Transport in Plants

Transpiration and Structure of Stomata

Transpiration is the process in which plants release the water inside it in the form of moisture or water vapor. Parts of plants like stems, small pores on leaves, flowers evaporates the water to the atmosphere. Basically, Transpiration is the process in which water is evaporated in the atmosphere from plant leaves and other parts. Some amount of water is consumed by roots and rest is evaporated in the atmosphere. Let us study more about types of Transpiration and Stomata of leaves.

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Types of Transpiration

Transpiration can be of different types depending upon the specialized organ from where it is occurring.

  • Stomatal transpiration: It is the loss of water through specialized pores in the leaves. It accounts for around 80 to 90% of the total water loss from the plants.
  • Cuticular transpiration: Cuticle is an impermeable covering present on the leaves and stem. It causes only around 20% transpiration in plants.It is further reduced due to a thicker cuticle in xerophytes.
  • Lenticular Transpiration: Lenticels are the tiny openings present on the woody bark through which transpiration occurs.
  • Mechanism of water loss: Leaves also absorb visible and invisible radiations of the Sun and gets heated up. The water vaporizers and is given out in the atmosphere. This results in cooling down of the temperature of the leaves. Transpiration is basically regulated by the opening and closing of stomata.

Structure of Stomata

Stomata are the tiny pores present in the epidermal surface of leaves. The pores are guarded by two kidney-shaped cells known as guard cells. The inner wall of guard cell towards the stomata is thicker as compared to the outer walls. Also, the peculiar arrangement of the microfibrils of the guard cells also aids in opening and closing of the stomatal aperture.

Transpiration in plants

Source: prepjunkie

The microfibrils are oriented radially rather than longitudinal. This help stomata to open easily. In a dorsiventral dicotyledonous leaf, the number of stomata is a greater on the lower surface as compared to the upper surface. This adaptation helps in reducing the loss of water. In isobilateral leaf in a monocotyledonous plant, the number of stomata is equal on both the surfaces.

Factors affecting Transpiration

  • Climatic factors like temperature, humidity, wind speed etc.
  • Plant factors like number and distribution of stomata.
  • Percent of open stomata.
  • Water status of the plant.
  • The structure of canopy of the tree.

Mechanism of Stomatal Movement

The factors which affect stomatal movement are-

  • Amount of light
  • The concentration of carbon dioxide
  • Water supply

The opening and closing of stomata operate as a result of Turgidity changes in the guard cells. During daytime, guard cells photosynthesis due to which osmotic pressure increases. The guard cells absorb water from the neighboring cells. Guard cells become turgid.  As a result, the outer thin walls of guard cells are pushed out and the inner thicker walls are pulled inwards resulting in stomata to open.

During night or in a condition of water scarcity, guard cells are in a flaccid state and remain closed. Transpiration is the main driving force for the ascent of sap (rising of water in the tall trees through xylem vessels) which depends upon the following physical properties of water.

  • Cohesion-It is the attraction between water molecules.
  • Adhesion– The water molecules get attached to the surface of the tracheary elements of xylem.
  • Surface tension– the ability of water surface to behave like a stretched membrane

These properties give water high tensile strength and high capillarity. Because of this, the water is able to rise in vessels and tracheids of xylem of tall trees. As the water is lost from the leaves during transpiration, a pulling action is generated due to which the water rises high in the tall trees. The force generated by transpiration can create pressure sufficient to lift what over 130 M high.

Transpiration and Photosynthesis – a Compromise

Transpiration is an essential phenomenon.

  • It’s pulling action helps in absorption and transportation of water in the plant.
  • It supplies water for photosynthesis.
  • Transpiration cools the leaf surface.
  • It maintains turgidity of the cells.

Water and Carbon dioxide are essential for photosynthesis. Stomata are kept open for exchange of gases during the day but it leads to a lot of loss of water and plants get depleted of water due to continuous transpiration. C4 plants might have evolved to reduce the loss of water due to transpiration as they can maintain a constant supply of CO2 even after the closing of stomata.

Solved Questions for You

Q1. Transpiration is a necessary evil in plants. Explain.

Ans: Transpiration causes huge loss of water. Reduces photosynthesis, lowers growth and may cause wilting of the plan. In spite of all these disadvantages, it is necessary as it provides the pulling action for a\water to rise in the trees. It also maintains the temperature.

Q2.  The C4 plants are twice as efficient as C 3 plants in terms of fixing CO 2 but lose only half as much water as C3 plants for the same amount of CO 2 fixed. Explain.

Ans: This is because C4 plants regulate a constant supply of CO2 and keep their stomata closed for some period of time. Due to this loss of water is reduced.

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