Question

The condition under which transpiration would be most rapid is (a) High humidity (b) Excess of water in soil (c) Low humidity, high temperature, guard cells are turgid (open) and moist soil (d) Low velocity of wind

Short answer

The condition under which transpiration would be most rapid is (c) Low humidity, high temperature, guard cells are turgid (open) and moist soil.

Step-by-step solution

Understanding the factors affecting transpiration

Transpiration rate in plants is influenced by humidity, soil water availability, temperature, and guard cell state. Low humidity, High temperature, turgid (open) guard cells, and moist soil promote transpiration.

Evaluating the options

Option (a) states high humidity which actually reduces the rate of transpiration because the concentration gradient of water vapour between the inside of the leaf and the outside air is decreased. So, it is incorrect. \n\nOption (b) suggests an excess of water in the soil. While this condition ensures that the plant has enough water to transpire, it is not the most optimal condition for maximum transpiration rate. \n\nOption (c) suggests low humidity, high temperature, turgid (open) guard cells, and moist soil. These conditions favour rapid transpiration because low humidity and high temperature increase the concentration gradient for water vapour loss from the plant to the atmosphere, and open guard cells allow transpiration to occur. \n\nOption (d) suggests low velocity of wind which will not facilitate rapid transpiration as wind also affects the rate of transpiration.

Choosing the correct answer

From evaluating the options, we can see that the conditions of low humidity, high temperature, turgid guard cells and moist soil all favor transpiration from the leaf surface. These conditions create an ideal environment for water vapour to move out of the leaf cells and into the atmosphere. Therefore, the correct answer is the option that combines all these conditions, which is option (c): low humidity, high temperature, guard cells are turgid (open) and moist soil.

Key concepts

Transpiration Rate in Plants

Transpiration is the process by which water is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. The rate at which transpiration occurs can significantly impact a plant's water balance and overall health.

Several factors contribute to the rate of transpiration in plants. These include the surrounding humidity, temperature, wind velocity, and the plant's ability to regulate water loss through structures known as guard cells. It's essential to understand that while plants need to transpire to transport nutrients and cool themselves, excessive transpiration can lead to water stress and reduced growth.

Understanding how these factors interplay helps gardeners, farmers, and scientists provide better care for plants and manage agricultural water use more effectively.

Guard Cells Role in Transpiration

Guard cells are of critical importance in the process of transpiration. These specialized cells flank the stomata, which are the tiny openings on the leaf surfaces. The turgidity—or the state of being swollen with water—of guard cells controls the opening and closing of stomata.

When guard cells are turgid due to ample water, they create an opening for the stomata, allowing for gaseous exchange and water vapor to escape, which is the transpiration process. Conversely, when the plant is conserving water or under stress, the guard cells become less turgid and close the stomata to reduce water loss. The balance between the need for CO2 for photosynthesis and the need to conserve water makes the role of guard cells complex but essential.

Humidity and Transpiration

Humidity plays a pivotal role in the rate of transpiration. When the air is humid, the concentration gradient of water vapor between the inside of the leaf and the external environment is lower, meaning there is less driving force for water to evaporate from the leaf. As a result, transpiration rates decrease in high humidity conditions.

On the other hand, low humidity increases the concentration gradient, enabling more water vapor to leave the leaf and thus speeding up transpiration. Understanding this relationship is vital for controlling water use in plants, especially in greenhouse settings or areas with high or low atmospheric humidity.

Temperature Influence on Transpiration

Temperature is another critical factor influencing transpiration. As temperatures rise, water held in the earth and within plant tissues warms up, reducing its bond with surrounding molecules and making it easier for evaporation to occur.

High temperatures can lead to a higher transpiration rate by promoting the evaporation of water from the leaf's internal structures. However, if temperatures are too extreme, they can cause the closure of stomata to protect the plant from excessive water loss, ironically leading to a decrease in transpiration rate. Therefore, a certain range of temperatures is favorable for optimal transpiration without risking plant health or excessive water loss.