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All living organisms must carry out a set of basic functions in order to maintain themselves. One of these processes involves the transportation of materials throughout the organism. Therefore, organisms need to exchange materials with the environment, which can be seen on the small scale of cells transporting protein and other materials among one another or on a larger scale such as the water cycle, where water is continuously moving on, above, and below the surface of Earth. In both examples, the movement of materials from one area to another is a dynamic process regulated by both environmental and biological variables.
Transpiration: A Driver for Water Transport
Vascular plants, from the smallest ferns to the giant redwoods of California, transport water and the water-soluble materials throughout the plant by a system of bundles of vascular tissues that run from the roots to the tips of the plant. Specifically, water and nutrients are absorbed by the root hairs and transferred by osmosis into the plant's xylem, one of the two large vascular systems found in plants. Water is then transported to the tallest point of the plant and outwards into the leaves where photosynthesis takes place. Interestingly, only 1% of the water taken up by the plants is used for photosynthesis. The other 99% of the water is not directly used by the plant and is lost from the plant due to evaporation or guttation, also known as transpiration. Evaporation is the movement of water to the air, where guttation specifically refers to secretion of droplets of water from the pores of plants. Both processes combined make up transpiration in plants.
The leaves of plants play an important role in releasing the by-products of photosynthesis through their stomata, which are openings that allow the exchange of materials between the plant and the atmosphere. The stomata are bordered by guard cells, which regulate when the stomata open and close. This is the active site where the majority of water is lost due to evaporation, as well as the site where gases are exchanged with the atmosphere. Although it might at first seem like a bad strategy for a plant to lose water due to evaporation, it is inevitable in order to maintain the transportation of materials and water in the plant while maximizing gas input. Transpiration creates a lower concentration of water, thus a lower osmotic potential in the leaf. These differences in water concentration are responsible for driving the movement of water into the plant's leaves and also in releasing water into the atmosphere.
Water potential drives the uptake of water from the root hairs and also in transporting water to the tips of leaves. Water potential is the measure of the free energy of water, where water molecules move from areas of higher water potential to areas of lower water potential. When evaporation is high in leaves, this creates areas with lower water potential, or areas with less water, so water from the roots and stem is driven to the leaves. The cohesion and adhesion properties of water molecules enable this movement of water. Cohesion is the attraction of water molecules to each other and adhesion is the attraction of water molecules to other materials, such as the xylem walls. When water molecules exit through stomata, they pull water molecules underneath, causing water to move towards lower water potential.
Plant species differ widely in their physical characteristics as well as in their morphology and functions in the ecosystem. These differences among plant species and also similarities among distantly related plant species are shaped over time through evolution and more specifically by selection imposed by herbivores, pollinators, and other climatic and environmental factors. Therefore, differences in transpiration rates is influenced by both the environment and the plant species itself. The main environmental factors that drive the transpiration rate is temperature. Higher temperatures increase the rate of transpiration because water is lost via evaporation more quickly. Plants living in hot environments are prone to losing more water than plants located in cooler climates. Factors such as water availability, wind, sunlight, and others also help influence the transpiration rate in plants.
Plants living in hot and arid environments have specific adaptations that help them control water loss, such as the ability to store or conserve water. A group of plants in these environments, also known as Crassulacean acid metabolism or CAM plants, have evolved strategies, such as only opening their stomata for gas exchange at night in order to reduce water loss1. Some other plant features specific to arid environments include reduced leaf surface area, fewer stomata, or having hairs on their leaves to conserve water. However, there is a tradeoff with limiting water loss and having an optimal rate of transpiration needed for photosynthesis. The rate of photosynthesis relates to a plant’s rate of growth and energy acquisition, which is related to rate of water intake and loss, so it is extremely important for plants to be able to balance this tradeoff. On the other hand, in environments where water is not a limiting resource, such as tropical rainforests, plants face different selective pressures that drive differences in transpiration rates. In these environments, natural selection may instead favor plant species that can transport water more quickly in order to outgrow their competing neighbors or grow tall enough to avoid being eaten by herbivores.
Assessing Transpiration Rates
The transpiration rates can be assessed indirectly by using a potometer, which is a device that measures the rate of water uptake of a leafy plant. The assumption of the potometer measurement is that transpiration will cause the water uptake, the amount of which can be quantified. In addition, scientists can determine relative transpiration rates of plants by observing the leaf structures, such as the sizes and numbers of stomata per unit area.
Examining plant transpiration rates can teach us not only how plants adapt to different environments, but can also provide information about how to best grow crops in different environmental conditions to increase food production and adjust our use of plants to adapt to global climate change and population growth. For example, transpiration rates are altered by global warming and other causes and can affect the global water cycle, which in turn can affect ecosystems as well as human populations2. Thus, understanding these changes would be necessary to develop strategies to remediate their negative effects. Moreover, studying transpiration rates of different crops can help identify crops with high water-use efficiency to increase the food production per unit of water and reduce the need for irrigation3.
- IP, TIng. Crassulacean acid metabolism. Ann REv Plant Physiol. 1985, Vol. 36, 595-622.
- Schlesinger WH, Jasechko S. Transpiration in the global water cycle. Agricultural and Forest Meteorology. 2014, Vols. 189-190, 115-7.
- Coupel-Ledru A, Lebon E, Christophe A, Gallo A, Gago P, Pantin F, Doligez A, Simonneaua T. Reduced nighttime transpiration is a relevant breeding target for high water-use efficiency in grapevine. PNAS. 2016, Vol. 113, 32: 8963-8.