Prep Guide: Transpiration

Key Concepts (Recall & Understanding)

Transpiration: The loss of water in the form of water vapor from the aerial parts of a plant, primarily through stomata.
Significance of Transpiration: Creates transpirational pull for ascent of sap, helps in mineral transport, and has a cooling effect on the plant.
Stomata: Pores on the leaf surface, flanked by two guard cells, that regulate gas exchange and water loss.
Potassium Ion Exchange Theory: Explains stomatal opening and closing. In light, K+ ions are actively pumped into guard cells, causing them to become turgid and open the stoma. In darkness or water stress, K+ ions leave, guard cells become flaccid, and the stoma closes.
Factors Affecting Transpiration:
- External: Light intensity, temperature, humidity, wind speed.
- Internal: Stomatal density, leaf area, cuticle thickness.
Adaptations to Reduce Transpiration: Sunken stomata, thick cuticle, rolled leaves, reduced leaf surface area (spines).
Guttation: The loss of water as liquid droplets from the leaf margins through hydathodes, caused by root pressure.
Bleeding: The exudation of sap from a cut or injured part of a plant, also due to root pressure.

Application Corner

Why is it better to water plants in the morning or evening rather than in the middle of a hot, sunny day?
- Answer: In the middle of a hot, sunny day, the rate of transpiration is at its highest. Much of the water given to the plant will be quickly lost to the atmosphere. Watering in the cooler parts of the day reduces this rapid water loss, allowing the plant to absorb and retain more water.
A plant with broad leaves is likely to transpire more than a plant with needle-like leaves. Explain why.
- Answer: Transpiration rate is directly related to the surface area available for water evaporation. Broad leaves have a much larger surface area compared to needle-like leaves, leading to a significantly higher rate of transpiration.

Analytical Thinking

Odd One Out: Transpiration, Guttation, Bleeding, Photosynthesis.
- Odd One: Photosynthesis.
- Category: The rest are processes related to the loss or movement of water/sap from a plant.
Scenario: A Ganong's potometer is used to measure the rate of water uptake by a shoot. If the experiment is conducted in a dark, humid room, how would the movement of the air bubble compare to an experiment conducted in a bright, windy room? Explain your reasoning.
- Answer: The movement of the air bubble would be much slower in the dark, humid room. Darkness would cause the stomata to close, and high humidity would reduce the water potential gradient between the leaf and the air. Both factors drastically reduce the rate of transpiration and, consequently, the rate of water uptake measured by the potometer. In a bright, windy room, the opposite would occur, leading to a rapid rate of transpiration and fast movement of the bubble.

Key Case Study

Xerophytes: Masters of Water Conservation

Xerophytes are plants adapted to survive in arid environments like deserts, where water is scarce and transpiration rates are potentially very high. Cacti are a prime example. They exhibit a range of adaptations to minimize water loss. Their leaves are reduced to spines, which drastically lowers the surface area for transpiration. Photosynthesis is carried out by their fleshy, green stems, which are covered by a thick, waxy cuticle. Their stomata are often sunken in pits and, in many cases, they use a special type of photosynthesis (CAM) where they only open their stomata at night to take in CO2. This case study perfectly illustrates how plant structure and function can be modified through evolution to overcome the environmental challenge of water scarcity.

Assertion-Reason Practice

Assertion (A): Guttation occurs most readily during the night or early morning. Reason (R): During the night, transpiration is low, and root pressure builds up, forcing water out through hydathodes.

(a) Both (A) and (R) are true and (R) is the correct explanation of (A). (b) Both (A) and (R) are true but (R) is not the correct explanation of (A). (c) (A) is true but (R) is false. (d) (A) is false but (R) is true.

Answer: (a) Both (A) and (R) are true and (R) is the correct explanation of (A). Guttation is a direct result of root pressure, which becomes effective only when the pull from transpiration is low, a condition that typically occurs at night.

HOTS (Higher-Order Thinking Skills) Question

While transpiration is often described as a "necessary evil" because of the significant water loss, it is also vital for the plant's survival. Imagine a hypothetical plant that has completely impermeable leaves and loses no water to transpiration. What would be the major physiological drawback of this adaptation?

Answer: The major drawback would be the loss of the transpirational pull. This pull is the primary driving force for the ascent of sap, which transports water and essential dissolved minerals from the roots to the leaves and other aerial parts of the plant. Without transpiration, the plant would have to rely solely on the much weaker force of root pressure, which is insufficient to lift water to the top of tall plants. Consequently, the plant would suffer from a severe lack of water and mineral nutrients in its leaves, which would inhibit photosynthesis and ultimately lead to its death.

Specimen Prep - Transpiration