Transpiration
Short Questions and Answers
One mark questions with answers
1. Maximum transpiration occurs in
(a) mesophytes
(b) hydrophytes
(c) xerophytes
(d) algal cells
Answer: (a) mesophytes
2. The loss of water through cuticle may reach upto
(a) 5%
(b) 10%
(c) 20%
(d) 40%
Answer: (b) 10%
3. Stomatal closure in response to leaf stress is mediated by
(a) IAA
(b) IBA
(c) ABA
(d) 2, 4 D
Answer: (c) ABA
4. In which of the following plants, stomata open during night and close during day?
(a) maize
(b) Vallisneria
(c) cacti
(d) lily
Answer: (c) cacti
5. Ascorbic acid regulates
(a) stomatal opening
(b) stomatal closing
(c) both a and b
(d) none of the above
Answer: (b) stomatal closing
6. When plants are exposed to light, the guard cells accumulate large amounts of potassium ions which is due to
(a) active exchange process
(b) negative exchange process
(c) passive exchange process
(d) none of the above
Answer: (a) active exchange process
7. Which of the following is used for measuring the rate of transpiration?
(a) cobalt chloride paper
(b) 4 leaf experiment
(c) potometer
(d) bell jar
Answer: (c) potometer
8. At low atmospheric pressure, the rate of transpiration
(a) decreases
(b) gradually increases
(c) decreases rapidly
(d) remains unaffected
Answer: (b) gradually increases
9. When half of the leaves of a plant are removed at random, which one of the following is likely to happen?
(a) magnitude of transpiration increases, transpirational flux decreases
(b) magnitude of transpiration decreases, transpirational flux increases
(c) both magnitude of transpiration and transpirational flux increases
(d) both magnitude of transpirtation and transpirational flux decreases
Answer: (b) magnitude of transpiration decreases, transpirational flux increases
10. Which one of the following factors is most important in the regulation of transpiration?
(a) humidity
(b) light
(c) temperature
(d) wind
Answer: (a) humidity
Two marks questions with answers
1. Mention two differences between evaporation and transpiration.
Answer: Evaporation is a physical process which is controlled by the environmental condition like relative humidity and air current whereas transpiration is both a physical and physiological process.
In evaporation water vapours are formed at the free surface of water In case of transpiration water vapours are formed mostly in the internal tissues of the plant.
2. How does carbon dioxide concentration affect the stomatal movement?
Answer: Stomatal closure occurs if concentration of carbon dioxide in the external atmosphere increases, both in light and in dark. It is because high concentration of carbon dioxide reduces the pH of the guard cells which promotes conversion of sugar into starch. Stomata open in the light because in light carbon dioxide is utilised in photosynthesis, hence, its concentration decreases.
3. Mention two differences between transpiration and guttation.
Answer: Water is lost in the form of vapours in transpiration whereas in guttation, water exudes in the form of liquid drops.
In transpiration water is lost in the purest form whereas guttation fluid contains many inorganic and organic salts.
4. Transpiration is a ‘necessary evil’. Explain.
Answer: Transpiration has been described as a necessary evil because it cannot be avoided though potentially harmful. The stomata remain open for exchange of gases. This also results in the loss of water as vapours. Consequently, the water level in the soil is reduced causing wilting of plants. The stomata however cannot be closed to prevent water loss because this would also stop gaseous exchange needed for respiration and photosynthesis.
5. Mention three advantages of transpiration.
Answer: (1). Transpiration helps in rapid upward movement of sap. Transpiration pull in upward direction is responsible for mass movement of water in upward direction.
(2). It helps in absorption and translocation of solutes.
(3). Transpiration regulates plant temperature. It has a significant role in the cooling of leaves.
Three marks questions with answers
1. What are the different types of transpiration?
Answer: The three types of transpiration are: Stomatal transpiration: Stomata are minute pores on the epidermis of leaves. In herbaceous plants, stomata are also found on the epidermis of green stem. They form a connecting link between internal tissue and surrounding atmosphere. Stomatal transpiration accounts for 80%-90% total water loss of the plant.
Cuticular transpiration: Cuticle is a wax-like layer of cutin that covers the epidermis of leaves and stems. Cuticles present on the surface of the epidermis reduces the direct evaporation of water. If the cuticle is thin, up to 20% of total transpiration may take place through it. But as its thickness increases, the extent of water vapour loss is significantly reduced. Cuticular transpiration accounts for about 10% of the total amount of transpiration from leaves under ordinary condition.
Lenticular transpiration: Lenticels are small openings in the cork of woody stem, twigs and fruits. Water vapours are lost through these openings. Loss of water through lenticels present on woody stem and fruit is called lenticular transpiration. It accounts for 1% of total water loss of a plant.
2. In a windy day transpiration increases initially but later decreases gradually. Explain giving reasons.
Answer: In still air water vapours (diffusing out of the leaf) form a highly saturated atmosphere around the leaves. This reduces the steepness of vapour pressure gradient and the rate of transpiration is also reduced. Breeze (gentle wind) removes humid air present around the leaves and the rate of transpiration increases. Stronger winds cause bending and fluttering of the leaves, forcing water vapours present in the intercellular spaces to come out. Therefore, the rate of transpiration increases.
Winds of very high velocity or storms increase the rate of transpiration initially. Due to excessive loss of water the guard cells become flaccid. As a result, the stomata close at a later stage and transpiration stops.
3. What is the importance of potassium ions in the opening and closing of stomata?
Answer: S. Imamura and M. Fujino (1959) found a direct correlation between stomatal movement and K+ ion concentration of guard cells. Fujino proposed that stomatal opening and closing are a result of an active transport of K+ ion into the guard cells. They showed the accumulation of K+ in the guard cells during stomatal opening. By using the electron probe micro analyzer, it has been found that opening of stomata in light is accompanied by the transport of K+ into the guard cells from the adjacent cells. It has been found that anions which balance the positive charge of K+ are those of organic acid especially malic acid. The influx of K+ into the guard cells is accompanied by the synthesis of malic acid.
4. Define wilting. What are the different types of wilting?
Answer: Wilting may be defined as drooping of leaves due to loss of turgidity. It occurs when transpiration exceeds the amount of water absorbed by the plant. On bright warm days the rate of transpiration exceeds the rate of absorption, creating water deficiency in the cells of the leaf. Thus, there is a partial loss of turgidity that does not cause visible wilting. Such a process is called incipient wilting.
In dry and hot summer afternoon, the leaves wilt due to high rate of transpiration, causing loss of turgor. The wilted leaves and twigs recover at night when water absorption is more than transpiration. This is called as temporary wilting.
Sometimes, the amount of available soil water is so low that it is not useful to the plants. At this stage there is a general loss of turgidity and plants do not recover even after watering. This condition is referred to as permanent wilting.
5. How does the atmospheric humidity affect the rate of transpiration?
Answer: The rate of transpiration depends upon the difference in the vapour pressure between the internal atmosphere of the leaf and air outside the leaf. Since relative humidity is the actual amount of water vapours in the atmosphere at any given time, the rate of transpiration is mainly dependent on it.
Low humidity (i.e., low vapour pressure) outside the leaf favours transpiration because in such a situation, vapour pressure gradient between the internal leaf atmosphere and the external atmosphere is very sharp.
As the concentration of water vapours in the external atmosphere rises, the vapour pressure gradient becomes less steep and the rate of transpiration also decreases.
The vapour pressure of the atmosphere decreases with altitude just like atmospheric pressure (it means the air is drier at high altitudes). Therefore, high altitude plants often show xeromorphic adaptations to reduce transpiration.