1. Overview
Abiotic stresses such as drought, salinity, heat, cold, and nutrient deficiency
trigger complex biochemical reprogramming in plants. These responses involve
reactive oxygen species (ROS) signaling, antioxidant systems, osmolyte synthesis,
metabolic pathway shifts, and molecular regulation mechanisms.
2. Reactive Oxygen Species (ROS) Generation and Detoxification
Major ROS:
- Superoxide radical (O₂⁻)
- Hydrogen peroxide (H₂O₂)
- Hydroxyl radical (OH•)
- Singlet oxygen (¹O₂)
ROS are generated primarily in chloroplasts, mitochondria, and peroxisomes
during stress conditions such as drought and high light intensity.
Detoxification Pathways:
- Superoxide dismutase (SOD) converts O₂⁻ → H₂O₂
- Catalase (CAT) and ascorbate peroxidase (APX) convert H₂O₂ → H₂O
- Glutathione-ascorbate cycle maintains redox balance
3. Antioxidant Enzymes and Non-Enzymatic Defense Systems
Enzymatic Antioxidants
- Superoxide dismutase (SOD)
- Catalase (CAT)
- Peroxidases (APX, GPX)
- Glutathione reductase (GR)
Non-Enzymatic Antioxidants
- Ascorbic acid (Vitamin C)
- Glutathione (GSH)
- Carotenoids
- Flavonoids
- Tocopherols (Vitamin E)
These molecules protect cellular membranes, proteins, nucleic acids,
and lipids from oxidative damage.
4. Osmolyte Biosynthesis and Stress-Induced Metabolic Adjustments
Compatible Solutes (Osmoprotectants):
- Proline
- Glycine betaine
- Trehalose
- Sugars (sucrose, raffinose)
Osmolytes stabilize proteins, maintain cell turgor, and protect membranes
during drought and salinity stress.
Metabolic shifts include increased glycolysis, altered TCA cycle activity,
and reallocation of carbon and nitrogen metabolism.
5. Biochemical Markers of Nutrient Deficiency
Nitrogen Deficiency:
- Reduced chlorophyll content
- Lower nitrate reductase activity
Iron Deficiency:
- Chlorosis (yellowing of leaves)
- Reduced cytochrome activity
Phosphorus Deficiency:
- Reduced ATP synthesis
- Increased acid phosphatase activity
Biochemical markers are widely used in horticultural diagnostics
and precision nutrient management.
6. Metabolic Adaptation to Drought and Heat Stress
- Increased ABA (abscisic acid) synthesis
- Heat shock protein (HSP) accumulation
- Membrane lipid remodeling
- Enhanced antioxidant capacity
- Adjustment in photosynthetic enzyme activity
Heat stress destabilizes proteins and membranes; plants respond by
upregulating molecular chaperones and protective metabolites.
7. Applied Horticultural Relevance
- Breeding stress-tolerant varieties
- Biochemical screening of genotypes
- Precision nutrient management
- Improving yield stability under climate change
8. Free Learning Resources