The Wilting Tomato and the Salty Soil
A farmer irrigates tomato plants with water containing a high concentration of dissolved salts. The plants wilt during the day even though the soil is visibly wet. Leaf margins show browning, and root tips appear shriveled. A soil test shows high solute concentration in the soil solution. The farmer's neighbor suggests adding more fertilizer to 'give the plants nutrients so they can absorb more water.' The farmer also notices that the wilting is worse on hot, windy days and that stomata on the leaves are partially closed during midday. The plants are in a field with no shade.
1. Explain, in terms of water potential, why the plants wilt despite wet soil.
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Water moves from higher water potential to lower water potential. In normal soil, soil water potential is higher (less negative) than root cell water potential, so water enters the roots. With high dissolved salts, the solute potential of the soil solution becomes very negative, lowering the total water potential of the soil. If soil water potential falls below that of root cells, the osmotic gradient reverses and water leaves the roots rather than entering. The plant cannot take up water even though the soil is wet; this is physiological drought. The shriveled root tips and wilting are consistent with water loss from root and leaf cells, and the browning leaf margins reflect ion toxicity and dehydration.
2. Evaluate the neighbor's advice to add more fertilizer.
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Adding more fertilizer would worsen the problem. Fertilizer adds dissolved solutes, further lowering the solute potential of the soil solution and making soil water potential even more negative. That increases the osmotic gradient pulling water out of the roots. The plants are already suffering from high solute concentration; adding more solutes compounds the osmotic stress and can cause further ion toxicity. The correct response is to leach the soil with low-salt water, improve drainage, and reduce irrigation with saline water, not to add fertilizer.
3. Why is wilting worse on hot, windy days, and how does midday stomatal closure interact with the problem?
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Hot, windy conditions increase the vapor pressure deficit between the leaf interior and the air, raising transpiration rate. Higher transpiration pulls more water through the plant, but because root water uptake is blocked by low soil water potential, the water column cannot be replenished, so leaves lose turgor faster. Midday stomatal closure is a protective response that reduces transpiration, but it also reduces CO2 uptake, limiting photosynthesis. The closure helps conserve water but does not solve the underlying problem that water cannot be absorbed from the saline soil. Together, high evaporative demand and blocked uptake make midday wilting most severe.