Transpiration is the process by which water moves through a plant and evaporates from its aerial parts, such as leaves, stems, and flowers. Humidity refers to the amount of water vapor present in the air surrounding the plant. This article explains why increased atmospheric humidity decreases the rate at which plants transpire, clarifying how plants manage their water balance in different environmental conditions.
How Plants Release Water
Plants absorb water primarily through their roots, drawing it from the soil. This water travels upwards through specialized vascular tissues called xylem, which act like tiny pipes throughout the plant. The journey culminates as water reaches the leaves, where the majority of water release occurs.
Small pores located mostly on the underside of leaves, known as stomata, are the primary points of water vapor exit. Each stoma is flanked by two guard cells that regulate its opening and closing, controlling the exchange of gases and water vapor with the atmosphere. These openings are crucial for both water release and the uptake of carbon dioxide needed for photosynthesis.
Water inside the leaf forms a continuous column from the roots, extending into the intercellular spaces of the leaf tissue. As water evaporates from the moist surfaces within these spaces, it creates a negative pressure or tension, pulling more water up from below. This continuous flow is referred to as the transpiration stream.
The movement of water vapor out of the stomata is driven by a difference in water concentration. Water naturally moves from an area where its concentration is higher to an area where it is lower. Inside the leaf, the air spaces are saturated with water vapor, representing a high water vapor concentration.
Humidity’s Direct Impact
Humidity decreases transpiration due to the water vapor concentration gradient. Inside a plant’s leaf, the air spaces are saturated with water vapor, meaning the concentration of water molecules is very high. Outside the leaf, the concentration of water vapor in the atmosphere varies depending on the humidity level.
Transpiration occurs when water vapor diffuses from the high concentration inside the leaf to the lower concentration in the surrounding air through the stomata. This movement is similar to how a scent spreads from a perfume bottle into a room. The speed of this diffusion depends directly on the steepness of this concentration gradient.
When the outside air is dry, there is a significant difference in water vapor concentration between the inside of the leaf and the external environment. This steep gradient encourages a rapid outward diffusion of water vapor, leading to a high rate of transpiration.
Conversely, when the humidity in the air is high, the concentration of water vapor outside the leaf becomes much closer to the concentration inside the leaf. This reduces the steepness of the water vapor concentration gradient. With a smaller difference in concentration, water vapor diffuses out of the stomata much more slowly.
Imagine a crowded room (inside the leaf) trying to empty into a less crowded hallway (outside air). If the hallway is almost as crowded as the room (high humidity), people will move out much slower than if the hallway is nearly empty (low humidity). This diminished gradient directly results in a decreased rate of transpiration, as the driving force for water vapor movement is weakened.
Consequences of Reduced Transpiration
Reduced transpiration impacts the plant’s ability to acquire nutrients. Water moving through the plant carries dissolved minerals from the soil. When transpiration slows, this upward flow of nutrient-rich water diminishes, potentially limiting the delivery of essential elements for growth and development.
Transpiration also regulates a plant’s temperature, much like sweating cools animals. As water evaporates from the leaf surface, it absorbs latent heat, effectively cooling the plant. In conditions of high humidity, this evaporative cooling effect is reduced because less water evaporates.
If the plant cannot cool itself efficiently, especially in warm environments, it may experience elevated leaf temperatures. Prolonged periods of higher leaf temperatures can stress the plant and potentially damage its cellular machinery, affecting overall plant health.
A further indirect consequence of reduced transpiration relates to photosynthesis. Stomata, while allowing water vapor to escape, are also the entry points for carbon dioxide, a key ingredient for photosynthesis. When transpiration rates are low due to high humidity, plants may keep their stomata open for shorter durations to conserve water.
However, prolonged stomatal closure to conserve water also restricts the uptake of carbon dioxide. This limitation on CO2 availability can reduce the rate of photosynthesis, impacting the plant’s ability to produce sugars for energy and growth.