The Waterlogged Polyhouse: Wilting in Saturated Soil
In a polyhouse near Bhubaneswar, tomato plants of a hybrid variety are irrigated daily. After 72 hours of continuous rain the field stands under 5 cm of water and the soil temperature is 26 °C. On the fourth morning, plants in the lowest row are wilted at 11:00 even though the soil is saturated. A porometer records stomatal conductance falling from 320 to 45 mmol m⁻² s⁻¹, and leaf water potential is −1.9 MPa against −0.7 MPa in well-drained beds. Root segments from the flooded row, stained with triphenyl tetrazolium chloride, show almost no red formazan compared with control roots. In a well-drained plot nearby, droplets appear at the leaf margins at 06:00 and vanish by 09:00. A potometer attached to a decapitated, well-watered shoot shows only a slow rise of sap; when the same shoot is placed in bright sun the rise becomes rapid.
1. Resolve the paradox: why do these plants wilt when the soil is full of water? Use the TTC staining, ion transport and water potential in your answer.
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TTC is reduced to red formazan only by dehydrogenase enzymes of living, respiring cells, so the near-absent staining is direct evidence that most roots in the flooded row are dead or severely hypoxic. Waterlogging fills the soil pores with water, and oxygen diffuses roughly ten thousand times more slowly in water than in air, so root respiration shifts towards anaerobic fermentation, ATP yield per glucose collapses, and the ATP-driven proton pumps that load ions into the xylem stop working. Without ion secretion the root cannot build the osmotic gradient that keeps its water potential below that of the soil. In a saturated soil the soil water potential is close to 0 MPa, so the driving force for osmotic entry of water is almost gone, and damaged roots also lose hydraulic conductance through aquaporins and through suberised or rotting tissue. Hence the paradox is only apparent: the plant lacks water because the roots cannot absorb it, not because the soil lacks it. The fall in stomatal conductance is a secondary ABA response from the stressed roots and does not restore uptake.
2. Explain the droplets seen at 06:00 in the well-drained plot, why they disappear by 09:00, and what the two potometer readings show about root pressure versus transpiration pull.
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The droplets are guttation: liquid water forced out through hydathodes at the leaf margins and tips. At night the stomata are closed and the air is humid, so transpiration is almost nil, yet the roots keep secreting ions into the xylem. That osmotic gradient draws water in and generates a positive hydrostatic pressure, root pressure, which pushes liquid water out at the weakest points of the leaf. By 09:00 the stomata open and transpiration begins, putting the xylem under tension, so the positive pressure is relieved and the droplets evaporate. The potometer separates the two forces by scale. On the decapitated shoot only root pressure can move the bubble, and it moves slowly; in bright sun the same shoot is driven by transpiration pull and the bubble moves rapidly. The judgement to draw is that root pressure is real but small: it cannot raise water more than a metre or two, cannot supply a transpiring plant, and is a special night-time phenomenon that demonstrates active ion pumping by roots rather than being the main engine of ascent of sap.
3. The farmer proposes draining the field at 14:00 on day 4 and expecting the crop to recover by evening. Predict the actual recovery course and name two risks of draining at that time.
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Recovery will not be same-day. Draining restores air to the pore space, but the existing roots are largely dead, so water uptake stays low until new rootlets and root hairs form, usually four to seven days. Stomata stay partly closed until xylem ABA is metabolised and roots resume ion pumping, so leaf water potential should improve over 24–48 hours without reaching the −0.7 MPa of the well-drained beds at once. Two risks follow from draining at 14:00. First, the shoot is suddenly exposed to high evaporative demand while uptake capacity is still poor, so leaves may wilt more severely during that first afternoon; light shading or a small post-drainage irrigation reduces this. Second, warm, oxygenated, previously waterlogged soil favours oomycete root rots such as Pythium and Phytophthora, which invade already damaged roots, and nitrate may have leached. The practical judgement is to drain, keep the root zone moist but never saturated, watch stomatal conductance and leaf water potential rather than soil moisture alone, and treat for root rot if disease pressure is high.