A Wilting Tomato in Saline Irrigation Water
A grower irrigates tomato plants with water containing 60 mM NaCl. Control plants receive the same volume of fresh water. After two weeks, saline-treated plants are shorter, have smaller leaves, and wilt at midday even though soil moisture is adequate. Leaf analysis shows sodium accumulating in older leaves, chloride rising throughout, and potassium lower than in controls. Stomatal conductance is reduced by about 40%. Root pressure measured at night is lower in the saline group. The grower's first instinct is that the plants are simply short of water and proposes increasing irrigation volume with the same saline source.
1. Explain the mechanism by which saline water causes midday wilting despite wet soil, and evaluate the grower's proposed fix.
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The wilting is osmotic, not a shortage of water in the soil. Dissolved salt lowers the water potential of the soil solution, so the water potential gradient between soil and root is reduced and the plant must generate a more negative root water potential to take up the same water. Midday transpirational demand then exceeds what the roots can supply, so turgor falls and leaves droop even though the soil is wet. Increasing the volume of the same saline water adds more salt to the root zone, lowering soil water potential further and worsening the problem; it may also raise the water table and cause salt accumulation at the surface as it evaporates. The correct response is to leach accumulated salt with low-salinity water, improve drainage, and reduce evaporative demand.
2. The saline plants have lower leaf potassium. Why does this matter, and how does it connect to the reduced stomatal conductance?
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Sodium and potassium are chemically similar cations, so high external Na+ competes with K+ uptake and can displace K+ from binding sites. Potassium is the main cation driving guard cell turgor: K+ influx into guard cells draws in water and opens stomata. If K+ status is poor, the plant cannot regulate guard cell turgor normally. The observed 40% drop in stomatal conductance is partly a deliberate response (closing stomata limits transpiration and conserves water under osmotic stress, mediated by abscisic acid) and partly a consequence of ion imbalance. The net effect is reduced CO2 uptake and therefore lower photosynthesis, which explains the stunted growth and smaller leaves better than water shortage alone.
3. Night-time root pressure is lower in the saline group. What does this tell you, and what would you predict for guttation?
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Root pressure is generated by active ion transport into the xylem, which pulls water in osmotically. Lower root pressure suggests the roots are less able to accumulate ions in the xylem, consistent with Na+ interference and possibly membrane damage or energy cost of ion exclusion. Prediction: guttation (dew-like droplets at leaf margins, driven by root pressure) should be reduced or absent in the saline plants. This is a useful non-destructive check the grower can make at dawn. It also confirms the problem is at the root/uptake level, not only in the leaves.