Correct option is A
Explanation-
Reactive Oxygen Species (ROS) such as superoxide (O₂⁻), hydrogen peroxide (H₂O₂), and hydroxyl radicals (•OH) are generated as by-products of various metabolic pathways in plants—especially under stress conditions. ROS can damage lipids, proteins, and nucleic acids, but plants also use them in signaling. Therefore, plants have complex antioxidant systems (both enzymatic and non-enzymatic) to scavenge and manage ROS.
Statement A: "H₂O₂ is relatively more stable and travels relatively long distances." - True
Among ROS, hydrogen peroxide (H₂O₂) is relatively more stable than superoxide (O₂⁻) or hydroxyl radicals (•OH), which are highly reactive and short-lived. H₂O₂ is membrane-permeable and can act as a signaling molecule, moving from the site of production to distant parts of the cell or even between cells.
Statement B: "ROS is scavenged only through enzymatic reactions." - False
ROS scavenging occurs via both enzymatic and non-enzymatic mechanisms:
Enzymatic: Superoxide dismutase (SOD), catalase, ascorbate peroxidase (APX), glutathione reductase.
Non-enzymatic: Antioxidants like ascorbate (vitamin C), glutathione, tocopherols (vitamin E), carotenoids.
Incorrect — "only through enzymatic" is too restrictive.
Statement C: "Ascorbate-glutathione cycle is associated with the scavenging of ROS." - True
The ascorbate-glutathione (AsA-GSH) cycle is a major pathway for ROS detoxification in plant cells.
It removes H₂O₂ through a sequence of redox reactions involving:
1. Ascorbate peroxidase (APX) converts H₂O₂ to water using ascorbate.
2. Monodehydroascorbate reductase (MDAR) and dehydroascorbate reductase (DHAR) regenerate ascorbate.
3. Glutathione reductase (GR) regenerates glutathione.
Statement D: "Monodehydroascorbate reductase is not an antioxidant enzyme." - False
Monodehydroascorbate reductase (MDAR) is indeed an antioxidant enzyme. It is involved in the regeneration of ascorbate in the AsA-GSH cycle, maintaining the pool of reduced ascorbate needed to scavenge ROS.
Correct statement - A and C
Correct option is a - A and C
