Transport in Plants: Question 9
Syllabus 8.3
On a hot, dry, windy afternoon, a gardener notices that the leaves and stem of an unwatered potted plant have become soft and have drooped downward, even though the same plant looked upright and healthy that morning.
Which statement best explains why the plant has wilted?
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Worked solution
Step 1: Identify the two rates involved
A plant’s leaves and stem stay firm when the rate at which the plant absorbs water through its roots keeps up with the rate at which it loses water by transpiration. Wilting happens when this balance is upset.
Step 2: Apply this to the conditions described
High temperature, low humidity and wind are all conditions that increase the rate of transpiration. At the same time, the soil is described as unwatered, limiting how much replacement water the roots can absorb. With water being lost faster than it is being replaced, the leaves and stem lose their normal firmness and droop, which is wilting.
Step 3: Why the other options are wrong
- B reverses the relationship: faster uptake than loss would not cause drooping, and nothing in the scenario suggests uptake has increased faster than transpiration.
- C invents a xylem blockage that is not supported by the scenario; the explanation lies in the balance between water loss and uptake, not a physical blockage.
- D ignores the role of water balance entirely, attributing wilting directly to heat damage rather than to transpiration outpacing uptake.
Final answer
The plant has wilted because the hot, dry, windy conditions increased its rate of transpiration beyond the rate at which its roots could absorb replacement water, option A.