OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

How does light intensity affect the rate of transpiration?

1. Increases transpiration rate

2. Decreases transpiration rate

3. No effect on transpiration rate

4. Only affects transpiration at night

The correct answer is: Increases transpiration rate

Light intensity has a significant impact on the rate of transpiration, primarily because it influences the process of photosynthesis and the opening of stomata. When light intensity increases, photosynthesis rates typically rise, leading to higher oxygen production and the consumption of carbon dioxide. To facilitate photosynthesis, plants need to open their stomata, which are small openings on the leaves that allow gas exchange. As the stomata open in response to the increased light, water vapor inside the leaf can escape into the atmosphere, enhancing the process of transpiration. This is important for cooling the plant and helping to transport nutrients and water from the roots through the xylem. Consequently, when light intensity increases, it directly correlates to an increased transpiration rate due to enhanced stomatal opening and greater water vapor loss. In contrast, under lower light conditions, stomata tend to close to conserve water, which would decrease the transpiration rate. Therefore, the relationship between light intensity and transpiration is positive—greater light leads to increased rates of transpiration due to the physiological responses of the plant.