Introduction
Abiotic stresses are non-living environmental factors that negatively affect plant growth, development, and yield.
These stresses include drought, salinity, extreme temperatures, waterlogging, and nutrient imbalances.
They disrupt physiological, biochemical, and molecular processes in plants and are responsible for significant yield losses globally (30–50% in major crops).
Breeding for abiotic stress tolerance aims to develop varieties that maintain high productivity under adverse environmental conditions, ensuring food security.
Major Abiotic Stresses and Their Effects
1. Drought Stress
Definition: Drought occurs when water availability is insufficient for normal plant growth due to low rainfall or poor irrigation.
Effects on Plants: Reduced cell expansion (stunted growth), leaf rolling, wilting, early senescence, reduced photosynthesis, impaired translocation of assimilates, and shortened grain-filling period leading to lower yield.
Physiological Responses: Osmotic adjustment (accumulation of proline, glycine betaine, sugars), stomatal regulation to reduce water loss, and deeper root system development.
Breeding Strategies: Escape mechanism (early-maturing varieties), avoidance mechanism (deep roots and efficient water uptake), tolerance mechanism (osmotic adjustment, antioxidant activity).
Techniques: Field screening under controlled drought, selection based on leaf rolling, relative water content, stomatal conductance, and marker-assisted selection for drought QTLs like DRO1 in rice.
Effects on Plants: Reduced cell expansion (stunted growth), leaf rolling, wilting, early senescence, reduced photosynthesis, impaired translocation of assimilates, and shortened grain-filling period leading to lower yield.
Physiological Responses: Osmotic adjustment (accumulation of proline, glycine betaine, sugars), stomatal regulation to reduce water loss, and deeper root system development.
Breeding Strategies: Escape mechanism (early-maturing varieties), avoidance mechanism (deep roots and efficient water uptake), tolerance mechanism (osmotic adjustment, antioxidant activity).
Techniques: Field screening under controlled drought, selection based on leaf rolling, relative water content, stomatal conductance, and marker-assisted selection for drought QTLs like DRO1 in rice.
2. Salinity Stress
Definition: Salinity stress occurs due to excessive accumulation of soluble salts, mainly Na⁺ and Cl⁻, causing osmotic and ionic stress.
Effects: Reduced germination, growth inhibition, ion toxicity (enzyme inhibition, membrane damage), nutrient imbalance (K⁺ deficiency).
Physiological Adaptations: Ion exclusion, tissue tolerance through vacuolar compartmentalization, osmolyte accumulation.
Breeding Strategies: Selection of tolerant genotypes (barley, cotton, sugar beet), screening for Na⁺/K⁺ ratio, chlorophyll content, osmotic adjustment, molecular breeding using HKT genes, transgenic approaches expressing AtNHX1 or SOS1.
Effects: Reduced germination, growth inhibition, ion toxicity (enzyme inhibition, membrane damage), nutrient imbalance (K⁺ deficiency).
Physiological Adaptations: Ion exclusion, tissue tolerance through vacuolar compartmentalization, osmolyte accumulation.
Breeding Strategies: Selection of tolerant genotypes (barley, cotton, sugar beet), screening for Na⁺/K⁺ ratio, chlorophyll content, osmotic adjustment, molecular breeding using HKT genes, transgenic approaches expressing AtNHX1 or SOS1.
3. Temperature Stress
(a) High Temperature / Heat Stress
Effects: Protein denaturation, reduced pollen viability, increased respiration, reduced photosynthesis, and yield loss.
Breeding Strategies: Select for pollen fertility under high temperature, identify genotypes producing heat-shock proteins (HSPs), develop early/late sown cultivars, marker-assisted selection for thermotolerance genes.
Breeding Strategies: Select for pollen fertility under high temperature, identify genotypes producing heat-shock proteins (HSPs), develop early/late sown cultivars, marker-assisted selection for thermotolerance genes.
