Explain how and why wilting occurs.

8.1 Transport in Plants – How and Why Wilting Occurs

1. Main Vascular Tissues

• Xylem – dead, hollow vessels and tracheids that conduct water and dissolved mineral ions from the roots to the shoot.
• Phloem – living sieve‑tube elements and companion cells that transport organic nutrients (mainly sucrose, also amino acids) from **source** tissues (e.g., mature leaves) to **sink** tissues (growing roots, young leaves, fruits, storage organs).

2. Position of Xylem and Phloem in Roots, Stems and Leaves

These arrangements are required for the “identify in diagrams” assessment objective.

• Root – central stele: xylem in the centre, phloem surrounding it, both encased by the pericycle and cortex.
• Stem (dicot) – vascular cylinder: xylem on the inner side, phloem on the outer side, with a thin layer of cambium between them (site of secondary growth).
• Leaf (dicot) – vein cross‑section: xylem on the upper (adaxial) side, phloem on the lower (abaxial) side, both protected by bundle‑sheath cells.

3. Water Uptake by Roots

• Root hairs – extensions of epidermal cells that increase the absorptive surface area up to 10 000 × that of a bare root; they lie in the cortex and are the main entry point for water and mineral ions.
• Pathway of water (root → stem → leaf)
  1. Water enters root hairs (apoplast).
  2. Moves through the cortex either apoplastically (between cells) or symplastically (via plasmodesmata).
  3. Crosses the Casparian strip of the endodermis – forced to pass a plasma membrane, allowing selective uptake of ions.
  4. Enters the xylem vessels of the central stele.
  5. Travels upward through the stem’s xylem.
  6. Leaves the xylem into leaf veins and then into the mesophyll where transpiration occurs.

4. Forces Driving the Ascent of Water (Cohesion‑Tension Theory)

Force	Role in Water Movement
Root pressure	Positive pressure generated by active uptake of ions; pushes water up the xylem, noticeable in many herbaceous plants at night.
Cohesion	Hydrogen‑bond attraction between water molecules, creating a continuous column.
Adhesion	Attraction of water molecules to the hydrophilic walls of xylem vessels, preventing the column from breaking.
Tension (negative pressure)	Created by evaporation of water from stomata; pulls the water column upward.

5. Transpiration

• Definition – loss of water vapour from aerial parts of the plant, mainly through stomata.
• Leaf adaptations that increase transpiration
  • Large, thin leaf lamina providing a high surface‑area‑to‑volume ratio.
  • Extensive intercellular air spaces that facilitate vapour diffusion.
  • Stomata surrounded by guard cells that open in response to light, low internal CO₂ and adequate water status.
• Environmental factors that raise the transpiration rate
  • High temperature
  • Low relative humidity
  • Wind or moving air
  • High light intensity

6. What Is Wilting?

Wilting is the visible loss of turgor pressure in plant cells, causing leaves and stems to become limp.

Cellular turgor pressure (P) can be approximated by the osmotic equation:

\[ P = \frac{RT}{V}\,\ln\!\left(\frac{C_{\text{inside}}}{C_{\text{outside}}}\right) \] where \(R\) = gas constant, \(T\) = absolute temperature, \(V\) = cell volume, and \(C\) = solute concentration. When water leaves the cell, \(V\) falls, the logarithmic term decreases, and \(P\) drops → the plasma membrane pulls away from the cell wall (plasmolysis) and the organ wilts.

7. Causes of Wilting (Water Loss > Water Uptake)

Cause	Mechanism	Resulting Effect
Insufficient soil moisture	Soil water potential becomes less negative than that of root cells, reducing the gradient for uptake.	Root uptake slows or stops → turgor falls.
High transpiration rate	Hot, dry, windy conditions increase evaporation from stomata.	Water loss exceeds uptake → more negative xylem tension.
Root damage or disease	Physical injury or pathogen blockage prevents water entry.	Uptake is limited even if soil water is adequate.
Salt (osmotic) stress	High solute concentration in the soil lowers its water potential.	Water moves out of root cells → dehydration.

8. Sequence of Events Leading to Wilting

1. Soil water potential becomes less negative than the water potential inside root cells.
2. Root uptake declines; the continuous water column in the xylem becomes thinner.
3. Transpiration continues, generating a stronger tension gradient that pulls water upward.
4. Leaf mesophyll cells lose water; the cytoplasm contracts and the plasma membrane detaches from the cell wall (plasmolysis).
5. Turgor pressure falls below the level needed to keep cells rigid → leaves and stems become limp (wilting).

9. Reversibility of Wilting

• Recoverable wilting – if water is supplied before permanent damage, cells re‑absorb water, turgor is restored and normal growth resumes.
• Irreversible wilting – prolonged water deficit damages membranes, denatures proteins and can cause cell death, leading to necrosis of the affected tissue.

10. Practical Tips for Preventing Wilting (Cultivation)

• Maintain consistent soil moisture; water early in the day to reduce evaporative loss.
• Apply a mulch layer to minimise soil evaporation.
• Provide shade or windbreaks during periods of extreme heat or wind.
• Use balanced fertilisers and avoid excess salts that lower soil water potential.
• Prevent soil compaction to keep root hairs and the cortex well‑aerated.

11. Quick Revision Summary

Key Point	Why It Matters for the Syllabus
Root hairs increase surface area	Maximise water uptake – essential for maintaining a water potential gradient.
Cohesion‑tension theory	Explains upward water movement without a mechanical pump.
Stomatal control	Balances CO₂ intake for photosynthesis with water loss.
Wilting = loss of turgor	Visible symptom of a negative water balance; a core exam topic.
Source → sink transport in phloem	Shows the direction of organic nutrient movement, a required point in 8.1.
Re‑watering restores turgor (if not prolonged)	Demonstrates the reversibility of wilting and the importance of timely irrigation.