The Salted Slug and the Lettuce Bed
A gardener notices that lettuce seedlings along the edge of a path are wilting badly, while seedlings 2 m further into the bed look healthy. The path was gritted with salt the previous winter, and soil tests show the edge soil has a much higher solute concentration than the inner soil. The gardener waters the edge seedlings heavily with fresh water, but they wilt further before recovering two days later. A second observation: the same edge seedlings have smaller, darker green leaves than the inner seedlings. The gardener wonders whether the salt is poisoning the plants or affecting water uptake, and whether the heavy watering helped or harmed them in the short term.
1. Explain, using cell-level reasoning, why the edge seedlings wilted and why heavy watering initially made the wilting worse before recovery.
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High soil solute concentration lowers the water potential of the soil solution relative to the root hair cell sap. Water moves by osmosis from higher to lower water potential, so the gradient now favours water leaving root hair cells, or at least reduces net uptake. Root hair cells lose turgor, the tissue becomes flaccid, and the plant wilts. The heavy watering dilutes the soil solution only slowly because the salt is bound in the soil and the water drains away; in the short term the added water may briefly create a very dilute surface layer with the concentrated layer below, and the plant continues to lose water. Recovery occurs once leaching has actually lowered the solute concentration around the roots, restoring a water potential gradient into the root hairs, so turgor returns. The key judgement is that salt is not directly toxic here; it acts on water potential and osmosis.
2. The smaller, darker green leaves on the edge seedlings are also explained by the same salt effect. Justify this and state one piece of evidence you would collect to confirm it.
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Reduced water uptake limits cell expansion, since turgor pressure is the driving force for cell elongation in growing leaves. Cells that cannot reach full turgor stay smaller, so leaves are smaller overall. Darker green colour is consistent with reduced leaf expansion: the same amount of chlorophyll is packed into a smaller leaf area, raising chlorophyll per unit area. It is also consistent with mild water stress causing slower growth rather than chlorosis. Evidence to collect: measure leaf area and chlorophyll per unit area (or simply leaf mass per area) on edge versus inner seedlings, and measure soil solute concentration or water potential at both positions. If smaller leaves correlate with lower soil water potential and higher chlorophyll per unit area, the osmotic explanation is supported rather than a nutrient deficiency or direct ion toxicity.