A mature dicotyledonous tree undergoes a complete girdling (ringing) experiment in which a continuous ring of outer bark, cortex, and phloem tissue down to the vascular cambium is removed around the lower trunk. Several days after the treatment, carbohydrate accumulation is observed above the ringed area, while root cellular respiration decreases and roots eventually begin to die before the leaves show signs of wilting. Which of the following best explains why root decay precedes foliage wilting in this experiment?
- Organic nutrients manufactured in the leaves are translocated downwards via the phloem to sustain root metabolic activities, whereas xylem vessels located inner to the cambium remain intact to continue upward water transport.Answer
- BWater and inorganic ions are transported unidirectionally downwards through the phloem to the roots, which halts immediately when the outer bark is severed.
- CGirdling breaks the continuous transpiration pull and cohesion-tension column within phloem sieve tubes required to draw water from the soil.
- DPhloem vessels transport mineral salts upward to the leaves while xylem translocates manufactured carbohydrates bidirectionally to the root system.
Answer
Organic nutrients manufactured in the leaves are translocated downwards via the phloem to sustain root metabolic activities, whereas xylem vessels located inner to the cambium remain intact to continue upward water transport.
In dicotyledonous stems, phloem is situated peripheral to the vascular cambium while xylem lies deeper toward the center. Girdling interrupts the downward translocation of photosynthetic products (sucrose) in the phloem, depriving root tissues of essential respiratory substrates. Because xylem remains intact inner to the cambium, water and dissolved mineral transport up to the leaves continues temporarily, allowing leaves to stay turgid until root cellular collapse eventually stops water uptake.
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Key Concept
Differential Vascular Function in Girdling Experiments