Question

Under what conditions the rate of transpiration increases? (a) Increase of humidity (b) Increase of atmospheric pressure (c) Decrease of temperature (d) Decrease of humidity

Short answer

The rate of transpiration increases under the condition of decrease of humidity.

Step-by-step solution

Analyzing Condition (a) - Increase of Humidity

Humidity refers to the moisture content in the air. Higher humidity means more moisture in the air. When the air is already saturated with moisture, there is less gradient for moisture to move from the plant to the air, leading to a decrease in transpiration. So, an increase in humidity usually decreases transpiration rate.

Analyzing Condition (b) - Increase of Atmospheric Pressure

Atmospheric pressure does not directly affect transpiration. However, it might influence other factors such as wind movement and humidity, which can impact the rate of transpiration. Generally, an increase in atmospheric pressure alone doesn't lead to an increase in transpiration rate.

Analyzing Condition (c) - Decrease of Temperature

Temperature affects the evaporation of water. When temperature decreases, evaporation tends to slow down. Therefore, a decrease in temperature is expected to lower the rate of transpiration.

Analyzing Condition (d) - Decrease of Humidity

A decrease in humidity means less moisture in the air. This increases the moisture gradient from the plant to the air, facilitating the movement of water from the plant to the air. Hence, a decrease in humidity usually boosts the rate of transpiration.

Key concepts

Humidity

The term humidity refers to the amount of water vapor present in the air. It plays a significant role in plant transpiration, which is the process of water movement through a plant and its evaporation from aerial parts. When humidity is high, the air is saturated with water, leaving little room for more moisture to escape from the plants.
This results in a reduced moisture gradient between the plant and the surrounding atmosphere, leading to lower rates of transpiration.
Conversely, when humidity decreases, the air becomes drier, creating a larger difference in moisture levels between the plant and the air. This enhanced moisture gradient facilitates the movement of water from inside the plant to the outside air, thus increasing the rate of transpiration. Understanding this relationship is crucial for managing plant growth and optimizing conditions in controlled environments such as greenhouses.

Atmospheric Pressure

Atmospheric pressure is the force exerted by the weight of the air above a particular point. It can influence various environmental factors that indirectly affect plant transpiration.

• When atmospheric pressure increases, it may suppress wind movement, leading to higher humidity.
• A reduction in pressure might promote greater air circulation, enhancing evaporation rates.

However, an increase in atmospheric pressure alone does not directly cause an increase in the rate of transpiration.
Instead, its effects are subtler and usually mediated through altering conditions like air movement and humidity levels around the plant.
Consequently, while atmospheric pressure is a significant environmental parameter, it alone does not drastically change the transpiration rate but can do so in combination with other factors.

Temperature

Temperature is a key factor that influences the rate of transpiration in plants. As temperature increases, so do the energy levels of water molecules, promoting more rapid evaporation from the stomatal openings on the leaves. This energy boost makes it easier for water to transition from the liquid phase to the gas phase.
As a result, higher temperatures typically lead to higher transpiration rates.
Conversely, a decrease in temperature reduces the energy available for water molecules to convert into vapor, causing a slowdown in the rate of evaporation and consequently, lower transpiration rates. Therefore, maintaining the optimal temperature range is essential for maximizing plant growth without stressing the plant's water resources.

Moisture Gradient

The concept of the moisture gradient is crucial in understanding transpiration mechanics. It is essentially the difference in water concentration between two points—in this case, within the plant and the surrounding air.
A higher moisture gradient means a larger difference in water content between the plant and the air, facilitating more efficient water movement from the plant into the atmosphere.

• When there is a high moisture gradient, transpiration rates increase because the water naturally moves from an area of higher concentration (inside the plant) to one of lower concentration (the drier air).
• Conversely, a low moisture gradient indicates minimal difference in moisture levels, resulting in reduced transpiration rates.

The moisture gradient is an essential factor that controls how efficiently plants can release excess water, and is a key element in plant-water relations that is influenced by both environmental conditions such as temperature and humidity.