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Transpiration or the Loss of Water Vapor

Transpiration, or the loss of water vapor from the leaves and stems of plants, is a process that assists in evaporative cooling, gas exchange, and the absorption and distribution of minerals and water throughout the plant. The process is fundamental to photosynthesis because it supplies a plant with the water needed to synthesis glucose. Glucose in turn provides the energy and material for maintenance, growth, repair, reproduce, and structure of a plant. Transpiration is the inevitable consequence of gas exchange. Plant leaves are the primary organ of photosynthesis and the site of the exchange of oxygen and carbon dioxide (CO2). Due to the low permeability of a leaf’s waxy cuticle to CO2, pores through the leaf’s epidermis are needed to facilitate gas exchange. Known as stomata, these pores pose a problem to plants as they also allow the release of water vapor.

In fact, up to 99% of the water absorbed by roots can be lost through transpiration. This correlation establishes an intractable problem for plants and other organisms: having gas exchange without water loss. To conserve water, plants have developed specific parameters that resist transpiration: cuticle resistance, stomatal resistance and boundary layer resistance. The cuticle is the hydrophobic, waxy layer present on all above-ground tissue of a plant and serves as a barrier to water movement out of a leaf. Special cells called guard cells control each stoma’s opening or closing. When stomata are open, transpiration rates increase; when they are closed, transpiration rates decrease. Stomata are triggered to open in the presence of minimal light so that carbon dioxide is available for the light-dependent process of photosynthesis. Stomata are the only way plants can control transpiration rates in the short-term.

The boundary layer is a thin layer of still air hugging the surface of the leaf. For transpiration to occur, water vapor leaving the stomata must diffuse through this motionless layer to reach the atmosphere. The larger the boundary layer, the slower the rates of transpiration. Many factors such as temperature, sunlight intensity, pH, wind and …… influence transpiration rate. Wind alters a plant’s rate of transpiration by removing the boundary layer, a still layer of water vapor hugging the surface of leaves. Wind also sweeps away airborne water particles near the plant, increasing the water potential of the atmosphere. A hydrated leaf would have a RH near 100%, just as the atmosphere on a rainy day would have. Any reduction in water in the atmosphere creates a gradient for water to move from the leaf to the atmosphere. The lower the RH, the less moist the atmosphere and thus, the greater the driving force for transpiration. The removal of the boundary layer in combination with the increase in the nearby atmosphere’s water potential should increase a plant’s transpiration rate. These two driving factors are catalyzed by the ability for wind to change vapor pressure.

Wind moves air about rapidly, thereby causing it to expand. This process creates room for extra water vapor and evaporation will continue to occur while the wind is blowing. I decided to investigate the effect of wind speed on plant transpiration after reading an article that discussed how scientists are simulating specific environmental factors to increase the rate of plant growth. The scientist’s aim for the simulation was to help ameliorate the effects of global warming by experimenting on how fast trees are capable of growing. The article briefly discussed that wind was created with various vents to help increase the tree’s evapotranspiration rate. I began to wonder to what extent wind influences the transpiration rate of a plant and whether there is an optimum wind speed. As a result, I asked the question: To what extent does wind speed influence the transpiration rate of a Philodendron cordatum trimming?

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