Oct. 20, 2021
Oct. 7, 2021
Transpiration is a vital plant process that, for many years, received little attention. Given its importance in plant health and the global water cycle, the lack of attention is surprising. However, now transpiration applications are beginning to find a prominent place, especially in crop science and management.
A plant does not use most of the water that it absorbs. About 97-99% of the water is lost through transpiration. Transpiration is defined as the physiological loss of water in the form of water vapor, mainly from the stomata in leaves, but also through evaporation from the surfaces of leaves, flowers, and stems.
There are three main types of transpiration, based on where the process occurs:
Stomatal transpiration: Stomata make up only 3% of the leaf surface area, but most water loss happens through these openings due to the necessities of photosynthesis. Stomata are open to let carbon dioxide in for photosynthesis; however, this also causes the water in the mesophyll tissue in leaves to evaporate if the air outside is drier due to factors like high temperature.
Cuticular transpiration: The leaf surface has a waxy cuticle through which water vapor can evaporate. Water loss here is lower compared to stomatal transpiration, except when the stomata are closed.
Lenticular transpiration: Lenticels, small openings in some plants’ bark, are another area where some water loss can be seen. This type of transpiration sees the lowest amounts of water loss.
Several external and internal factors interact to regulate the rate of transpiration at the plant level.
Figure 1: Transpiration causes water uptake by producing “a decreasing gradient of water potential (ψ) from the soil through the plant to the atmosphere.” Kupers 2020. (Image credit: https://www.researchgate.net/publication/339133813_The_Soil_Moisture_Niche_in_a_Moist_Tropical_Forest_-_A_Demographic_Approach)
Solar radiation is the most important factor, as stomata are open in only daylight, and this is when transpiration can occur.
The Cohesion-Tension theory, which explains how transpiration moves water in the plants, shows how the external and internal plant atmosphere are connected.
Loss of water vapor at the leaves creates negative water pressure or potential at the leaf surface. Water potential describes the tendency of water to move from one place to another. The water potential is lower in the leaves than in the stem, which is lower than the water potential in the roots. Since water moves from an area of high to lower water potential, water is drawn up from the roots to the leaves; see Figure 1.
In addition, the adhesion of water molecules to the xylem walls and cohesion/attraction between water molecules pull water up to the leaves in tall trees.
Ultimately, for transpiration to occur, the water vapor pressure deficit of the surrounding air must be lower than the water potential of the leaves. Transpiration rates are higher when the relative humidity of air is low, which can occur due to windy conditions or when the temperatures are high. At higher relative humidity, there is less transpiration.
Carbon dioxide levels in the air that control the stomata opening will also influence transpiration rates.
In addition, various biochemical and morphological features of plants will also affect transpiration rate:
In ecosystems, other factors, such as species composition and density of plants, will also play a role in determining largescale transpiration rates.
There are two important aspects to transpiration: the movement of water through the plant to the leaves and the movement out of the leaves into the atmosphere. These two aspects are crucial on the plant and global scales.
The rate at which water moves through the plants due to transpiration plays an important role in maintaining plant water balance. This has many benefits for plants.
1. Uptake of Nutrients, is one of the main benefits of the Cohesion-Tension mechanism, triggered by transpiration, which pulls water out of the soil into the roots. This moves water and other nutrients absorbed by roots to the shoots and other parts of the plant. Hence, transpiration is very important for the survival and productivity of plants. In agriculture, the rate of transpiration determines yields.
2. Plant survival due to heat and drought stress will depend on transpiration rate, as too much water loss can leave the plants dehydrated. Since water is a limiting factor in many cases, much of crop research is focused on trying to improve plant water use to increase productivity in combination with photosynthesis. Transpiration and water use efficiency are intricately connected with photosynthesis through stomata.
Between the three types of photosynthesis, water use efficiency is the least in C3 plants, better in C4 plants, and best in CAM plants. CAM, or crassulacean acid metabolism, is found in plants in arid areas that have developed adaptations to reduce transpiration through leaves. However, there is a wide range of water use efficiency in each type based on species.
3. Through evaporative cooling, plant transpiration brings down the temperature of leaves, the largest plant organ.
4. Water balance in plants is also maintained by transpiration. Plants absorb a lot of water and transpiration is a means by which excess water is removed. Much of the water uptake is used for photosynthesis, cell expansion, and growth, but a single tree that is 20 meters high can take up between 10 liters to 200 liters daily, depending on its species. Euperua purpurea, an Amazonian overstory tree, absorbs 1,180 kg day-1, and even a simple corn plant can absorb 200 liters in the summer.
5. Turgor pressure keeps the plant cells full and turgid, due to the transpiration stream of water from roots to shoots. This has many uses for plants:
Transpiration is an important component of the global water cycle. The amount of water that is lost by plants is great enough to influence the atmosphere.
Figure 2 “ Average disposition of 4200 billion gallons per day of precipitation in the conterminous United States. (source: Data from U.S. Geological Survey, 1990),” USGS. (Image credits: https://geochange.er.usgs.gov/sw/changes/natural/et/)
Transpiration along with evaporation is called evapotranspiration. In the USA, it was estimated that nearly 67% of rainfall returns to the atmosphere in the form of evapotranspiration, while 29% enters the oceans as runoff, 2% makes up groundwater, and 2% is used by people; see Figure 2.
The contribution of transpiration to evapotranspiration is a hotly debated topic. Various studies estimate that transpiration from land surfaces accounts for 61-75% of evapotranspiration. On a global scale, when evaporation from the oceans, lakes, wetlands, and soil is also considered, transpiration still accounts for 10-15% of the entire global evaporation. Transpiration varies based on vegetation, with the maximum amount coming from rainforests (70%) and the minimum coming from steppe and desert (51%).
Differences in transpiration can be attributed to geographic location, season, time of day, and cloud cover. Anthropogenic activities resulting in increased carbon dioxide levels, land use, deforestation, and climate change also alter the transpiration rate.
Higher temperatures due to climate change are speeding up evapotranspiration. This increases the amount of water vapor in the atmosphere, leading to more intense and frequent rains in some places, especially in coastal areas. Colder regions are experiencing warmer and longer growing periods, which also produces higher levels of transpiration.
With added pressure from climate change, measuring transpiration is becoming an important part of many disciplines, including the following:
The increased role that transpiration plays requires the use of devices that can give precise, rapid results. To allow for in situ measurements, tools also have to be portable and small. The CI-340 Handheld Photosynthesis System, manufactured by CID-BioScience Inc., combines all of these qualities. Tools like Leaf Area Meters and Plant Canopy Imagers are also needed to measure factors that influence transpiration. With the help of this new technology, it is possible to utilize plant-scale transpiration measurements to aid global needs.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
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