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Water-Deficit Signaling
MCQ questions and answers.
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MCQ Questions
1. During progressive soil drying, which sequence most accurately represents the early root-to-shoot water-deficit signaling pathway involving abscisic acid?
A.
Root water deficit increases ABA synthesis or accumulation, ABA reaches guard cells, and stomatal closure reduces transpirational water loss
B.
Root water deficit first causes stomatal opening, which increases ABA breakdown in guard cells
C.
Root drying directly increases cytokinin transport to leaves, causing stomatal closure independently of ABA
D.
Root water deficit prevents ABA movement to leaves, so stomata close only after irreversible leaf dehydration
Correct Answer:
Root water deficit increases ABA synthesis or accumulation, ABA reaches guard cells, and stomatal closure reduces transpirational water loss
Explanation:
Water deficit promotes ABA accumulation and signaling, including root-to-shoot communication. In leaves, ABA acts on guard cells to promote stomatal closure, thereby limiting further water loss.
2. Why can a rise in leaf ABA concentration cause rapid stomatal closure before extensive loss of leaf water content occurs?
A.
ABA activates guard-cell signaling that changes ion transport and reduces guard-cell turgor
B.
ABA is converted into cellulose, which physically blocks stomatal pores
C.
ABA stimulates guard cells to accumulate potassium and water, increasing pore aperture
D.
ABA destroys chloroplasts in guard cells, stopping all leaf gas exchange permanently
Correct Answer:
ABA activates guard-cell signaling that changes ion transport and reduces guard-cell turgor
Explanation:
ABA functions as an early water-deficit signal. In guard cells, it promotes ion efflux and associated water loss, lowering guard-cell turgor and causing stomatal closure.
3. A plant mutant cannot perceive ABA in guard cells but can synthesize and transport ABA normally. Under water deficit, which response is most directly impaired?
A.
ABA-induced stomatal closure
B.
Synthesis of water in the roots
C.
Absorption of carbon dioxide by chloroplasts through active pumping
D.
Conversion of all leaf proteins into storage proteins
Correct Answer:
ABA-induced stomatal closure
Explanation:
ABA perception in guard cells is required to initiate the signaling cascade that changes ion-channel activity and reduces stomatal aperture. Normal ABA production alone is insufficient if guard-cell perception is defective.
4. Which observation would provide the strongest evidence that ABA participates in root-to-shoot signaling during soil water deficit?
A.
Detached leaves close their stomata only when supplied with ABA at concentrations comparable to those in water-stressed plants
B.
Water-stressed roots grow more rapidly than well-watered roots even when their ABA content is unchanged
C.
Leaves lose water faster after exposure to darkness, regardless of root treatment
D.
Roots absorb nitrate more slowly when exposed to high light
Correct Answer:
Detached leaves close their stomata only when supplied with ABA at concentrations comparable to those in water-stressed plants
Explanation:
This result would show that ABA can act as a transferable chemical signal capable of inducing the shoot response associated with root water deficit. The comparison with physiologically relevant concentrations strengthens the inference.
5. Under water deficit, why is stomatal closure considered both protective and potentially limiting for the plant?
A.
It conserves water but also restricts carbon-dioxide entry and may reduce photosynthesis
B.
It increases water loss while simultaneously increasing carbon-dioxide entry
C.
It prevents water conservation but enhances photosynthesis by opening internal air spaces
D.
It permanently blocks xylem transport while increasing leaf hydration
Correct Answer:
It conserves water but also restricts carbon-dioxide entry and may reduce photosynthesis
Explanation:
ABA-mediated stomatal closure decreases transpiration and helps maintain plant water status. However, closed stomata reduce carbon-dioxide diffusion into leaves, which can restrict photosynthetic carbon fixation.
6. A water-stressed plant has high ABA concentration in its leaves, but its stomata remain open. Which explanation most directly accounts for this result?
A.
The guard cells may be insensitive to ABA or unable to transmit the ABA signal
B.
High ABA necessarily causes stomata to open by increasing guard-cell turgor
C.
ABA acts only in roots and cannot influence guard-cell behavior
D.
Stomatal aperture is controlled exclusively by leaf temperature and cannot respond to chemical signals
Correct Answer:
The guard cells may be insensitive to ABA or unable to transmit the ABA signal
Explanation:
Stomatal closure depends on ABA perception and downstream signaling in guard cells. A defect in either perception or signal transduction can prevent closure even when ABA accumulates.
7. Which experimental comparison best distinguishes ABA-dependent stomatal closure from closure caused solely by a direct physical reduction in leaf water content?
A.
Compare stomatal responses of ABA-insensitive and normal plants exposed to similar soil water deficits
B.
Compare the leaf colors of plants grown under red and blue light
C.
Measure root length only in plants supplied with different nitrogen concentrations
D.
Compare seed size in plants grown under high and low humidity
Correct Answer:
Compare stomatal responses of ABA-insensitive and normal plants exposed to similar soil water deficits
Explanation:
If normal plants close stomata but ABA-insensitive plants do not under comparable water deficits, the difference supports an ABA-dependent signaling mechanism rather than a purely physical effect of dehydration.
8. Which change in guard-cell ion transport is most consistent with ABA-induced stomatal closure during water deficit?
A.
Enhanced export of ions from guard cells followed by water efflux and reduced guard-cell turgor
B.
Enhanced accumulation of potassium and organic solutes followed by water influx
C.
Complete inhibition of all membrane transport followed by permanent pore opening
D.
Increased proton-driven uptake of solutes followed by guard-cell swelling
Correct Answer:
Enhanced export of ions from guard cells followed by water efflux and reduced guard-cell turgor
Explanation:
ABA signaling activates ion efflux pathways in guard cells. The resulting loss of solutes lowers water potential, promotes water خروج from guard cells, decreases turgor, and closes the stomatal pore.
9. Why may ABA levels decline after a plant is rewatered following a period of water deficit?
A.
Improved water status reduces the stimulus for ABA accumulation and permits ABA inactivation or redistribution
B.
Rewatering converts ABA directly into chlorophyll in guard cells
C.
Water causes ABA to polymerize into an insoluble structural component of xylem
D.
Rewatering prevents all chemical signaling between roots and shoots
Correct Answer:
Improved water status reduces the stimulus for ABA accumulation and permits ABA inactivation or redistribution
Explanation:
ABA accumulation is linked to water-deficit conditions. After rewatering, the signal is reduced through decreased synthesis together with metabolism, inactivation, or redistribution of ABA, allowing stress responses to relax.
10. A researcher applies ABA to the leaves of a well-watered plant while keeping its roots hydrated. If stomata close, what conclusion is most justified?
A.
ABA can act directly in the shoot as a chemical signal for stomatal regulation
B.
Only root dehydration can cause stomatal closure, so the observation is impossible
C.
ABA must have increased water absorption by the roots before reaching the leaves
D.
The treatment proves that stomatal closure is unrelated to guard-cell turgor
Correct Answer:
ABA can act directly in the shoot as a chemical signal for stomatal regulation
Explanation:
Exogenous ABA applied to leaves can induce stomatal closure without root dehydration. This demonstrates that ABA is capable of functioning locally in the shoot, particularly in guard-cell water-deficit signaling.
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