Transpiration in Plants : Its Factors and Significance MCQ - Practice Questions with Answers

• Transpiration is the evaporative loss of water by plants. It occurs mainly through stomata. 
• Transpiration has more than one purpose; it
  • creates transpiration pull for absorption and transport of plants
  • supplies water for photosynthesis
  • transports minerals from the soil to all parts of the plant
  • cools leaf surfaces, sometimes 10 to 15 degrees, by evaporative cooling
  • maintains the shape and structure of the plants by keeping cells turgid
• There are three types of transpiration:

1.  Stomatal Transpiration: Water vapour diffuses out through minute pores (stomata) present in soft aerial part of plant is known as Stomatal Transpiration. Of the total water loss, near about 85 – 90% of water is lost by the stomatal transpiration.
2. Lenticular Transpiration: Sometimes water may evaporate through certain other openings present on the older stems. These openings are called Lenticels and the transpiration that takes place through the term is known as Lenticular Transpiration. The water lost by lenticular transpiration was about 0.1% of the total transpiration loss.
3. Cuticular Transpiration: Loss of water may also take place through the cuticle, but the amount so lost is relatively small and makeup only about 5 to 10 percent of the total transpiration. This type of transpiration depends upon the thickness of the cuticle and presence or absence of wax coating on the surface of the leaves. Xerophytic plants generally have a very thick cuticle and wax coating on the leaves and stem in order to check cuticular transpiration.
    

1. Relative Humidity:  The rate of transpiration is inversely proportional to the relative humidity, i.e., the rate of transpiration is higher when the relative humidity is lower and lower when the relative humidity is higher.

2. Atmospheric Temperature: A high temperature opens stomata even in darkness. Besides producing a heating effect, it lowers the relative humidity of the air and increases vapour pressure inside transpiring organ. But very high temperature may cause desiccation and closure of stomata. Very low temperature also closes the stomata and hence decreases the rate of transpiration.

3. Light: In the majority of plants stomata open in the presence of light and close in darkness. A strong light further increases transpiration probably due to its heating effect. 

4. Atmospheric Pressure: Low atmospheric pressure enhances evaporation, produces air currents and increases the rate of transpiration.

5. Availability of Water: The rate of transpiration depends upon the rate of absorption of soil water by roots. This is further influenced by a number of soil factors like soil water, soil particles, soil temperature, soil air, etc. A decrease in water uptake by the root causes partial dehydration of the leaf cells resulting in closure of stomata and wilting.

6. Leaf Area (Transpiring Area): A plant with large leaf area will show more transpi­ration than another plant with less leaf area. However, the rate of transpiration per unit leaf area decreases in a canopy due to density of foliage, shading effect and decrease of air movement inside the canopy.

7. Root/Shoot Ratio: A low root/shoot ratio decreases the rate of transpiration while a high ratio increases the rate of transpiration. The latter is due to the fact that an extensive root system is more efficient in water uptake from soil. Increased availability of water also increases transpiration.

8. Leaf Structure:

• Thickness of Cuticle: decreases transpiration
• Number and Position of Stomata
• Sunken Stomata: device to reduce the rate of transpiration by providing an area where little air movement occurs.
• Mesophyll: Compact mesophyll (as having more of palisade tissue and fewer intercellular spaces) reduces transpiration while a loose mesophyll (having more of spongy tissue and larger intercellular spaces) increases transpiration.
• Leaf Modifications: prickles, leaf spines, scaly leaves, phyllodes, phylloclades (instead of leaves), are all modifications found in xerophytes to reduce transpi­ration.