MODULE 7B - STRESS PHYSIOLOGY

Stress

any environmental condition that exerts disadvantageous influence on plant growth.

• Drought

• Temperature

• Radiation

• Flooding

• Wind

• Basic Classes of Plant Stressors

Physical (5)

• Salinity

• Nutrient limitation

• Soil pH

• Soil organics

• Atmospheric gases

• Basic Classes of Plant Stressors

Chemical (5)

• Competition

• Herbivory

• Pathogens

• Allelopathy

• Mycorrhizae

• Basic Classes of Plant Stressors

Biotic (5)

• Air Pollution

• Pesticides

• Heavy metals

• Fires

• Invasion

• Basic Classes of Plant Stressors

Anthropogenic (5)

Strain

magnitude of physiological change that occurs in plant in response to environmental stress

• Avoidance

• Tolerance

Strain (2)

Avoidance

severe reduction in metabolism during stress, and plants often enter into a dormant state

Tolerance

maintenance of high metabolic activity similar to that attained under optimum conditions

Resistance

plant’s fitness to the unfavorable environment

• Acclimation

• Adaptation

Resistance (2)

Acclimation

resistance increases as a result of exposure to prior stress

Adaptation

genetically determined level of resistance acquired over generation by selection

Intensity

magnitude of stress

• Chronic stress

• Acute stress

Intensity (2)

Chronic stress

when the pressure placed by a stressor on a physiological process is mild but it is maintained for extended period of time

Acute stress

when intense pressure is placed on a physiological process by sudden exposure to extreme conditions

Dose

magnitude of stress times the length of exposure

• Water Deficit / Drought

• Heat Stress / Heat Shock

• Salinity / Salt Stress

• Flooding / Waterlogging (Oxygen deficit)

• Polluting Gases

• Heavy Metal Stress

Dose (6)

Water Deficit / Drought

• Decreased leaf area

• Enhancement of root extension

• Stomatal closure due to increase ABA

• Osmotic adjustment through synthesis of compatible solutes

• Induction of CAM (acclimation response)

• desiccation avoidance

• desiccation tolerance

• dual root systems

(3) Plant Strategies for coping with Water Deficit / Drought

Heat Stress / Heat Shock

• usually occurs at temperatures

  • 45°C for tissues

  • 120°C for seeds

  • 70°C for pollen

• impairs thermal stability of membranes and proteins, and modifies membrane composition and structure causing leakage of ions

• Induction of ___ shock proteins

• reflective hairs

• leaf rolling

• small leaves

(3) leaf adaptations against excessive heating

Heat Shock Proteins

• may act as protective agents

• may play a role in normal metabolism

act as protective agents

enables plant to tolerate temperature previously lethal

play a role in normal metabolism

act as chaperonins essential for post- translational folding and assembly of proteins

Saline soils

soils characterized by high concentration of soluble salts; have a pH below 8.5

• osmotic effect

• ion effect (nutrient imbalance)

(2) main components of Salinity / Salt stress

it lowers the osmotic potential (making it more negative) and the soil water potential

Effect of high salt concentration on water potential

It prevents the flow of water and minerals from the soil to the plant roots.

Consequence of lowered soil water potential

Specific Ion effects

it occurs when Na+, Cl- or SO4 2- accumulates in cells that may inactivate enzymes and protein synthesis

1. Restricting the entry of toxic ions at root level

2. Transporting the toxic ions to stem, leaf sheath or older leaves

3. Excretion of salt through salt glands, salt-hairs or bladders

4. Sequestration of the toxic ions to vacuole or cell wall

Predominant salt-tolerance mechanisms operating in plants (4)

Exclusion

Restricting the entry of toxic ions at root level

Plant level compartmentation

transporting the toxic ions to stem, leaf sheath or older leaves

In most halophytes

Excretion of salt through salt glands, salt-hairs or bladders

Cell level compartmentation

Sequestration of the toxic ions to vacuole or cell wall

Mechanisms of Salt Tolerance

• Ion regulation and compartmentalization (in vacuoles)*

• Induced biosynthesis of compatible solutes/ osmoregulatory compounds**

  • proline, glycine, betaine, polyols

• Induction of abscisic acid and jasmonic acid synthesis

• Induction of antioxidative enzymes

• Salt exclusion*

• Salt secretion

*Avoidance mechanism

**Tolerance mechanism

• Glycophytes

• Halophytes

Salt Functional Types (2)

Glycophytes

plants whose growth decrease when exposed to salinity level greater than 10 mM

Halophytes

plants that tolerate higher salinity than glycophytes (up to 50 mM) before a reduction in growth occurs

Flooding / Waterlogging (Oxygen deficit)

Observed in ___ or ___ soils

Constraints in Flooded/Waterlogged Soils

• Roots are injured in soils where there is low O₂ (hypoxia) or absence of O₂ (anoxia)

• Low redox potential

• reduction in nitrate

• reduction of Fe and Mn causing toxicity

• accumulation of methane and ethylene

Mechanisms of Submergence Tolerance

• Induction of anaerobic stress proteins

• Morphologically by stomatal closure, leaf shedding or slowing growth

• Specialized structures for acquiring O2

  • aerenchyma

  • pneumatophores

Polluting gases

• such as SO₂, NO, NO₂, CO, CO₂, O₂, C₂H₄ cause stomatal closure

• pollutive gases dissolved in rainwater – “acid rain”

Polluting gases

Detoxification

Detoxification

• sulfite and bisulfite metabolized to sulfate

• toxic oxygen detoxified by antioxidants like glutathione, tocopherol, ascorbate, superoxide dismutase, catalase, perox

Heavy metal stress

• usually associated with acidic soils

• Micronutrients: Co, Cu, Fe, Mn, Zn, Mo, Ni Toxic even at low conc.: Cd, Pb, Cr, Hg, Ag, Au

• binding to cell wall

• restricted influx thru plasma membrane

• compartmentation in vacuole

• chelation in cytoplasm, membrane or cell wall

tolerate heavy metals through: (4)

Heavy metal stress

• Sequestration / compartmentation

• Induction of chelating proteins

Induction of chelating proteins

• phytochelatins

• metallothioneins (MTs)

• Tolerance Mechanisms

• Avoidance Mechanisms

Plant Adaptation to Heavy Metal Soils (2)

Tolerance Mechanisms

• Production of metal- binding organic acids and polypeptides

• Isolation of metals in vacuoles

• Production of metal

• Enhanced membrane repair or alteration

Avoidance Mechanisms

• Exclusion of metal by

  • mycorrhizal association with roots

  • exudation of organic chelators into the soil

  • binding to cell walls

  • limiting ion transport to rhizodermis and cortex

Avoidance Mechanisms

• Uptake followed by storage in

  • vacuole and apoplasmic space

  • epidermal cells

  • trichomes

Metallophytes

plants that can grow in high metal concentrations

• Accumulators

• Indicators

• Excluders

Metallophytes (3)

Accumulators

amass metals primarily in their shoots, both at low and high metal concentrations in the soil

Indicators

plants with metal concentrations in tissues corresponding to environment concentration

Excluders

maintain low metal concentrations in their shoots even if the external concentration is high