Transpiration is derived from trans, the Latin noun that means “across,” and spiration, which comes from the Latin verb spīrāre, meaning “to breathe.” It means “the ACT of breathing across”. Transpiration is considered the process of water movement through a tree and its evaporation from aerial parts, such as leaves, stems, and flowers. Transpiration plays a vital role in the existence of plants since all trees require water to absorb and transport nutrients and other essential minerals from the soil through root nodules to various parts of the plant. However, trees do not use all the water that they consume. Studies show that the remaining 97% to 99.5% is lost through the process of transpiration and guttation.
Types of Transpirations
Tree transpiration functions as an excretory system by eliminating excess water from its tissues through the aerial parts of the plants. There are 3 types of transpirations: Stomatal, Lenticular and Cuticular Transpiration.
Stomatal transpiration
Leaf surfaces are dotted with pores called stomata, and in most plants, they are mostly found on the undersides of the foliage. The stomata are bordered by guard cells and their stomatal accessory cells that open and close the pore. Together these are known as the stomatal complex. Stomatal transpiration account for around 80 to 90% of the total water loss from the plants. The Stomata only makes up to 3% of the leaf surface area, nonetheless most water loss occurs through these openings due to the necessities of photosynthesis. The Stomata are open to allow 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 hot temperature.
Cuticular transpiration
Cuticular transpiration is the transpiration of gases or vapor directly through the external membranes. This takes place through the thin cuticle covering the leaves of the tree. 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. In common land plants cuticular transpiration is only 3-10% of the total transpiration. In herbaceous shade loving plants where the cuticle is very thin, the cuticular transpiration may be up to 50% of the total.
Lenticular transpiration
Another area where some water loss can be seen are lenticels which are small openings in some plants’ bark. Lenticular transpiration is the evaporation of water from a plant’s lenticels This type of transpiration sees the lowest amounts of water loss. The lenticular transpiration is only 0.1% of the total transpiration. Thanks to the continuous elimination of water from the plant body, there is a balance of water maintained within the plant. Lenticular transpiration maintains osmosis and keeps the cells rigid.
How does the process of transpiration work?
1. The roots absorb the water from the soil.Water is passively transported into the roots and then into the xylem (Xylem is the transport tissue in vascular plants which transports water and nutrients from roots to stems and leaves).
2. The forces of cohesion and adhesion cause the water molecules to form a column in the xylem i.e., the water travels from the roots to the leaves.
3. Water moves from the xylem into the mesophyll cells, evaporates from their surfaces and leaves the plant by diffusion through the stomata.
In summary the water movement starts from the soil to the roots through the xylem to the leaves to the stomata and finally the air.
Benefits of transpiration
The degree at which water travels through trees due to transpiration plays a significant role in maintaining the tree water balance. This provides the tree with numerous benefits.
1. Nutrients uptake
Nutrients uptake is one of the main benefits of the Cohesion-Tension mechanism, caused by transpiration, which draws water out of the soil into the roots. This transports nutrients and water absorbed by roots to the shoots and other parts of the tree. This is consequently why transpiration is very essential for the survival and productivity of trees. In agriculture, the rate of transpiration determines harvests.
2. Ensures the survival of trees during heat and drought stress
To survive in drought conditions, trees need to decrease transpiration to limit their water loss. Trees are dependent on the transpiration rate, as too much water loss can leave the trees dehydrated. Several trees that live in dry climates have evolved to have smaller leaves and therefore fewer stomata. Various tree species completely shed their leaves during a drought, to prevent further water loss. The general rule is that fewer leaves cause less water loss through transpiration. These extreme leaf adaptations can also protect the trees from thirsty and hungry birds and animals
3. Evaporative cooling
Trees cool the air by a process known as ‘transpiration cooling’. As trees emit water into the atmosphere from their leaves via transpiration, the surrounding air is cooled as water goes from liquid to gas (vapor). Transpiration ensures trees receive the required cooling and that the temperatures of the leaves are lowered during summer.
4. Improves water balance in trees
The water balance in trees is maintained by transpiration. Plants absorb a lot of water and transpiration is a means by which excess water be regulated and removed. Much of the water uptake is used for photosynthesis, cell expansion, and growth. Did you know a single tree that is 20 meters high can take up between 10 litres to 200 litres daily, depending on its species?
5. Transpiration helps with rain
Transpiration also has environmental benefits such as providing a cooling effect, especially in hot climates or seasons. Some studies show that collective of trees especially in forests can contribute to the formation of rain. Once water is evaporated (in gaseous form) through the tree’s leaves or stroma and moves upwards and accumulates as clouds. The clouds then condense into droplets (rain).
6. Turgor pressure
Another benefit of transpiration is turgor pressure which keeps tree cells full and turgid. This allows the tree to maintain shape, form, and structure. Without water, trees would become droopy and wilt. Turgor pressure is also essential for numerous cell functions. It causes stomata to take in carbon dioxide for photosynthesis (especially important on mornings of sunny days). The more the stomata open, the more trees transpire and lose water. Therefore, watering your trees early in the morning will support plant energy, especially on scorching summer days.
We hope this blog will help you have a better understanding on how trees function and help you appreciate the incredible wonder of trees. If you are embarking on a landscape project anytime soon, do not hesitate to contact us and we’ll help select the best trees for your landscape project.