Phloem Transport in a Girdled Tree
A 12-year-old eucalyptus has a band of bark and cambium removed around its entire trunk, 1.5 m above the ground. The tree is otherwise healthy and well watered. Over eight weeks, leaves remain green and continue photosynthesising, but the bark above the girdle swells and a sugary exudate leaks from the upper cut edge. Roots begin to decline by week six, and leaf growth slows. A gardener concludes that the girdle blocked water transport to the leaves, and that the swelling is caused by water accumulating above the cut.
1. Use transport-tissue structure and function to evaluate the gardener's conclusion and explain each observation.
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Evaluating the conclusion. The gardener is wrong. Water travels in xylem, which lies inside the cambium and was not removed, so water supply to the leaves is intact - consistent with leaves staying green and photosynthesising for eight weeks. If water transport had been blocked, wilting would have appeared within days, not weeks.
Explaining the observations.
- Swelling and sugary exudate above the girdle: phloem sieve tubes were severed. Sucrose loaded in leaves continues to be pushed down by pressure flow and accumulates above the cut, raising solute concentration, drawing in water by osmosis, and causing the swelling and leakage.
- Root decline by week six: roots depend on phloem-delivered sucrose for respiration. Cut off from the supply, root respiration fails and roots die back - the expected consequence of phloem disruption, not xylem disruption.
- Slowed leaf growth: reduced root function limits mineral uptake and cytokinin supply, and accumulating sugars can feedback-inhibit photosynthesis.
Conclusion. The evidence points to phloem transport being blocked while xylem transport continues.
2. Predict what would be observed if the same girdle were made but the tree were kept in darkness. Justify your prediction.
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Prediction. Little or no swelling and no sugary exudate would form above the girdle, and the tree would decline faster overall.
Justification. Pressure flow requires sugar loading at the source. In darkness there is no photosynthesis, so no sucrose is loaded into phloem at the leaves and no pressure gradient develops. Without a sugar flux, nothing accumulates above the cut, so the characteristic swelling and leakage do not occur. The tree would instead survive on stored starch until that is exhausted, then show decline in both roots and shoots. This is a useful test of the mechanism: the girdle symptom depends on an active source, not merely on a severed tissue.