Water Potential Arithmetic in a Wilting Pot
At 8 a.m. a potted tomato plant is upright. The student measures water potential and samples expressed sap: soil water potential is −0.45 MPa; root cell solute potential is −0.60 MPa and pressure potential is +0.25 MPa; leaf cell solute potential is −1.10 MPa and pressure potential is +0.30 MPa. At noon the same leaf reads a water potential of −1.40 MPa. In the evening the student adds a soluble fertiliser; the soil solution now has a water potential of −0.90 MPa. Next morning the plant is wilted. The student concludes that the fertiliser burned the roots and that water moves from a region of lower solute potential to one of higher solute potential. The teacher asks for a water-potential explanation. After watering, soil water potential returns to −0.05 MPa and the plant recovers within two hours.
1. Calculate root water potential and leaf water potential, and arrange soil, root and leaf in order of water potential to show the direction of water movement.
Read the worked answer
Water potential is the sum of solute potential and pressure potential.
Root: −0.60 + 0.25 = −0.35 MPa. Leaf: −1.10 + 0.30 = −0.80 MPa.
In order from highest to lowest water potential: root −0.35 MPa, soil −0.45 MPa, leaf −0.80 MPa. Water moves from higher water potential to lower water potential. Because the root is higher than the soil, water enters the root from the soil. Because the leaf is lower than the root, water moves from the root to the leaf. The same arithmetic shows that the leaf is the lowest point in this chain, which is consistent with transpiration drawing water upward.
2. Explain the wilting after fertiliser and the recovery after watering using water-potential values. Why is burning not the main explanation?
Read the worked answer
After fertiliser, soil water potential falls to −0.90 MPa. The root is at −0.35 MPa, which is now higher than the soil. Water therefore moves out of the root into the soil solution, not into the plant. Root cells lose water, turgor falls, and the shoot wilts. The direction of movement is set by the water potential difference, not by the fertiliser being poisonous.
After watering, soil water potential rises to −0.05 MPa. The root at −0.35 MPa is now lower than the soil, so water enters the root again and turgor is restored. Recovery within two hours supports an osmotic explanation.
Burning implies direct toxicity or death of root tissue. That can happen at very high fertiliser concentration, but the rapid, reversible wilt before watering is explained by the water potential gradient. If the soil were flushed only to −0.20 MPa, the root at −0.35 MPa would still be lower than the soil, so water would enter and wilting would not be expected.
3. The student claims that water moves from lower solute potential to higher solute potential. Correct this statement using the data, and explain why the leaf can have a lower water potential than the root.
Read the worked answer
The claim is incorrect. Water movement is governed by water potential, which includes both solute potential and pressure potential. Water moves from higher water potential to lower water potential. Solute potential is only one component, and a pressure potential can offset it.
The leaf has a lower water potential than the root because its solute potential is much more negative, −1.10 MPa compared with −0.60 MPa, and its pressure potential does not fully offset that difference. The leaf water potential is −0.80 MPa, while the root is −0.35 MPa. Transpiration from the leaf concentrates solutes and lowers the leaf water potential, maintaining the gradient that pulls water from the root. The root, in turn, has a higher water potential than the soil after fertiliser is added, which is why water leaves the root and the plant wilts.