Midday Wilting in a Wet, Salty Tomato Root Zone
At a greenhouse near Kadapa, 35-day-old tomato plants wilt from 11:00 to 15:00 even though drip irrigation keeps the root zone near field capacity. A pressure chamber reads leaf water potential of −1.75 MPa at 13:00 and −0.55 MPa at 05:00. Stomatal conductance falls from 320 to 90 mmol m−2 s−1 over the same interval. Root-zone electrical conductivity is 4.6 dS m−1, air temperature is 36 °C, and vapor pressure deficit reaches 3.8 kPa. Leaves show no marginal necrosis, and roots are white and firm, not rotted. The grower must decide whether to apply more water, leach the root zone, or shade the crop. A colleague says the plants have lost turgor because they are not absorbing water, but the soil is wet. The next irrigation is scheduled in 6 hours.
1. Interpret the leaf water potential and stomatal conductance data. Is the wilting primarily a soil-water deficit, an osmotic limitation, or a hydraulic/atmospheric demand problem? Justify with the numbers.
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It is not primarily a soil-water deficit, because the root zone is near field capacity. The predawn value of −0.55 MPa shows that the plants are not fully rehydrated overnight; a well-watered tomato crop would normally be closer to −0.2 to −0.4 MPa. The root-zone EC of 4.6 dS m−1 adds an osmotic component, making water uptake harder even when soil moisture is high. At midday, leaf water potential falls to −1.75 MPa while VPD reaches 3.8 kPa and stomatal conductance drops from 320 to 90 mmol m−2 s−1. That pattern indicates high atmospheric demand and stomatal closure, with limited water supply to the leaf. The primary problem is hydraulic/atmospheric demand combined with an osmotic limitation, not lack of soil water. Applying more water would not correct the osmotic component and could worsen root-zone conditions.
2. Explain why a bagged-leaf measurement before dawn would help distinguish osmotic adjustment from root-zone salinity.
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Bagging a leaf before dawn stops transpiration, so the leaf water potential equilibrates with the stem xylem water potential. At that time, the plant is closest to equilibrium with the root zone. If the bagged predawn value is more negative than the soil matric potential measured with a tensiometer, the difference indicates an osmotic component, consistent with the high EC. If the bagged value matches the soil matric potential, salinity is not the main cause. Measuring the osmotic potential of expressed sap would confirm whether solutes are lowering water potential inside the plant. The comparison must be made before dawn because daytime transpiration creates water-potential gradients that obscure the root-zone signal. Bagging alone does not identify the solute, but it separates water-status effects from atmospheric demand.
3. Recommend a 48-hour action plan and state one risk of each option.
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Do not apply more water immediately. First, leach the root zone with enough water to move salts below the root zone, using about 20–30% more water than the root-zone volume and checking that drainage is adequate. Risk: leaching can remove nutrients and, if drainage is poor, create waterlogging and root hypoxia. Second, install 30–40% shade cloth or run fogging/misting during the midday peak to reduce VPD. Risk: shade reduces photosynthesis and can increase humidity-related disease if kept on too long. Third, recheck EC, predawn leaf water potential and soil moisture after 24 hours before repeating any treatment. If EC remains high, repeat leaching in smaller amounts rather than adding more water without drainage. The aim is to lower the osmotic limitation while reducing atmospheric demand, not to increase soil water content.