(b) Low Temperature / Cold Stress
Effects: Impaired germination, seedling establishment, membrane damage due to ice crystals, delayed growth and flowering.
Breeding Strategies: Selection of cold-tolerant germplasm in rice, maize, barley; breeding for membrane stability, antifreeze protein production; wide hybridization with wild relatives like Oryza rufipogon.
Breeding Strategies: Selection of cold-tolerant germplasm in rice, maize, barley; breeding for membrane stability, antifreeze protein production; wide hybridization with wild relatives like Oryza rufipogon.
4. Flooding / Waterlogging
Definition: Excess water around roots reduces oxygen availability, causing hypoxia or anoxia.
Effects: Root suffocation, reduced nutrient uptake, accumulation of toxic metabolites, lodging, and yield reduction.
Breeding Strategies: Development of submergence-tolerant rice (e.g., Swarna-Sub1 with Sub1A gene), selection for aerenchyma formation to improve oxygen diffusion, screening maize and wheat for waterlogging tolerance.
Effects: Root suffocation, reduced nutrient uptake, accumulation of toxic metabolites, lodging, and yield reduction.
Breeding Strategies: Development of submergence-tolerant rice (e.g., Swarna-Sub1 with Sub1A gene), selection for aerenchyma formation to improve oxygen diffusion, screening maize and wheat for waterlogging tolerance.
5. Nutrient Deficiencies and Toxicities
Effects: Deficiency reduces growth and grain quality (Zn, Fe, N, P); toxicity due to Al³⁺, Fe²⁺, or boron.
Breeding Strategies: Screening under nutrient-deficient conditions, selecting genotypes with high nutrient use efficiency, biofortification (Zn- and Fe-rich rice/wheat), use of tolerance genes like PSTOL1 for phosphorus deficiency.
Breeding Strategies: Screening under nutrient-deficient conditions, selecting genotypes with high nutrient use efficiency, biofortification (Zn- and Fe-rich rice/wheat), use of tolerance genes like PSTOL1 for phosphorus deficiency.
Breeding Approaches for Abiotic Stress Tolerance
1. Conventional Breeding: Pedigree selection, recurrent selection, hybridization under stress, use of landraces and wild relatives.
2. Physiological & Morphological Selection: Traits like deep roots, leaf rolling, stomatal regulation, chlorophyll stability, osmotic adjustment.
3. Molecular Breeding: Marker-assisted selection for QTLs/genes controlling stress tolerance, genomic selection for polygenic traits.
4. Biotechnological Approaches: Transgenic expression of stress-responsive genes (DREB, HSP, NHX, PSTOL1), genome editing (CRISPR-Cas9).
5. Pre-breeding & Wide Hybridization: Introgression of tolerance genes from wild relatives, combining multiple stress tolerances in elite cultivars.
2. Physiological & Morphological Selection: Traits like deep roots, leaf rolling, stomatal regulation, chlorophyll stability, osmotic adjustment.
3. Molecular Breeding: Marker-assisted selection for QTLs/genes controlling stress tolerance, genomic selection for polygenic traits.
4. Biotechnological Approaches: Transgenic expression of stress-responsive genes (DREB, HSP, NHX, PSTOL1), genome editing (CRISPR-Cas9).
5. Pre-breeding & Wide Hybridization: Introgression of tolerance genes from wild relatives, combining multiple stress tolerances in elite cultivars.
Examples of Success in Abiotic Stress Breeding
| Crop | Stress Type | Tolerant Variety/Line | Mechanism / Key Gene |
|---|---|---|---|
| Rice | Flooding | Swarna-Sub1 | Sub1A gene |
| Rice | Drought | IR64-Drought | Deep root system, drought QTLs |
| Wheat | Heat | Heat-tolerant lines | Pollen fertility under high temperature |
| Barley | Salinity | N/A | Natural salt tolerance used as donor |
| Maize | Drought | Drought-tolerant hybrids | QPM and CIMMYT selections |
