biology : class11 Reactive Oxygen Species : Functions, Stress Response, Dormancy : Abscisic Acid MCQ Questions and Answers
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Reactive Oxygen Species
MCQ questions and answers.
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MCQ Questions
1. During abscisic acid-induced stomatal closure under drought, which sequence most accurately describes the role of reactive oxygen species?
A.
ABA stimulates ROS production, ROS activate Ca2+ channels, and the Ca2+ signal promotes anion efflux and guard-cell turgor loss
B.
ABA removes ROS from guard cells, preventing ion-channel activation and causing stomatal opening
C.
ROS directly hydrolyze guard-cell starch into sucrose, causing water uptake and stomatal closure
D.
ABA converts hydrogen peroxide into oxygen, and the released oxygen closes stomata by increasing guard-cell respiration
Correct Answer:
ABA stimulates ROS production, ROS activate Ca2+ channels, and the Ca2+ signal promotes anion efflux and guard-cell turgor loss
Explanation:
ABA can induce ROS generation in guard cells. ROS, particularly hydrogen peroxide, act as signaling molecules that promote Ca2+ influx and downstream ion efflux, reducing guard-cell turgor and closing the stomatal pore.
2. Which observation would most strongly support the conclusion that hydrogen peroxide acts as a signaling molecule rather than only as a damaging by-product during ABA-induced stomatal closure?
A.
A low, transient hydrogen peroxide increase occurs before cytosolic Ca2+ elevation and stomatal closure
B.
Hydrogen peroxide accumulates only after guard cells have lost all membrane integrity
C.
Hydrogen peroxide concentration remains unchanged after ABA treatment but lipid peroxidation increases
D.
Removing hydrogen peroxide from the medium increases the ABA-induced Ca2+ signal
Correct Answer:
A low, transient hydrogen peroxide increase occurs before cytosolic Ca2+ elevation and stomatal closure
Explanation:
A transient ROS increase preceding Ca2+ signaling and stomatal closure is consistent with hydrogen peroxide functioning upstream as a signal. Damage alone would more likely be associated with prolonged, excessive ROS accumulation and loss of cellular integrity.
3. A plant mutant has normal ABA synthesis but lacks a guard-cell NADPH oxidase required for ABA-stimulated ROS production. Which phenotype is most likely under drought?
A.
Enhanced stomatal closure because ROS toxicity directly destroys guard-cell membranes
B.
Delayed or reduced stomatal closure because the ROS-dependent signaling step is impaired
C.
Normal stomatal closure because ROS are produced only in chloroplasts and cannot affect guard cells
D.
Permanent stomatal closure because ABA cannot be degraded in the absence of NADPH oxidase
Correct Answer:
Delayed or reduced stomatal closure because the ROS-dependent signaling step is impaired
Explanation:
Guard-cell NADPH oxidases can generate ROS in response to ABA. If this ROS burst is absent, downstream Ca2+-dependent ion-channel regulation and turgor loss are weakened, reducing or delaying stomatal closure.
4. Why is catalase overexpression expected to weaken some ABA-induced stomatal responses even though it improves protection against oxidative damage?
A.
Catalase converts hydrogen peroxide into water and oxygen, reducing a ROS signal needed for downstream guard-cell signaling
B.
Catalase converts ABA into hydrogen peroxide, preventing ABA from binding its receptor
C.
Catalase blocks all Ca2+ transport by removing oxygen from the cytosol
D.
Catalase increases superoxide formation, which permanently activates stomatal opening
Correct Answer:
Catalase converts hydrogen peroxide into water and oxygen, reducing a ROS signal needed for downstream guard-cell signaling
Explanation:
Hydrogen peroxide has a dual role: excessive accumulation causes oxidative damage, whereas controlled accumulation participates in ABA signaling. Increased catalase can lower hydrogen peroxide below the level required for effective guard-cell signal transduction.
5. Which relationship best explains how plants maintain ROS signaling during ABA-mediated stress responses without allowing ROS to become predominantly toxic?
A.
They completely eliminate ROS immediately after ABA perception
B.
They balance ROS production with antioxidant systems, allowing localized and transient ROS accumulation
C.
They convert all ROS directly into ABA, making antioxidant enzymes unnecessary
D.
