What is Transpiration? Why it is said be a necessary evil?

The loss of water by evaporation from a plant surface is called transpiration.

Over 90% of water escapes through the open stomata, while about 5 is lost directly from the epidermal cells. The combined area of stomatal pores is on average only 1-2% of the total leaf surface.

Transpiration rates are greatest when the leaf cells are fully turgid, stomata are open and relative humidity in the atmosphere is low.

Transpiration_Anessary Evil

Upward movement of water in plants is attributed to two processes:

•          i.            Root pressure    (ii)   Transpiration

1.       Root Pressure

Root pressure is capable of moving water upward in a plant, but not in the quantity and to the heights necessary for most plants. So we are left with the hypothesis that water is pulled up through the plant body due to transpiration.

2.       Transpiration

Although water is used in the maintenance of turgidity and the possible translocation of dissolved minerals, water use in plants is inefficient and can endanger their survival. So water loss by transpiration becomes necessary because of these and some other reasons (cooling effect by evaporation) it is said that transpiration is a necessary evil.

What are the functions of roots in plants and how these help in the uptake of water and salts?

FUNTIONS:

Functions of roots in plants

Roots perform the following functions in plants:

         i.            These anchor the plants in soil.

       ii.            These absorb water and salts from soil.

      iii.            These provide conducting tissues for disturbing these substances to the tissues of the stem.

For better understanding of uptake of water and salts, the internal structure of root should be taken into account.

Anatomy of Root

The centre of the root in most of the cases is occupied by vascular tissue, The xylem composed of conducting elements, the Tracheids and vessels occupies the centre of the root is continuous with the xylem tissues in the stem. The phloem tissue is closely associated to the xylem tissue. The xylem and phloem elements are surrounded by layer of living cells,, the Pericycle. The vascular tissue and the epicycle form a tube of conducting cells called stele. Just outside the stele is a layer of cells called endodermis. This endodermis acts as watertight jacket around the conducting vascular elements because water with its dissolved substances cannot pass around the endodermal cells via their walls.

Outside the endodermis, several layer of large thin walled living cells with intercellular spaces among them are present. This is called as cortex. The air spaces form interconnected air channels necessary for internal aeration. The cell wall of cortical cells are highly permeable to water and its dissolved solutes. The cortex is surrounded by a layer of almost flattened cells. It is epidermis. Some epidermal cells develop long projections called root hairs that extend out among the soil particles around the root. The root hairs increase soil-root contact and enhance water absorption and the volume of soil penetrated.

Uptake of Water and Salt

Root hairs provide large surface area for absorption. The cytoplasm of the root hairs has higher concentration of salts than the soil water, so water moves by osmosis into the root hairs. Salts also enter root hairs by diffusion or active transport. After their entry into the root hairs, water and salts must move through the epidermis and cortex of the root and then into the xylem tissue in the centre of the root.

What are the Apoplast and Symplast water pathways?

There are two pathways through which water travels from the outside of the root to the inside. These pathways are as follows:

(i)                  Apoplast Pathway    (ii)   Symplast Pathway

• 1)      Apoplast pathway

Interconnected walls and water filled xylem elements should be considered a single system, which is called Apoplast. When water travels along cell walls and through intercellular spaces to reach the core of the root then we call this pathway as Apoplast pathway.

• 2)      Symplast Pathway

The rest of the plant living part (other than Apoplast) is termed as Symplast. In the Symplast pathway, water moves through Plasmodesmata.

 (rod like connections or bridges by which cytoplasm of the neighbouring cells is linked with each other).

What are different types of transpiration?

Types of Transpiration

There are three types of transpiration:

        i.            Stomatal Transpiration

Evaporation of water through stomata is called stomatal transpiration. More than 90% of water is lost through the stomata although stomata openings surface. Stomatal transpiration involves two processes:

• a)      Evaporation of water from cell wall surface bordering the inner cellular. Spaces, or air spaces of the mesophyll tissue.
• b)      Diffusion of the water vapours from the intercellular spaces into the atmosphere by way of the stomata.

      ii.            Cuticular Transpiration

The loss of water as a vapour, directly from the surface of leaves of leaves and herbaceous stems through the cuticle is called Cuticular Transpiration. Only a small fraction of water is lost by Cuticular Transpiration.

    iii.            Lenticular Transpiration

The loss of water through the lenticels in the bark is called Lenticular Transpiration. Lenticels are small openings present in the bark.

What is Transpiration? Why it is said be a necessary evil?

The loss of water by evaporation from a plant surface is called transpiration.

Over 90% of water escapes through the open stomata, while about 5 is lost directly from the epidermal cells. The combined area of stomatal pores is on average only 1-2% of the total leaf surface.

Transpiration rates are greatest when the leaf cells are fully turgid, stomata are open and relative humidity in the atmosphere is low.

Transpiration_Anessary Evil

Upward movement of water in plants is attributed to two processes:

         i.            Root pressure    (ii)   Transpiration

1.       Root Pressure

Root pressure is capable of moving water upward in a plant, but not in the quantity and to the heights necessary for most plants. So we are left with the hypothesis that water is pulled up through the plant body due to transpiration.

2.       Transpiration

Although water is used in the maintenance of turgidity and the possible translocation of dissolved minerals, water use in plants is inefficient and can endanger their survival. So water loss by transpiration becomes necessary because of these and some other reasons (cooling effect by evaporation) it is said that transpiration is a necessary evil.

The opening and closing of stomata regulates the transpiration. What is its mechanism?

Most plants keep their stomata open during the day and close them at night. The regulation of transpiration through stomata depends upon guard cells. Each stoma is surrounded by two guard cells, which are attached to each other at their ends. The inner concave sides of guard cells are thicker than the outer convex sides.

Mechanism

Initially, it was thought that concentration of glucose in guard cells is responsible for opening and closing stomata. When guard cells become turgid, their shapes are like two beans and stoma between them opens. When the guard cells loose water and become flaccid, their inner sides touch each other and the stoma closes.

Recently, it is has been revealed that opening and closing of stomata depends upon the movement of Potassium ions in and out of guard cells. The blue wavelengths of daylight cause the K⁺ to flow into the guard cells, from the surrounding epidermal cells. Water passively follows these ions into the guard cells. The guard cells become turgid and open. During the night time, the K⁺ flows back to the surrounding epidermal cells, which also lead to loss of water. Guard cells become flaccid and stomata close.

Define the term Water Potential

Water molecules posses’ kinetic energy, which means that in liquid or gaseous form thy move about rapidly and randomly from one place to another So greater the concentration of the water molecules in a system the greater in the total kinetic energy of water molecules. This is called water potential.

Water always moves from an area of higher potential to an area of lower water potential. The relationship between the concentration of solute and water potential. The relationship between the concentration of solute and water potential is inverse i.e. where there is a lot of solute the water potential is low.