4.2.2: Transpiration

 

The Process of Transpiration in Plants

In addition to evaporation, plants are a significant pathway for water to enter the atmosphere. This specific process is called transpiration, which is the movement of water vapor into the air through microscopic pores on leaf surfaces known as stomata.

The same three requirements for evaporation also apply to transpiration. Specifically, there must be available water, a vertical moisture gradient, and energy. A considerable amount of energy—approximately 600 calories—is required to change just one gram of liquid water into a gas to be transpired by the leaf. Because the transfer of water from the plant to the air uses this energy as latent heat, it removes heat from the leaf itself. Transpiration, like evaporation, is a cooling process. Consequently, it is an essential mechanism for transporting heat from the surface of the Earth into the air above.

Figure \(\PageIndex{1}\): Transpiration moves water from soil to atmosphere while transferring latent heat, cooling plants and contributing to atmospheric energy flow. (CC BY-SA 4.0; Sohini Dutt via AI Image Generated by Gemini).

 Figure \(\PageIndex{1}\) explains transpiration, the process by which water moves from soil through a plant and is released into the atmosphere as water vapor. It also shows that energy is transferred during this process as latent heat.

On the left side, solar radiation is shown reaching the leaves. Labels indicate that solar energy warms the leaves and provides the energy that helps drive the movement of water and the evaporation of water from leaf surfaces.

At the bottom and center of the diagram, water is shown entering the plant from the soil through the roots. Arrows labeled uptake show water moving into the roots. Additional arrows labeled capillary action flow show water moving upward through the stem toward the leaves. This part of the figure shows that water is transported internally from the soil to the leaves.

On the right side, a leaf cross-section shows several internal layers, including the upper epidermis, palisade mesophyll, spongy mesophyll, and lower epidermis. Small openings called stomata are shown in the lower epidermis, surrounded by guard cells. These structures regulate the release of water vapor and gas exchange.

Arrows show water vapor leaving the leaf through the stomata. Labels identify this as transpiration. During this process, liquid water changes into water vapor and enters the atmosphere.

The escaping water vapor is shown as scattered molecules moving more freely than liquid water. The diagram indicates that the energy used for evaporation is stored in the water vapor as latent heat and carried into the atmosphere.

Overall, the figure shows that transpiration connects solar energy, water movement in plants, and atmospheric moisture. Solar energy helps drive evaporation from leaves, water moves upward from soil through the plant, and latent heat is transferred into the atmosphere with the water vapor.

The water that plants transpire originates as moisture stored within the soil, which is then extracted by plant roots and moved through the plant body. How much water can be held in this soil moisture zone depends on the soil’s physical characteristics, specifically its texture and structure. For example, a coarse-textured soil, like one made mostly of sand, cannot hold as much moisture as a soil with a much finer texture. 

The significance of plant transpiration is immense when viewed at scale. For instance, a single adult oak tree is capable of releasing 100 gallons of water into the atmosphere every day, while an acre of corn can transpire up to 4,000 gallons daily. This massive movement of water highlights the heavy demand for moisture, which is increasingly met by extracting groundwater in regions where rainfall is insufficient for farming.

Figure \(\PageIndex{2}\) illustrates the full spatial extent of the High Plains Aquifer (commonly referred to as the Ogallala Aquifer). The aquifer boundary spans approximately 174,000 square miles across the central United States. The High Plains aquifer system is one of the largest freshwater aquifers in the world, serving as the primary groundwater source for agricultural irrigation, municipal water supply, and drinking water in the Great Plains region. It consists mainly of silt, sand, gravel, and clay deposited during the Miocene to Quaternary periods, with the Ogallala Formation forming the principal geologic layer.

A critical issue facing many nations is the rapid exhaustion of these groundwater supplies. When water is pumped for agriculture faster than nature can refill the source, it leads to the depletion and potential physical collapse of underground aquifers. This crisis is currently unfolding in the drier parts of the United States, specifically involving the Ogallala Aquifer.

The Ogallala Aquifer Crisis

Source and Extent: The aquifer is replenished by water entering the ground in the Rocky Mountains and stretches for hundreds of miles beneath the Great Plains.

Usage: It serves as the primary water source for both large-scale farming and local municipal needs across the Great Plains region.

Sustainability Gap: Current extraction rates significantly exceed the natural rate of recharge.

Economic Risk: There is a growing concern that if the aquifer runs dry due to over-pumping, it will result in a catastrophic collapse of the agricultural economy in the Great Plains.

Figure \(\PageIndex{2}\): Ogalala Reservoir. (Public Domain ; USGS)