8.1 Transport in Plants: Wilting

This section explains the process of wilting in plants, focusing on the mechanisms of water transport and the factors that lead to water loss.

What is Wilting?

Wilting is the drooping or loss of rigidity in non-woody parts of a plant, such as leaves and stems. It occurs when the plant loses excessive amounts of water through transpiration, and the water potential in the plant tissues becomes negative.

How Water is Transported in Plants

Water transport in plants is a crucial process facilitated by several mechanisms:

• Transpiration: The evaporation of water from the aerial parts of the plant, mainly through stomata in the leaves.
• Xylem: A vascular tissue that transports water and minerals upwards from the roots to the rest of the plant. Water movement in the xylem is driven by transpiration pull.
• Cohesion-Tension Theory: This theory explains how water moves up the xylem. Water molecules are cohesive (they stick to each other) and adhesive (they stick to the xylem walls). Transpiration creates a negative water potential in the leaves, pulling water up the xylem due to cohesion.
• Root Pressure: The positive pressure that develops in the xylem of roots due to the continuous uptake of water by the root cells. Root pressure helps to push water upwards, especially when transpiration is low.

Why Wilting Occurs

Wilting happens when the rate of water loss through transpiration exceeds the rate of water uptake by the roots. This imbalance leads to a decrease in the water potential within the plant tissues.

Here's a breakdown of the key factors contributing to wilting:

• High Transpiration Rate: Factors like high temperature, low humidity, high wind, and bright sunlight increase the rate of transpiration.
• Low Water Uptake by Roots: If the soil is dry, the roots may not be able to absorb water quickly enough to replace what is lost through transpiration.
• Stomatal Closure: In some cases, the plant may close its stomata to reduce water loss. While this reduces transpiration, it also limits carbon dioxide uptake for photosynthesis, which can negatively impact the plant's overall health.

The Process of Wilting

1. Water Loss: Transpiration causes water to evaporate from the leaf surfaces.
2. Reduced Water Potential: The loss of water reduces the water potential in the leaf cells.
3. Cell Flaccidity: As the water potential decreases, the plant cells lose turgor pressure (the pressure of the cell contents against the cell wall).
4. Leaf Drooping: The loss of turgor pressure causes the cells to become flaccid, leading to the drooping of leaves and stems.

Factors Affecting Wilting

Factor	Effect on Wilting
Temperature	Higher temperature increases transpiration rate, leading to more wilting.
Humidity	Lower humidity increases transpiration rate, leading to more wilting.
Wind	Higher wind speed increases transpiration rate, leading to more wilting.
Light Intensity	Higher light intensity increases transpiration rate, leading to more wilting.
Soil Water Availability	Low soil water availability reduces water uptake by roots, increasing wilting.

Preventing Wilting

To prevent wilting, it's important to ensure that plants have access to sufficient water. This can be achieved by:

• Regular Watering: Providing adequate water to the soil.
• Mulching: Applying a layer of organic material around the base of plants to reduce water evaporation from the soil.
• Providing Shade: Protecting plants from excessive sunlight and heat.