Stress Physiology

• Stress:
• Stress in physics is any force applied to an object. Stress in biology is any change in environmental conditions that might reduce or adversely change a plant’s growth or development.
• Such as freeze, chill, heat, drought, flood, salty, pest and air pollution etc.
• Resistance: resistance is the ability adaptive or tolerant to stresses.

• Resistance includes adaptation, avoidance and tolerance.
• Adaptation is permanent resistance to stress in morphology and structure , physiology and biochemistry under long-term stress condition.

​

• a well-developed aerenchyma in hydrophytes,
• a pattern for stomata movement in CAM plant.

​

• Avoidance is a manner to avoid facing with stress using neither metabolic process nor energy.
• Very short lifecycle in desert plants. Dormancy during the cool,hot, and drought conditions.
• Tolerance is a resistant reaction to reduce or repair injury with morphology , structure, physiology, biochemistry or molecular biology, when plant counters with stresses.

​

• Hardening is a gradual adaptation to stress when the plant is located in the stress condition.

• Section 1. Water stress in plant
• 1.1 Resistance of plant to drought
• Drought injure:

Soil drought, no rain for long time and no-available water in the soil.

Air drought, RH<20% in atmosphere，transpiration>>water absorption. If longer, soil drought occurs.

• Drought injury is actually in physiology.

• Metabolism relevant to water sensitive to range of water
• Inhibit (-) promotion (+)

MPa

0

-0.5

-1.0

-1.5

-2.0

Cell elongation(-)

Cell wall synthesis(-)

Protein synthesis(-)

Chlorophyll synthesis(-)

ABA synthesis(+)

Seed germination(-)

Stomatal opening(-)

CO₂ assimilation(-)

respiration(-)

Proline accumulation(+)

• Symptoms in plant facing to drought：stun, red color in base，small cell and leaf area，leaf yellowish and abscission. Young leaves or/and reproductive organs wilt to death.
• 1.1.1 Mechanism of drought injure
• 1.1.1.1 Membrane damage.
• Like senescence, biomembrane changes in states, such as hexagonal phase and become leaked.

Hydrophilic groups of lipid aggregate together

drought

Re-watering

• 1.1.1.2. Metabolic disorder
• （1）Redistribution of water among organs：

• (2)Photosynthesis decreases, while respiration rises after lowering
• Starvation to death。
• a. assimilate↓ SC↓ ，Photorespiration↑，electron transfer activity and PSP ↓.In sunflower, -1.1MPa，ET and PSP decrease obviously，-1.7 MPa, PSP is 0。
• b. inhibition by photoassimilate feedback.

• （3）Decrease in nuclear acids and proteins。
• Protease activity↑，free aa↑，RNAase activity↑，RNA hydrolysis ,DNA content falls down.
• （4）Pro accumulation：
• ① Pro from protein hydrolysis;②synthesis↑，③oxidation↓。
• Pro function：
• ① detoxification of NH₃;②bound water ↑ .

• （5）Changes in plant hormones，promoters↓，inhibitors↑，esp. ABA↑.
• （6）Poisonous agents accumulation。 NH₃ and amines↑.
• 1.1.1.3 Mechanical injure
• Cytoplasm is broken down
• Formation of －S－S－.

​

• 1.1.2 Mechanisms of resistance to drought and the methods to increase the resistance
• 1.1.2.1. Mechanisms of resistance

（1）Morphology： increase in water absorption and transportation , declination of transpiration.

• a. Developed root system and higher ratio of root to shoot——‘开源’

• b. Thick leaf , smaller leaf area and thick cuticle。
• c. Developed bundle and veins，smaller and more stomata

• （2）Physiology and biochemistry
• a. Stomatal regulation：
• ABA accumulation→stomatal closure →
• b. Increase in capacity of resistance to dehydration of cytoplasm
• Rapid accumulation of Pro, glycinebetaine Lea protein, dehydrin, osmotins and ion etc.

• 1.1.2.2. Methods to increase the resistance
• （1）Selection of cultivars with high resistance to drought，high yield and quality.
• （2）drought hardening：
• “蹲苗”、“饿苗”及“双芽法”。
• Seed priming special technology to control seed water absorption and re-drying slowly

• （3）Suitable fertilizer application：
• Application of more P、K to plants.

​

• （4）Chemical regents application
• Soaking in 0.25% CaCl₂ or 0.05%ZnSO₄ solution.
• Application of plant substance: ABA, CCC etc

​

• 2.2 Resistance of plant to flood
• Flood injury: moisture injury and flooding injury.

​

• Moisture injury is caused by soil space filled with water and without air.

