what is the mechainism in the process of transpiration?

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mechanism of Transpiration

Transpiration is the process by which plants give off water into the atmosphere in the form of water vapor. It is not a simple process of evaporation since it is regulated by the physiological activity of the protoplasm. The extent of loss of water due to transpiration is different for different plants; for example, 10 to 20 liters by an apple tree, and 3 to 4 liters by a maize plant per day.

Mechanism of Transpiration

Water may transpire directly through the cuticle, a waxy layer covering the leaf surfaces (cuticular transpiration), through stomata (stomatal transpiration) or through the lenticels (lenticular transpiration). Turgor pressure inside mesophyll cells of the leaf forces water outwards through the cell wall. Water is collected in the intercellular spaces. From intercellular spaces, water diffuses out of the stomata into the atmosphere. Transpiration is greatly influenced by three factors – guard cells, water vapor in the respiratory cavities and the intercellular spaces.

Importance

Transpiration plays and important role in the translocation of water and dissolved matter. Evaporation of water from the leaf causes a drop in turgor pressure. It will pull water from the neighboring cells and a continuous chain is formed through xylem tubes. This is known as cohesion theory of transpiration pull theory to explain the ascent of water from the roots. According to this, the water molecules cohere together and form a long continuous column in the vessels extending from leaf to the root. You can compare it with taking ink from an ink bottle using a dropper. Besides translocation of substances transpiration also facilitates other physiological activities of the plant.

During photosynthesis gases like carbon dioxide and oxygen also diffuse in the same fashion. When the concentration of carbon dioxide in the cell falls, it sets up a diffusion gradient between the air and intercellular spaces in the cell. This results in diffusion of carbon dioxide into the cell from the atmosphere. The concentration of carbon dioxide increases inside the leaf. This then comes out of the leaf to the atmosphere. Since diffusion occurs in all directions, large surface area is helpful in greater diffusion.

Transpiration

Figure 1: Transpiration of water from the plant and evaporation of water from the soil condense on the inside of the bag. (Source: Ming Kei College, Hong Kong.)

Transpiration is the term used to describe the transport of water through an actual, vegetated plant into the atmosphere. Transpiration is an important part of the evapotranspiration process, and a major mechanism of the water cycle in the atmosphere. Transpiration may also refer to the rate of the water vapor transport through the whole vegetative canopy (that is, through the group of plants).

Just as you release water vapor when you breathe, plants do, too—although the term "transpire" is more appropriate than "breath." During this process individual water molecules are released from the surface of the plant body through tiny structures called stomata. There are many more individual water vapor molecules inside the air spaces between the tissues of a plant than in the air surrounding the plant body. Consequently water vapor will always exit the plant along a concentration gradient. As more water vapor molecules exit the plant, the remaining water molecules tug on each other and will pull an entire column of water throughout the plant body through special tissues called xylem during the process of transpiration. One way to visualize transpiration is to put a plastic bag around some plant leaves. As Figure 1 shows, transpired water will condense on the inside of the bag. If the bag had been wrapped around the soil below it, too, then even more water vapor would have been released, as water also evaporates from the soil. During a growing season, a leaf will transpire many times more water than its own weight. An acre of corn gives off about 3,000-4,000 gallons (11,400-15,100 liters) of water each day, and a large oak tree can transpire 40,000 gallons (151,000 liters) per year.

Factors affecting transpiration

The amount of water that plants transpire varies greatly geographically and over time. There are a number of factors that determine transpiration rates:

Temperature: Transpiration rates go up as the temperature goes up, especially during the growing season, when the air is warmer due to stronger sunlight and warmer air masses. Higher temperatures cause the plant cells which control the openings (stoma), where water is released to the atmosphere, to open, whereas colder temperatures cause the openings to close.

Relative humidity: As the relative humidity of the air surrounding the plant rises the transpiration rate falls. It is easier for water to evaporate into dryer air than into more saturated air.

Wind and air movement: Increased movement of the air around a plant will result in a higher transpiration rate. This is somewhat related to the relative humidity of the air, in that as water transpires from a leaf, the water saturates the air surrounding the leaf. If there is no wind, the air around the leaf may not move very much, raising the humidity of the air around the leaf. Wind will move the air around, with the result that the more saturated air close to the leaf is replaced by drier air.

Soil-moisture availability: When soil moisture is lacking, plants can begin to senesce (premature ageing, which can result in leaf loss) and transpire less water.

Type of plant: Plants transpire water at different rates. Some plants which grow in arid regions—for example, cacti and succulents—conserve precious water by transpiring less water than other plants.

Transpiration and groundwater

Figure 2: In places where the water table is near the land surface, such as next to lakes and oceans, plant roots can penetrate into the saturated zone below the water table, allowing the plants to transpire water directly from the groundwater system. Here, transpiration of groundwater commonly results in a drawdown of the water table much like the effect of a pumped well (cone of depression—the dotted line surrounding the plant roots in the diagram). (Source: U.S. Geological Survey)

In many places, the top layer of the soil where plant roots are located is above the water table and thus is often wet to some extent, but is not totally saturated, as is soil below the water table. The soil above the water table gets wet when it rains as water infiltrates into it from the surface, but it will dry out without additional precipitation. Since the water table is usually below the depth of the plant roots, the plants are dependent on water supplied by precipitation. As Figure 2 shows, in places where the water table is near the land surface, such as next to lakes and oceans, plant roots can penetrate into the saturated zone below the water table, allowing the plants to transpire water directly from the groundwater system. Here, transpiration of groundwater commonly results in a draw down of the water table much like the effect of a pumped well (cone of depression—the dotted line surrounding the plant roots in the diagram).

It is often difficult to distinguish between evaporation and transpiration. So we use a composite term evapotranspiration