They store ROS permanently in vacuoles so that ROS cannot participate in signaling
Correct Answer:
They balance ROS production with antioxidant systems, allowing localized and transient ROS accumulation
Explanation:
ROS signaling depends on controlled concentration, location, and duration. Antioxidant enzymes such as catalase and peroxidases restrict excessive accumulation while permitting a transient ROS signal during ABA responses.
6. A seed treated with ABA shows increased hydrogen peroxide at the onset of dormancy. Which interpretation is most appropriate?
A.
The hydrogen peroxide increase may function as a regulated signal contributing to dormancy establishment, provided it remains controlled
B.
The hydrogen peroxide increase proves that the seed has irreversibly lost viability
C.
Hydrogen peroxide must be acting only as a respiratory substrate that directly replaces oxygen
D.
The increase necessarily indicates that ABA has been completely inactivated
Correct Answer:
The hydrogen peroxide increase may function as a regulated signal contributing to dormancy establishment, provided it remains controlled
Explanation:
In seeds, ABA-associated ROS changes can participate in signaling that favors dormancy and stress adaptation. A controlled ROS increase does not necessarily indicate damage; excessive or prolonged accumulation, however, can impair viability.
7. Which experimental result would best distinguish a signaling role for ROS from a purely damaging role during ABA treatment of seedlings?
A.
A ROS scavenger abolishes an ABA-induced gene-expression response without causing detectable reduction in cell viability
B.
ABA-treated tissues show membrane leakage only after extensive tissue death
C.
ROS levels increase in dead cells but not in living cells after ABA treatment
D.
Antioxidant treatment increases lipid peroxidation while reducing ROS fluorescence
Correct Answer:
A ROS scavenger abolishes an ABA-induced gene-expression response without causing detectable reduction in cell viability
Explanation:
If scavenging ROS prevents an ABA-responsive gene expression change while cells remain viable, ROS are implicated as necessary signaling intermediates rather than merely agents of nonspecific oxidative damage.
8. Under drought, excessive ROS accumulation in a leaf is most likely to compromise ABA-mediated stress adaptation by causing which effect?
A.
Oxidative damage to membranes and proteins, disrupting the cellular processes required for regulated stomatal control
B.
Permanent enhancement of antioxidant enzyme activity without any cellular injury
C.
Direct conversion of guard-cell anions into sugars, increasing stomatal aperture
D.
Complete prevention of ABA synthesis because ROS destroy all chloroplast pigments immediately
Correct Answer:
Oxidative damage to membranes and proteins, disrupting the cellular processes required for regulated stomatal control
Explanation:
ROS are beneficial as controlled signals but harmful when they accumulate excessively. Oxidation of membrane lipids and proteins can impair ion transport, membrane integrity, and the coordinated guard-cell responses needed for stomatal regulation.
9. A researcher applies ABA to two leaf samples. In sample X, ROS production is blocked; in sample Y, ROS production is normal but Ca2+ channels are inhibited. Which result is most consistent with ROS acting upstream of Ca2+ signaling?
A.
Both samples show normal Ca2+ elevation and normal stomatal closure
B.
Sample X shows reduced Ca2+ elevation, while sample Y may show ROS accumulation but reduced stomatal closure
C.
Sample X shows increased Ca2+ elevation, while sample Y shows no ROS formation
D.
Both samples show increased stomatal opening because ABA is converted into a ROS scavenger
Correct Answer:
Sample X shows reduced Ca2+ elevation, while sample Y may show ROS accumulation but reduced stomatal closure
Explanation:
If ROS lie upstream of Ca2+, blocking ROS production should reduce the ABA-induced Ca2+ signal. Blocking Ca2+ channels can leave ROS production intact but prevent the downstream ionic events required for stomatal closure.
10. Which statement most accurately describes the importance of ROS concentration during ABA-related stress responses?
A.
ROS are always harmful, so any ABA response requires their complete removal
B.
ROS are always beneficial, so increasing them continuously produces stronger stress tolerance
C.
A moderate, transient ROS signal can activate defense responses, whereas excessive or prolonged ROS causes oxidative injury
D.
ROS have no signaling role and influence stress responses only by changing the color of leaves
Correct Answer:
A moderate, transient ROS signal can activate defense responses, whereas excessive or prolonged ROS causes oxidative injury
Explanation:
The biological effect of ROS depends on their dose, duration, and cellular location. ABA-associated ROS can act as second messengers in stress signaling, but uncontrolled accumulation damages cellular components and reduces stress tolerance.
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