​

​

• flooding injury: whole plant or part of shoot is submerged to water while flooding

• 2.2.1 Injures of flood to plant
• Flood is actual deficiency in O₂
• Anything increases in soluble O₂, the injury will decrease. And anything decreases in soluble O₂, the injury will increase.
• Such as slowly streaming water less damage than static water.

• (1) Injury in morphology and anatomy by O₂ deficiency：growth↓，leaf yellowish (nutrition deficiency），root darkness（low Eh），epinasty（Eth), air root(IAA, Eth), stem hollow (tissue degradation caused by Eth ）.

​

• (2) Injury in metabolism by O₂ deficiency: photosynthesis ↓——stomatal block, inhibition of CO₂ entrance . Anaerobic respiration↑，toxicants: alcohol ，acetaldehyde，NH₃，lactate , H₂S。

• (3) Nutrition disorder：
• absorption ↓ ，soil N、P、K、Ca loss but H₂S、Fe、Mn ↑，microelements poison.
• (4) Changes in plant hormones：IAA and CTK ↓. ACC synthesis in root and release of Eth in shoot.
• (5) Mechanical damage and infection by harmful organism

• 2.2.2 Mechanism of resistance to flood
• Resistance is different in plants:hydrophytes>land plants，rice>rape>barley; O.sativa>O.japonica ，and in growth stages : seedling >other stages,
• (1) Tolerance in tissues：Well-developed aerenchyma 。
• (2) Tolerance in metabolism：mitochondria well develops in anaerobic conditions, succinic acid dehydrogenase↑，tolerance to ethanol ; PPP instead of EMP，NR↑，Glutamate dehydrogenase ↑。

• Section 2 Temperature stress
• Temperature stress: Low or high temperature, called frost injury or heat injury, respectively.
• 2.1 Frost ( freezing )injury
• The injury is caused by low temperature below freezing point (〈 0℃）,companied with frost.

• 2.1.1 Mechanism of freezing (frost )injury
• 2.1.1.1.Freezing:(intercellular and intracellular freezing)
• (1) Intercellular freezing

Freezing

Intercellular freezing occurs when temperature falls gradually.

ice

• （2）Intracellular Freezing :
• Intracellular freezing often occurs when temperature falls suddenly.
• Ice results in the direct injury in cytoplasm, biomembrane and organelle, and damages to cell compartmentation and metabolic disorder.
• Much more serious damage is caused by Intracellular Freezing than by Intercellular Freezing.
• 2.1.1.2 damage of protein:
• Sulfhydryl group hypothesis（disulfide bridge hypothesis ）

—SH HS—

S

​

S

Before freezing

​

​

frozen

​

​

​

defrozen

—SH H S—

—S—S—

—S—S—

—S—S—

—S—S—

HS—

SH

—S—S—

Illustration of sulfhydryl group hypothesis

• Supported Exp：
• (1) －S－S一increase and soluble －SH decrease after plant tissue faces to freezing.
• (2) Less－S－S－and －SH of protein in the resistant-freeze plants.
• (3) The plant with free－SH,glutathione, is more resistant to freeze.
• (4) Artificial －SH,mercapthanol increases resistance of plant to low temperature.

• 2.1.1.3.Damage of biomembrane
• Electric conductivity↑，cell material leakage↑，photochemical activity and ATP production ↓, while photoinhibition ↑，CF1 and PC depart from membrane.
• Change in state of lipid and protein denuturation

• 2.1.2 Chilling injury
• Chilling injury in tropical or subtropical plants is caused by temperature above 0℃ (freezing point )..
• Maize, cotton rice seedling——10℃。
• Rice pollen-mother cell division，23℃ for O. sativa and 20℃ for O. japonica.
• Banana tree——13℃。
• Oak tree——5℃。

• 2.1.2.1. Change in state of lipid

Electric conductivity as an index for resistance to low temperature in pruduction

• 2.1.2.2. Metabolism disorder
• （1）Uptake function of roots declines and water balance disorders
• Transpiration>water absorption. The plant loss water and leaf curl——青枯死苗（水稻）。
• （2）Photosynthetic rate lowers 。
• Photosynthesis< respiration, starvation to death——黄枯死苗。
• Rubisco losses activity under low temperature，PSP uncouples and free radicals breaks suddenly.

• （3）Aerobic respiration decreases and anaerobic respiration increases。
• Cytaa₃ activity ↓, respiratory electron transport and phosphorylation activities ↓. Ethanol poison.
• （4） Organic substance degrades。
• protease↑，protein↓，RNA、ATP ↓.

