200.
  • Under the microscope, the measured length of the element on the image is 1 cm.

Step-by-Step Calculation:

  1. Identify the magnification:
    Magnification (M) = ×200 means the observed length is 200 times larger than the actual length.
  2. Convert image length to actual length:
    Actual length = (Measured length on the image) ÷ (Magnification)
    = 1 cm ÷ 200
    = 0.005 cm
  3. Unit conversions:
    • 1 cm = 10 mm
    • Therefore, 0.005 cm = 0.005 × 10 mm = 0.05 mm
    • 1 mm = 1000 µm (micrometers)
    • Thus, 0.05 mm = 0.05 × 1000 µm = 50 µm

Result:

  • Actual length of the sieve tube element = 50 µm.
  • Biological Significance:
    • Measuring the length of sieve tube elements helps in understanding phloem structure and how it impacts the rate and efficiency of sap flow. Shorter or longer sieve tubes, along with the arrangement of sieve plates, can influence resistance to flow and ultimately affect the distribution of nutrients throughout the plant.

B) Number of Sieve Plates per Meter

Scenario:

  • A single sieve tube element is calculated to be 50 µm long.
  • We want to estimate how many sieve plate boundaries there would be in a 1-meter length of phloem.

Step-by-Step Calculation:

  1. Convert 1 meter to micrometers:
    1 m = 1000 mm
    1 mm = 1000 µm
    Therefore, 1 m = 1000 mm × 1000 µm/mm = 1,000,000 µm
  2. Determine how many 50 µm segments fit into 1 m:
    Number of sieve plates = (Total length in µm) ÷ (Length per element in µm)
    = (1,000,000 µm) ÷ (50 µm)
    = 20,000

Result:

  • There are approximately 20,000 sieve tube elements per meter, hence 20,000 sieve plates if each element is separated by a plate.
  • Biological Significance:
    • The number of sieve plates per unit length provides insights into the structural complexity of the phloem. More frequent sieve plates could mean more sites of potential resistance to flow, influencing the rate at which sugars and other assimilates travel. An understanding of this helps botanists and plant physiologists better understand the efficiency of translocation and how plants adapt to meet their metabolic demands.

7. Identifying Sources and Sinks

  • Nectary in a Flower: Sink (requires sugars for nectar production).
  • Developing Fruit: Sink (requires sugars and amino acids for growth and storage).
  • Potato Tuber with Sprouting Buds: Source (stored starch is converted to sucrose and mobilized to support shoot growth).
  • Forming Potato Tuber: Sink (accumulates and stores assimilates as starch).

8. Function of Sieve Plates

Sieve plates have pores that allow the phloem sap to move freely between sieve tube elements. They facilitate the continuous flow required for mass flow transport.

9. Key Terms

  • Mass Flow: Bulk movement of solutes in the phloem driven by hydrostatic pressure differences between source and sink regions.
  • Source: A plant organ or tissue that produces or releases sugars (e.g., mature leaves).
  • Sink: A plant organ or tissue that consumes or stores sugars (e.g., roots, developing fruits).
  • Sieve Tube Element: Specialized phloem cell that conducts sap.
  • Companion Cell: A specialized cell that maintains the metabolic functions and assists in loading/unloading assimilates in sieve tube elements.

Additional Useful Information

Phloem vs. Xylem:

  • Phloem: Transports primarily organic compounds (sugars, amino acids) and some signaling molecules. Operates under positive pressure.
  • Xylem: Transports water and minerals from roots to shoots, driven largely by transpiration and negative pressure.

Experimental Evidence for Phloem Transport:

  • Aphid Stylet Experiments: Aphids insert their stylets into the phloem; when the aphid’s body is removed, phloem sap continues to exude. By analysing the composition and pressure of this sap, scientists have confirmed the pressure-flow mechanism.

Practice Questions

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