Significance of transpiration - class-VII

Types of Potometer are:-

Approx. 98-99% of the water absorbed by a plant is lost in transpiration. Hardly 0.2% is used in photosynthesis while the remaining is retained in the plant during growth.

Transpiration generates tension in plants. This tension is the main pulling force which pulls water to great heights in vascular plants. Thus, transpiration leads to the ascent of sap. Without transpiration water and minerals will not rise from roots to aerial parts of plants and trees. Transpiration also causes surface cooling of leaves and hence regulates the temperature of the plant body. At the same time, it is essentially a wasteful process. As much as 99 percent of water absorbed by a plant goes waste into the atmosphere. Thus, transpiration has been called as a necessary evil.

In plants, gas exchange occurs through small pores in the leaf surface called stomata. Stomata can open to allow the exchange of gasses and close to reduce water loss from the plant. In the covering of the secondary structure of the stem of woody plants gas exchange is made through the lenticels of the periderm (small breaches of the cork). The cork of mature roots is also perforated by non-suberized pores called lenticels. These enable oxygen to reach the intercellular spaces of the interior tissues and carbon dioxide to be released into the atmosphere.

Transpiration is essentially similar to evaporation of water. It is the loss of water in vapour form through stomata. Water molecules tend to remain in the liquid state rather than moving to vapour state; so during evaporation water molecules absorb a lot of heat energy, this generates a cooling effect.

The plant uses only $1%$ of the absorbed water for its own needs the rest of the water is lost in the atmosphere through transpiration.

Most acceptable theory for the ascent of sap is the 'Physical force theory', called as 'Cohesion-Tension' or 'Cohesion-Transpiration pull theory'. This theory was put forward by Dixon and Joly. According to this theory ascent of sap in plants is a physical process, the lifting force for which is provided by the pulling force generated by transpiration. 

Stomata can regulate the rate of transpiration by simply controlling how open or closed they are (via the guard cells). Stomata that are open wider will facilitate a greater rate of transpiration, while stomata that are more narrowly opened will reduce the rate of transpiration.

When plants use solar energy for the synthesis of their food, only a small part of solar energy used,  rest of the part of this energy evaporate from the surface of leaves by the process of transpiration and due to its high latent heat. It is the best material for disposing excess  solar energy.

In weighing method of transpiration measurement there is loss in weight b the plant due to loss of water due to transpiration.

The forces of cohesion (attraction among water molecules) and adhesion (attraction between water molecules and the walls of the xylem vessels), together help to form a thin, unbroken column of water which leads to the mass flow of water in apoplastic pathways. These forces act due to the cohesive and adhesive properties of water. In plants, the capillarity is aided by the small diameter of the tracheary elements like the tracheids and the vessel elements. The proton pump has no role in the facilitation of water and minerals.

Curtis proclaimed that transpiration is a necessary evil. Stomata allows gaseous exchange. Carbon dioxide is taken in for its fixation by Calvin cycle and oxygen produced during the photolysis is given out. This gaseous exchange takes place by the process of diffusion. When the stomata open for the gaseous exchange, the water vapor is lost by the process of transpiration.

Water moves from high water potential to less water potential. Also it is assumed that transpiration loss of water equals water absorption. In saline water there will be less of transpiration loss as absorption will be less. Consequently there are less chances of wilting.

Stomata are usually confined in more number on the lower sides of the leaves. The lower surface is called as abaxial surface and the upper surface is called as the adaxial surface. Hence, maximum percentage i.e., 80- 95% of transpiration occurs from the lower surface.

Passive absorption is absorption by the pull from transpiration without the active involvement of root cells, The shoots and leaves of the plant experience transpiration due to evaporation of water from their surface.

Water is required to generate the reducing power, which is used to reduce carbon dioxide during photosynthesis. The water molecules are split up and oxygen released as a by-product. The protons are used to synthesize reducing power. Now all the water that is absorbed is never used up. In fact, the water that is actually used is a small fraction of water that is absorbed. Hence, transpiration ratio is calculated to find out the effectiveness of stomata in maximizing photosynthesis and minimizing loss of water.

According to transpiration pull theory, due to transpiration, the water column inside the plant comes under tension. This is called 'transpiration pull'. On account of this tension, the water column is pulled up passively from below to top of the plant(almost like a rope). A tension of one atmosphere is sufficient to pull water to a height of about $20$ meters.

Loss of water from the leaves causes water stress to develop from the top of the trees which forces more and more water to rise from the roots to the leaves. The column of the water is maintained due to cohesion and adhesion forces and the water moves up due to transpiration pull.

An action spectrum is a graph of the rate of a physiological activity plotted against the wavelengths of light. it shows which wavelengths of light is most effectively used in a specific reaction. In photosynthesis, chlorophyll is much more efficient at the blue and red wavelengths of light to carry out photosynthesis. Since transpiration is indirectly related to photosynthesis, its action spectrum would be similar to photosynthesis.