Lesson 9 of 10 / Nutrition and transport in plants
When a plant wilts
Why can a plant droop even while its roots are taking up water?
In this lesson: Explain wilting by comparing water loss, water uptake and cell turgor.
About 5 min
The key ideaIf water loss exceeds uptake, cells lose water and turgor. Soft tissues become flaccid and the plant wilts.
Loss 7 > uptake 4: a deficit of 3 arbitrary units per hour. Cells lose water and turgor, so soft tissues can wilt.
Blue: vacuole. Olive outer line: cell wall. The selected state contrasts turgor qualitatively, without implying that ordinary wilting requires plasmolysis. The illustrative rates do not predict an exact cell size or recovery time.
Stomatal closure reduces further water loss but restricts CO2 entry. Recovery is possible if tissues remain undamaged; severe stress may cause lasting harm.
Explanation
Water balance depends on both uptake and loss. A plant can still absorb water while losing it faster through transpiration. When loss exceeds replacement, the water content of tissues decreases. Hot, dry or windy conditions can increase this imbalance, especially when soil water is limited.
As living cells lose water, their vacuoles become smaller and the pressure of the cell contents against the cell wall decreases. This pressure is turgor. Reduced turgor makes soft tissues less firm, so leaves and young stems may droop.
Stomata may close during water stress, reducing further water loss but also restricting carbon dioxide entry for photosynthesis. This is a trade-off between conserving water and obtaining carbon dioxide. The response does not mean the plant no longer respires.
If water uptake later exceeds loss, cells may regain water by osmosis, recover turgor and become firm again. Recovery depends on the severity and duration of stress; severe damage may not be reversible. Wilting describes loss of tissue firmness, not automatic death of the entire plant.
Step by step
- 1
Compare the two rates
Do not infer water balance from uptake alone.
- 2
Explain the cell change
Connect water loss to reduced vacuole volume and pressure against the wall.
- 3
Connect cell to whole plant
Lower turgor reduces support in soft tissues, producing wilting.
Worked example
Uptake continues
An illustrative plant takes up 4 units of water per hour but loses 7. Explain why it can wilt despite continued uptake.
One way to explain it
It loses 3 more units per hour than it replaces, so tissues have a net water deficit. Cells lose water and turgor, making soft tissues flaccid and causing drooping.
Why this answer works
- Compare both rates: 7 is greater than 4.
- Link the deficit to cellular water loss.
- Explain turgor rather than saying only "the plant is dry".
Is this true? "A plant cannot wilt unless water uptake falls to zero."
Wilting can occur whenever water loss exceeds uptake sufficiently. Both processes can still be occurring.