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Plant bio
Abiotic stress
| Question | Answer |
|---|---|
| Plant stress | Stress: external factor that exerts a disadvantageous effect on a plant Usually measure effect on: Survival Crop yield Biomass Carbon or mineral uptake |
| Stress tolerance depends on: | Adaptation – genetically determined tolerance Acclimation – increase in tolerance in response to pre-exposure |
| Types of stress: | Abiotic: water deficit, salinity, heat and cold, O2 deficiency, light intensity. UV light Biotic: microbial pathogens, pests, weeds, parasitic plants - effects crop growth severely |
| Water deficit | Any water content of tissue below that of the most hydrated state |
| Developmental response to water deficit: decrease in leaf expansion and number | Rate of expansion is proportional to turgor pressure (Yp) and extensibility (m) |
| Water content and turgor pressure | Decreased cellular water content = lower turgor pressure Lower turgor pressure = decreased cell expansion Decreased cell expansion = decreased leaf size Decreased leaf size = decreased transpiration |
| Leaf number | In indeterminate plants, leaf number may also be reduced Note that decrease in expansion is an early indicator of water deficit, but the protective effects are quite slow (developmental) |
| Developmental response to water deficit: stimulation of root growth | The root biomass:shoot biomass ratio is determined by water availability If water availability exceeds photosynthetic capacity, shoot growth is favoured |
| If photosynthetic capacity exceeds water availability, root growth is favoured Soils tend to dry from above and plants respond to water deficit by directing photosynthate to downward root growth - Increased water uptake | |
| Developmental response to water deficit: stimulation of leaf abscission | Abscission = decreased transpiration |
| Rapid response to water deficit: reducing absorption of solar radiation | Leaf movements that reduce the absorption of solar radiation = decreased transpiration |
| Rapid response to water deficit: stimulation of stomatal closure | In response to acute water deficit, stomata are closed to reduce desiccation The response is mediated by abscisic acid (ABA) Guard cell closure = decreased transpiration ABA - apoplast - guard cell - guard cell closure |
| Rapid response to water deficit: osmotic adjustments | Increased cellular solute concentration leads to decreased water potential (Yw)*, facilitating absorption of water from the soil (water is absorbed as long as Yw is more negative than that of soil) and allowing extraction of difficult to extract water |
| Rapid response to water deficit: osmotic adjustments: compatible solutes | ie. amino acids, sugar alcohols |
| Rapid response to water deficit: changes in gene expression | ie. aquaporins = water transport Many of these genes are also induced in response to cold and salt stress |
| Rapid response to water deficit: changes in gene expression | Gene expression in response to osmotic stress is mediated by ABA-dependent and ABA-independent pathways |
| Heat stress and heat shock | Most plants cannot survive >45oC for extended periods Water deficit contributes to heat stress by reducing transpiration and thus cooling |
| Effects of heat stress | - inhibits respiration/photosynthesis (before) - heat - increased membrane fluidity - loss of membrane function At temps > temperature compensation point CO2 production exceeds CO2 fixation and there is a net loss of carbon |
| Responses to heat stress | Responses include: Leaf movements (as in water deficit) Synthesis of heat shock proteins (HSPs) |
| Cold stress | Chilling: temperature too low for normal growth, but no ice formation Chilling sensitive crop species: maize, French bean, rice, tomato, cucumber, cotton |
| Chilling injury | 25-35oC - 10-15oC - discolouration, lesions, water soaking, wilting |
| Chilling injury results from membrane dysfunction | decreased membrane fluidity - loss of membrane function - loss of photosynthesis, translocation, respiration, protein synthesis but not protein degradation |
| Freezing acclimation | - damage involves protoplast dehydration - acclimation involves ABA Arabidopsis abi1 mutants (ABA insensitive) and aba1 (ABA deficient) mutants do not acclimate to freezing. Acclimation is restored to aba1 mutants (but not abi1 mutants) by ABA treatment |
| Changes in gene expression following cold treatment | - >100 genes exhibit cold-induced expression ie. heat shock proteins = renaturation of denatured proteins |
| Cold tolerance | Transgenic plants with increased CBF1/DREB1b exhibit increased expression of cold-responsive genes and are more cold tolerant |
| Anoxia | - absence of oxygen - hydrophytes (flooding tolerant plants) have adaptations - physiological/morphological changes - seed distribution changes |
Created by:
reub8n
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