Transpiration process in plants: Definition, factors, types, mechanism


This article explains an important mechanism known as transpiration process in plants. It involves water movement or transportation of water through the plant structure. A major part of it then gets evaporated mainly from the leaves of plants and trees.

Introduction

Just like humans or other living organisms present on the earth, plants too require water to grow and to perform various functions. However, they use a very little amount of absorbed water. Rest amount gets evaporated in the environment just like the water getting evaporated from water bodies. Plants absorb water through their roots. It then gets transported to different parts and gets used up. Remaining water gets evaporated through plants stems, leaves and flowers. Plant transpiration is the process of water movement or transportation within the plant structure and then getting evaporated through aerial parts.

Why is water movement and evaporation necessary?

It is well known that evaporation provides a cooling effect. Similarly, in plants also it offers the same effect and protects from direct heat through sun rays. Also, it helps in water transportation or movement of water through the plant structure.

How can water evaporation retard growth?

Every living organism needs to maintain fluid balance in the body. In absence of it, the body may dehydrate and die. Similarly, plants too need some amount of water to sustain. If water is lost in the form of evaporation from its aerial parts in too high amount, plants and trees may not grow well. In short, if plants are absorbing too little water, and losing too high amount, they may dehydrate and dry up.

Sites of transpiration in plants

Transpiration in plants may take place from stems, leaves and flowers. However, leaves are the primary sites of the transpiration process. Leaf stomata comprise of guard cells which are two in number. Small pores are formed on the surface of leaves through these cells. These two cells are called guard cells because their function is to open and close stomates. Opening and closing occur in response to the stimuli taking place in the environment. Sunlight, adequate temperature, sufficient absorption of water through roots will open stomates. In absence of sunlight, too high temperature, high level of carbon dioxide and too little water absorption, guard cells close stomates. When stomates open up, the transpiration process increases and when stomates close, the rate of transpiration decreases.

How do plants control transpiration?

As leaves are the primary sites for transpiration, those plants and trees that live in the habitat with low humidity adapt themselves by decreasing the surface areas of their leaves. Lesser the surface area, lesser will be the water evaporation. Plants and trees that live where sunlight is scarce and humidity is high will grow their leaves with large surface areas. It is because in absence of adequate sunlight, risk of water evaporation from stomates decreases. For example, cacti and other desert plants lack leaves or if found are extremely small in size to prevent evaporation. Climatic conditions in desert regions are extreme, and thus, transpiration rates are high. This modification where there are no or minute leaves on plants helps them to save water for their growth and sustenance. Stomates present in them are sunken to avoid transpiration. Another modification they do is that guard cells only open their stomates during night time to absorb carbon dioxide. Throughout the day, stomates are closed to prevent or at least decrease transpiration.

