Explain how and why wilting occurs.
8.1 Transport in Plants – Wilting Explained
What is Wilting?
Wilting is the temporary loss of turgidity (fullness) in plant tissues, usually caused by a drop in water content. Think of a fresh‑cut flower that starts to droop when it runs out of water. The plant’s cells lose pressure, the stem bends, and leaves may curl. 🌱
Why Does Wilting Happen?
Wilting occurs when the plant cannot replace the water it loses through transpiration. The main reasons are:
- High temperature or low humidity increases transpiration rate.
- Root damage or drought limits water uptake.
- Excessive evaporation from the soil surface.
- Stomata stay open for too long, letting water escape faster than it can be absorbed.
The Water Transport System
Plants move water from the roots to the leaves through a continuous column of water in the xylem. The driving force is a difference in water potential, described by the equation:
$$\Psi_{\text{total}} = \Psi_{\text{solute}} + \Psi_{\text{pressure}}$$
Where:
- $\Psi_{\text{solute}}$ (solute potential) is negative because solutes lower the water potential.
- $\Psi_{\text{pressure}}$ (pressure potential) is positive inside cells due to turgor pressure.
Analogy: Water as Traffic
Imagine the plant as a city:
- Roots are the water reservoirs that fill the city.
- Stomata are the traffic gates that let water vapor leave.
- Leaves are the highways where water moves from the reservoir to the city’s outskirts.
- When the reservoir runs low or the gates stay open too long, traffic jams (water loss) happen and the city (plant) starts to look tired and droopy.
Key Factors in Wilting
| Factor | Effect on Water Potential |
|---|---|
| High Temperature | Increases evaporation → lowers $\Psi_{\text{solute}}$ in leaves. |
| Low Humidity | Accelerates transpiration → reduces $\Psi_{\text{pressure}}$. |
| Root Damage | Reduces water uptake → overall $\Psi_{\text{total}}$ drops. |
| Closed Stomata | Can prevent water loss but also stops CO₂ uptake, affecting photosynthesis. |
Preventing Wilting – Practical Tips
- Water the plant early in the morning so it can absorb water before the heat of the day.
- Mulch the soil to keep moisture in and reduce evaporation.
- Use a shade cloth or move the plant to a cooler spot during extreme heat.
- Check root health; transplant if roots are crowded or damaged.
- Observe the plant’s stomata behaviour: if it stays open during a hot day, it’s at risk of wilting.
Quick Quiz
1️⃣ What is the main driving force for water movement from roots to leaves? 2️⃣ How does high temperature affect the plant’s water potential? 3️⃣ Name one practical way to reduce wilting during a heatwave.
Answers: 1) Difference in water potential ($\Psi_{\text{total}}$). 2) Lowers $\Psi_{\text{solute}}$ and $\Psi_{\text{pressure}}$, increasing water loss. 3) Mulching or early watering. 🌞💧
Revision
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