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Physiological Importance
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MCQ Questions
1. A well-watered herbaceous plant is covered with a bell jar overnight, and droplets appear at the margins of its leaves by morning. Which physiological significance of root pressure is best demonstrated?
A.
It provides the main force for water transport through the xylem during intense daytime transpiration
B.
It can drive the upward movement of xylem sap and cause guttation when transpiration is low
C.
It directly forces water through the stomatal pores by increasing guard-cell turgor
D.
It increases evaporation from the leaf surface by maintaining a wet cuticle
Correct Answer:
It can drive the upward movement of xylem sap and cause guttation when transpiration is low
Explanation:
Root pressure develops when roots absorb ions and water enters the xylem osmotically. When transpiration is low, this positive pressure can push xylem sap upward and force water out through hydathodes as guttation.
2. Which observation most strongly supports the physiological importance of root pressure in restoring the continuity of water columns in xylem vessels?
A.
Exudation of xylem sap from a freshly cut stem of a well-watered plant
B.
Rapid wilting of a plant after its stomata open in dry air
C.
Increased carbon dioxide uptake by leaves during daylight
D.
Formation of a thicker cuticle on leaves exposed to sunlight
Correct Answer:
Exudation of xylem sap from a freshly cut stem of a well-watered plant
Explanation:
Sap exudation from a cut stem indicates positive pressure within the xylem. This pressure, generated partly by root activity, can help refill or restore water continuity in xylem conduits, especially when transpiration is weak.
3. Why is root pressure considered physiologically useful but insufficient to account for the ascent of sap in tall trees?
A.
Root pressure occurs only in leaves and is absent from roots
B.
Root pressure is generally too small and intermittent to lift water through the entire height of tall trees
C.
Root pressure moves water downward but never upward through xylem
D.
Root pressure operates only when xylem vessels contain air rather than water
Correct Answer:
Root pressure is generally too small and intermittent to lift water through the entire height of tall trees
Explanation:
Root pressure can contribute to short-distance upward movement, guttation, and repair of water columns, but its magnitude and occurrence are usually inadequate for transporting water to the crowns of tall trees. Transpiration pull is more important there.
4. A plant is transferred from a humid chamber to dry, windy air. Which change best explains why the relative contribution of root pressure to upward water transport becomes physiologically less important?
A.
The increased transpiration demand produces a much stronger transpiration pull
B.
Dry air immediately stops ion absorption by roots
C.
Wind eliminates the cohesion between water molecules in xylem
D.
Root pressure changes from positive pressure to atmospheric pressure in every root cell
Correct Answer:
The increased transpiration demand produces a much stronger transpiration pull
Explanation:
Root pressure is most evident when transpiration is low. In dry, windy air, evaporation increases and the tension generated by transpiration becomes the dominant force moving water upward, making root pressure relatively less significant.
5. Which sequence most accurately describes how root pressure can contribute to recovery from a limited xylem embolism during periods of low transpiration?
A.
Ion accumulation in roots lowers xylem water potential, water enters the xylem, and positive pressure helps push water into affected conduits
B.
Stomatal opening increases evaporation, which directly dissolves all air bubbles in xylem vessels
C.
Root pressure removes ions from the xylem, causing water to leave the vessels and compress the air bubbles
D.
Cuticular transpiration generates negative pressure that forces air out through the roots
Correct Answer:
Ion accumulation in roots lowers xylem water potential, water enters the xylem, and positive pressure helps push water into affected conduits
Explanation:
Active solute accumulation in the root xylem promotes water entry and raises hydrostatic pressure. This positive pressure may assist refilling or restoring continuity in some embolized xylem elements, particularly under low-transpiration conditions.
6. In a plant showing abundant guttation at night, which inference about root pressure is most justified?
A.
Root pressure is sufficiently positive to force xylem water through specialized leaf water-excreting structures
B.
Root pressure has completely replaced transpiration pull as the dominant force in all xylem transport
C.
Root pressure has caused stomata to open maximally despite darkness
D.
Root pressure has increased the vapor-pressure deficit between the leaf and atmosphere
Correct Answer:
Root pressure is sufficiently positive to force xylem water through specialized leaf water-excreting structures
Explanation:
Guttation occurs mainly when soil water is abundant and transpiration is low. Positive root pressure pushes xylem sap out through hydathodes, producing visible droplets at leaf margins or tips.
7. Which experimental result would indicate that root pressure has a role in maintaining upward xylem water movement rather than merely supplying water to root cells?
A.
A decapitated, well-watered stem releases xylem sap from its cut surface
B.
Root hairs increase their surface area during water shortage
C.
Leaves lose water more rapidly in moving air
D.
Guard cells accumulate potassium before stomatal opening
Correct Answer:
A decapitated, well-watered stem releases xylem sap from its cut surface
Explanation:
Sap release from a cut stem shows that pressure generated below the cut is transmitted through the xylem. This demonstrates that root-generated positive pressure can push water upward through the stem.
8. Why does root pressure often become physiologically apparent at night or during very humid weather?
A.
Reduced transpiration allows positive pressure generated in the root xylem to accumulate and become observable
B.
Darkness increases the cohesion of water molecules more than light does
C.
Humidity causes the xylem to actively secrete water into the atmosphere
D.
Night-time respiration reverses the direction of xylem transport in the stem
Correct Answer:
Reduced transpiration allows positive pressure generated in the root xylem to accumulate and become observable
Explanation:
At night or under humid conditions, the loss of water from leaves is small. Consequently, root-generated positive pressure is less opposed by transpiration-driven tension and may produce exudation or guttation.
9. A researcher removes the roots from a leafy shoot and observes that the shoot still transports water for some time in bright sunlight. What does this result imply about the physiological role of root pressure?
A.
Root pressure is unnecessary for every form of water movement in plants
B.
Transpiration pull can maintain water movement temporarily, whereas root pressure is not the sole driving force
C.
Root pressure is generated mainly in leaves rather than roots
D.
Xylem transport depends only on active pumping by living vessel elements
Correct Answer:
Transpiration pull can maintain water movement temporarily, whereas root pressure is not the sole driving force
Explanation:
Water may continue moving because evaporation from leaves creates tension in the xylem. This demonstrates that root pressure has supportive physiological roles, but it is not the only mechanism responsible for upward water transport.
10. Which combination correctly identifies two physiological situations in which root pressure is especially useful?
A.
Driving guttation during low transpiration and assisting the restoration of water continuity in xylem
B.
Maintaining maximum transpiration during drought and opening stomata in darkness
C.
Replacing photosynthesis in roots and transporting sugars through phloem
D.
Cooling leaves by evaporation and directly fixing atmospheric carbon dioxide
Correct Answer:
Driving guttation during low transpiration and assisting the restoration of water continuity in xylem
Explanation:
Root pressure is most evident when transpiration is low. Under such conditions, it can produce guttation and may help push water into or through xylem conduits, contributing to the restoration of water continuity.
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