Explaining the Effects of Temperature, Wind Speed, Humidity & Light Intensity (Cambridge O Level Biology)

The effects of environmental factors on transpiration rate

• The rate at which transpiration occurs is affected by factors in the environment, including:
  • Wind speed
  • Temperature
  • Humidity
  • Light intensity
• The table below explains how these factors affect transpiration rate

Factors affecting transpiration rate table

Factor	Relationship to transpiration rate	Explanation
Wind speed	As wind speed increases, the transpiration rate increases	When it is windy, water molecules that diffuse out of stomata are quickly blown away from the leaf; this creates a concentration gradient and more water vapour diffuses out of the leaf
Temperature	As temperature increases, the transpiration rate increases	When temperatures are high water vapour molecules have more kinetic energy and move around faster, so they are more likely to move out of the stomata by diffusion
Humidity	As humidity increases, the transpiration rate decreases	Humid air contains a high concentration of water vapour; this reduces the diffusion gradient between the inside of the leaf and the outside air, so less water vapour diffuses out of the leaf
Light intensity	As light intensity increases, the transpiration rate increases	Increased light intensity increases the rate of photosynthesis, so the stomata open to allow gas exchange; when this happens, water vapour diffuses out of the stomata

• If water is lost by transpiration more quickly than it can be replaced then wilting will occur
• During wilting the cells of the plant lose water, so they are no longer turgid; these cells can no longer support the weight of the plant, so the plant tissues become floppy and the plant stems may fall over
• Wilting can be beneficial to the plant; wilted leaves may become folded, reducing the exposure of stomata to the outside air and slowing further water loss
• Plants that remain wilted for extended periods will eventually die due to the death of leaves and the inability to photosynthesise

Plant wilting diagram

A wilted plant (left) cannot support itself and starts to collapse