Question

Assertion: Light is a very important factor in transpiration. Reason: It induces stomatal opening and darkness closing. Therefore, transpiration increases in light and decreases in dark.

Short answer

Yes, light is indeed a very important factor in transpiration. It induces the opening of stomata which results in increased transpiration, whereas darkness induces the closing of stomata which in turn decreases transpiration.

Step-by-step solution

Transpiration and its Factors

Transpiration is the process by which moisture is carried through a plant, from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. There are several environmental factors that affect the rate at which transpiration happens, one of which is light conditions.

Stomata and Light

Stomata are tiny openings or pores in plant tissue that allow for gas exchange. They are primarily found on the undersides of plant leaves. The opening and closing of stomata are regulated by the function of guard cells which surround each stoma. They open during the day to allow for the exchange of gases, and close at night to prevent water loss. Therefore, in light conditions, stomata are generally open which increases transpiration.

Transpiration in darkness

In darkness or at night, the stomata are generally closed because photosynthesis, the process that requires gas exchange through stomata, doesn't occur then. Therefore, the rate of transpiration decreases in darkness as the stomata are closed to minimize water loss.

Key concepts

Stomatal Regulation

Understanding how plants regulate their water loss is crucial, and it starts with stomatal regulation. Stomata are tiny adjustable openings on the underside of leaves which play a pivotal role in transpiration. Controlled by specialized cells known as guard cells, these pores can open or close according to the plant's needs.

During the day, guard cells actively absorb water and swell, opening the stomata to facilitate gas exchange for photosynthesis—a process essential for the plant's growth. This exchange allows carbon dioxide to enter while releasing oxygen, and incidentally, water vapor escapes in this process.

In the absence of light, such as during the night, guard cells lose their turgidity, leading to the closure of the stomata to conserve water, as gas exchange is not required. External stimuli such as humidity, soil moisture, and temperature also influence stomatal behavior, integrating environmental signals to maintain the plant's water balance.

Environmental Factors Affecting Transpiration

Apart from light, which induces stomatal opening, several environmental factors impact transpiration rates in plants. These factors combine to influence a plant's water retention and loss.

• Humidity: High atmospheric humidity discourages transpiration, as the moist air creates a negligible water gradient between the inside of the leaf and the outside.
• Temperature: Warmer temperatures increase water vapor pressure and thus, enhance the transpiration rate.
• Wind: Wind can remove the humid air layer surrounding the leaf, thus accelerating transpiration by increasing the water vapor gradient.
• Soil water availability: Plants with a sufficient water supply in their roots are more likely to keep stomata open, leading to increased transpiration.

Every plant species has adapted to its environment by developing various strategies to modulate transpiration in response to these changing external conditions.

Photosynthesis and Gas Exchange

The lifeline of a plant lies in the delicate balance it maintains through photosynthesis and transpiration. Photosynthesis is the process by which plants convert light energy into chemical energy by synthesizing glucose from carbon dioxide and water.

This essential process is tightly linked to gas exchange, which occurs through stomata. During photosynthesis, carbon dioxide (CO₂) is absorbed from the atmosphere, while oxygen (O₂) and water vapor are released as byproducts.

Transpiration's role in this is twofold—it helps in cooling the plant and generates a transpiration pull that aids in the transport of minerals and water from roots to leaves. Without the integrated process of photosynthesis and gas exchange, a plant's ability to produce food and sustain its water balance would be compromised.