Factors that influence rate of transpiration

  • Atmospheric conditions-
    Warm air increases the rate of transpiration. On the other hand, cool air in the environment decreases the rate of water evaporation. If the air is dryer, transpiration increases. If the atmospheric condition is windier, transpiration rate increases. Increase in the temperature in the atmosphere will increase the evaporation of water from the surface of the leaves.
  • Relative humidity-
    Air has a certain capacity of its own to hold the water vapour at different temperatures. However, it does not always happen that humidity is right for the given area. Sometimes it increases, and other times it decreases. The relative humidity is the amount of water vapour present in the air at that particular temperature. It is also referred to as RH. When RH decreases, air dries up, and thus, the rate of transpiration increases.
  • Number and size of parts-
    Since transpiration primarily takes place from the surface of leaves, the number and size of leaves matter a lot. If lots of leaves are present, the rate of transpiration will be high. If the number is less, transpiration will be less. If the surface area of leaves is larger, evaporation will take place at a faster rate when compared with those leaves which are tiny in size. If the stomata number increases, the rate of transpiration increases.
  • Presence of sunlight-
    In presence of sunlight, plants and trees prepare their own food using carbon dioxide. Small pores known as stomates present on leaves open up during photosynthesis process. Thus, there is an increase in the rate of transpiration. In the presence of sunlight, evaporation of water increases. During night time, when there is no presence of sunlight, these pores are closed, and thus, there is no evaporation or transpiration process.
  • Cuticle and trichomes-
    If there is a presence of cuticle on the leaves of plants and trees, the rate of transpiration is less. The reason is that cuticle is waxy, and thus, water cannot penetrate through it. Thus, there is no evaporation from the surface of the leaves. Some amount of evaporation does take place through stomata. Still, overall transpiration rate decreases. The cuticle also reflects back the sunlight and do not allow heating of the leaf. Lesser the heating, lesser will be the evaporation rate. Trichomes are very small hair-like structures on the surface of leaves. It prevents water evaporation by increasing the humidity on the surface of leaves.
  • The thickness of the boundary layer-
    Different species of plant shows the difference in the boundary layer which is nothing but stagnant air present around the surface of the leaves. If the boundary layer is thick, the rate of transpiration will be less and vice versa. Boundary layer increases when the surface area of leaves increases and when trichomes are present. Boundary layer plays an important role in evaporation of water vapour as they have to pass through it into the atmosphere. Thicker boundary layer makes it difficult for evaporation of water vapour in the atmosphere, and hence the rate of transpiration decreases.

Types

  • Foliar transpiration-
    It is the transpiration that takes place through the surface of leaves which is seen in around 90% of cases.
  • Cauline transpiration-
    It takes place through stems.
  • Cuticular transpiration-
    It takes place through cuticles present on the surface of leaves.
  • Lenticular transpiration-
    It takes place through the pores present on woody stems. This type of transpiration is rare.
  • Bark transpiration-
    As the name itself suggests, transpiration takes place from the bark which is the hard layer present on the stem.
  • Stomatal transpiration-
    Rate of transpiration taking place through this type is very high. In it, water evaporates from the stomatal pore located on the leaf's lower epidermal layer.

Mechanism

From the absorption of water from the soil to the evaporation in the atmosphere, below-mentioned steps are involved-
  • Water absorption from the soil is carried by the root portion of plants and trees. Roots are widely spread out in the soil and can deepen themselves as much as possible to extract the water from the soil. Root hairs extract water from the soil. They also absorb various essential minerals and salts.
  • From root hairs, water gets transported to roots, and from there, an upward movement of water takes place in plants and trees. It takes place by capillary action via xylem tissue in the stems which, in turn, transports water to the mesophyll cells.
  • Water is stored in the intercellular spaces of mesophyll cells. During transpiration process, water from these intercellular spaces of mesophyll cells gets transported into the substomatal cavity by the diffusion process. Water is diffused in the form of water vapours.
  • Guard cells located on the surface of leaves open up and from here, water vapour gets evaporated into the atmosphere. Stomata are located on the surface of leaves and are also present on the underside of the foliage. Guard cells and stomatal accessory cells border these stomata. They are responsible for opening and closing of the pores from where transpiration takes place.
  • Stomata open up usually during the day time in the presence of sunlight when plants want to prepare their own food through photosynthesis process. The stomatal opening allows gaseous exchange and allows plants to absorb carbon dioxide essential for photosynthesis process. However, more water molecules get lost when compared to absorbed carbon dioxide molecules.
  • Water in the form of water vapour flows into the atmosphere because the moisture present in the air is less than the moisture present in the leaves of plants and trees. Thus, the flow of water vapour is outwards from leaves into the atmosphere and not in the other direction.
  • The water absorbed by roots is used in only a small amount by the plants and trees. Around 97-99.5% of absorbed water gets lost in the atmosphere by evaporation of water vapour.

How is it measured?

There are many techniques to measure transpiration rate in plants. Some of them are photosynthesis systems, porometers, photometers and lysimeters.


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