SIGNIFICANCE OF TRANSPIRATION
Transpiration is regarded as a “necessary evil” Curtis (1926) or unavoidable evil (Steward 1959) because it cannot be avoided though potentially harmful. The stomata remain open for exchange of gases which also results in the loss of water as vapours. The loss of water leads to wilting, serious desiccation and often death of a plant. The stomata, however, cannot be closed to prevent water loss because this would also stop gaseous exchange needed for transpiration and photosynthesis. Despite the harmful effects, there are certain advantages of transpiration.
ADVANTAGES OF TRANSPIRATION:
- It helps in rapid upward movement of sap. The absorption of water and ascent of sap to various parts of the plant body is mostly due to transpiration. Transpiration pull in upward direction is responsible for mass movement of water in the upward direction.
- It helps in absorption and translocation of solute. As transpiration helps in the absorption of water and ascent of sap, the minerals dissolved in water are also absorbed along with water. The solute uptake is high in actively transpiring plants.
- It regulates plant temperature: Since transpiration involves the evaporation of water, it has a significant role in cooling of leaves. As leaves are heavily pigmented, they absorb large amount of direction solar radiation, only a small fraction of these radiations is utilized in photochemical reaction and the remaining radiation accounts for a significant heat gained by the leaves. Leaves also exchange infrared energy with their surroundings, both absorbing and radiating infrared radiation. But a leave radiates more infrared energy than it gains, thus there is a negative net infrared exchange. One important method of dissipating the heat load is by evaporation of water from the leave surface or transpiration. Transpiration, in fact, accounts for dissipation of approximately half of the net radiation balance. The remaining half is probably dissipated by convection from the leaf to the surrounding air.
- Mechanical tissue: The development of mechanical tissue which is essential for providing rigidity and strength to the plant, is favoured by the increase in transpiration.
- Root system: Transpiration helps in better development of root system, which is required for support and absorption of mineral salts.
- Quality of fruits: The ash and sugar contents of the fruits increase with the increase in transpiration.
- Resistance: Excessive transpiration induces hardening and resistance to moderate drought.
DISADVANTAGES OF TRANSPIRATION:
- Wilting: Wilting may be defined as drooping of leaves due to loss of turgidity. Wilting occurs when transpiration exceeds the amount of water absorbed by the plant. Wilting or loss of turgidity is quite common during noon due to transpiration. Wilting reduces photosynthesis and other metabolic activities
- Reduced growth: Transpiration reduces availability of water inside the plant. Deficiency of water leads to decreased growth and hence, the plant gives a stunted appearance.
- Reduced yield: According to Tumarov (1925) a single wilting reduces yield by 50%. It is because decreased availability of water inside the plant checks meristematic activity and hence, the formation of flowers, fruits and seeds.
- Abscisic acid: Water stress produces abscisic acid which prevents several plant processes and promotes abscission of leaves, flowers and fruits.
- Wastage of energy: Since 98-99% of water absorbed is lost through transpiration. The energy used in absorption and conduction of water goes waste.
COMPARISON OF OPENING AND CLOSING OF STOMATA IN LIGHT AND DARK
|GUARD CELLS IN LIGHT||GUARD CELLS IN DARK|
|1. Carbon dioxide accumulated in the intercellular spaces and that released in the process of respiration is utilized in photosynthesis.||1. Carbon dioxide released in respiration accumulates in the intercellular spaces and guard cells because it is not utilized in photosynthesis.|
|2. pH of the guard cells increases to 7.5 and becomes alkaline.||2. pH of the guard cells decreases to 5 and becomes acidic.|
|3. In the alkaline medium of guard cells, starch is hydrolyzed into the sugar in the presence of enzyme phosphorylase.||3. In the acidic medium of guard cells, sugar is converted into starch in the presence of enzyme phosphorylase.|
|4. The osmotic pressure of the guard cells, increases as the concentration of sugar increases in their cell sap.||4. The osmotic pressure of the guard cells, decreases due to conversion of sugar into starch.|
|5. Water from adjoining cells move into the guard cells due to increased concentration of their cell sap.||5. Water from the guard cells passes into the adjoining epidermal cells due to decrease in the osmotic concentration of the guard cells.|
|6. Due to endosmosis, the turgidity of guard cells increases resulting in the opening of stomata.||6. Due to exosmosis, the turgidity of guard cells decreases resulting in the closing of stomata.|
DIFFERENCE BETWEEN STOMATA AND HYDATHODES
|1. Stomata are present in the aerial parts of all the land plants.||1. Hydathodes occur in the leaves of only a few plants|
|2. They are found on the surfaces of both the leaves and the young parts of the stem, flowers, fruits etc.||2. They occur on the margin and tips of the leaves.|
|3. Guard cells contain chloroplast.||3. Cells bordering a water pore are usually achlorophyllous.|
|4. Guard cells may be surrounded by subsidiary cells.||4. Subsidiary cells are absent.|
|5. The turgor changes in guard cells lead to opening and closing of the stomata.||5. Hydathodes possess permanent pores because the guard cells surrounding them are immobile.|
|6. Stomata pass out water vapour.||6. Hydathodes send out liquid water.|
|7. Along with water vapours, stomata allow passage of carbon dioxide and oxygen.||7. Small quantities of solute also pass out dissolved in guttation fluid.|
|8. Each stoma leads internally to a substomatal cavity.||8. Hydathodes possess loosely arranged epithem cells below its pore.|
|9. They do not have any connection with a vein ending.||9. Hydathodes contain a vein ending.|