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Difficulty: HardTransport Mechanisms in Plants

A botanical research team performed a ringing (girdling) experiment on a woody dicotyledonous plant by removing a complete ring of bark down to the vascular cambium layer near the base of the main stem. After several weeks, a distinct swelling was observed immediately above the ring, while the roots gradually died due to sugar starvation. Which transport process and direction were disrupted to cause this specific outcome?

  1. Downward translocation of organic solutes via phloem tissueAnswer
  2. B
    Upward translocation of manufactured sugars via xylem vessels
  3. C
    Downward movement of mineral salts and water via xylem vessels
  4. D
    Bidirectional flow of inorganic nutrients via sieve tube elements

Answer

Downward translocation of organic solutes via phloem tissue
Ringing removes phloem, which is responsible for translocating organic food solutes manufactured during photosynthesis downward from leaves to roots. Blocking this path leads to solute accumulation above the ring, causing swelling, and starves roots of carbohydrates.

Step-by-Step Solution

1
Analyze the structural effect of ringing (girdling) a woody stem down to the cambium
Removing the outer layers (bark, cortex, and phloem) removes the phloem tissue while leaving the deeper xylem intact.
Phloem is located towards the periphery of vascular bundles in dicot stems.
2
Identify the primary substance and direction of transport affected by phloem removal
Organic products of photosynthesis (sucrose) synthesized in leaves are transported downward to root sinks via phloem.
Roots act as non-photosynthetic sink organs reliant on shoot sources for metabolic energy.
3
Evaluate the physiological consequences observed in the stem and roots
Accumulated sucrose and auxins above the girdle cause cell proliferation and swelling; root starvation occurs due to lack of carbohydrate supply.
Phloem transport blockade prevents organic food from crossing the ringed zone.

Key Concept

Phloem Translocation and Girdling Experiments
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Topics

Transport Mechanisms in Plants

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