• 2.1.3 Physiological reaction of plant to low temperature
• (1) Water content, metabolism, growth decrease .
• Total water content↓，bound water↑，free water and ratio (free water/bound water) ↓。
• (2) Protective substances increase。
• NADPH——reduces－S－S－ to － SH，ATP and sugar↑, bound water↑.

• (3) Unsaturated fatty acid increase in membrane。
• Unsaturated fatty acid↑ and saturated one ↓.

​

• (4) ABA↑，GA↓, dormancy appears.

​

• (5) Proteins-resistant to freezing accumulations.

​

• Freezing resistant protein —— Ice-Box——The genes expression induced by freeze——freeze-resistant protein.

• 2.1.2.4 Methods to increase the resistance to low temperature。
• (1) The resistant cultivars.
• (2) Low temperature hardening.
• (3) Chemical control.
• ABA ,CCC，PP₃₃₀，Amo-1618).
• (4) Others.
• PK application, keep warm with artificial things.

• 2.2 High temperature stress and heat resistance of plants。
• Cold-favored plants: some alga，bacteria and fungi，meets heat injury at 15－20℃ .
• Temperature-mediate plant: most of crops——35℃.
• Temperature-favored plants: some alga，bacteria 65－100℃，many CAM plants>50℃.
• Heat injury is a damage to the temperature- mediate plant by high temperature above 35℃.

• 2.2.1 Reasons for heat injure
• 2.2.1.1. Indirect damage
• （1）Starvation。
• Temperature compensation point: Pn is equal to zero at high temperature
• Respiration is much larger than photosynthesis.

Pn,Rd (μmolm^-2s^-1)

Temperature (℃)

Total photosynthetic rate

Pn

Respiration rate

Respiration is larger than photosynthesis under low temperature

• （2）Poisoning。
• Ethanol or acetaldehyde, free radicals
• （3）deficiency of biotins。
• Biotins，Vitamins
• （4）damage of nuclear acids and proteins.

• 2.2.1.2. Direct damage
• （1）Protein denaturation
• Configuration damage
• The degree in denaturation is positively related to water content in plant tissue.
• Dry seed is able to resist to 70－80℃。

​

• （2）Lipid liquefaction

High temperature

liquefaction

• 2.2.2 Mechanism of heat resistance
• (1) High stability of protein under heat stress。
• much－S－S－
• (2) Lower water content
• (3) High contents of saturated fatty acid.
• (4) High contents of organic acid。
• CAM——extremely heat-resistance ——a great number of organic acid.
• Lessen or protect them from NH₃ poison.

• (5)Form of heat shock proteins (HSPs or hsps)
• Heat shock proteins are a newly synthesizing set of proteins that organisms ranging from bacteria to humans respond to high temperature.
• Functions: protect or repair proteins, nuclear acids and biomembrane from heat injury.
• More than 30 HSPs, 15-27kD, some are chaperons

• Section3 Salt stress and resistance to salt
• Over 1% of salt content in reclaimed tideland (海涂地) ，0.2~0.25% of salt content in the northern basic soil (碱土). 1/5-1/3 of tatol cultivated land .
• 3.1Mechanism of salt injure
• 1. Physiological drought。
• 2. Single salt toxicity .Na⁺ and Cl^-，SO₄^－.
• 3. Metabolic damage：Ch1 and Rubisco↓，protein degradation↑,Pro↑，NH₄⁺ poison↑.

• 3.2 1Mechanism of resistance to salt
• 3.3 Methods resistant to salt
• (self-study)

Section 4 Resistance to plant diseases

• 4.1 Types of plant response to diseases.
• Three types: resistance, sensitivity and tolerance

• 4.2 Physiological damage of plant diseases to plants
• 1. The cell membrane permeability increases.
• 2. Metabolism disorders.
• Water metabolism(absorb, loss and transport). Photosynthesis,
• Respiration (PPP).
• Assimilate transport.

• 4.3 The resistance of plant to plant diseases

​

• 1.Formation of protective structure.
• 2.hypersensitive response. Synthesis of phytoalexins and fungitoxic proteins and pathogenesis related proteins (PRs)
• 3.immuno-induction.

• Section5 The role of plant in environmental protection
• 1.O₂ and CO₂ equilibrium;
• 2.Prevent water and soil loss.
• 3. Clean soil, water or other environmental conditions or detoxification.
• 4.Detect environmental conditions

​

• Section6 General response to stresses
• 1. Damage in biomembrane system
• 2. Disorder in metabolism
• 3. Functional proteins denuturation and stress protein synthesis
• 4. Osmotic substance synthesis
• 5. Change in plant hormones

​

• Questions:�1.How does chilling injury damage the plants in physiology and in which season does chilling injury occur frequently.
• 2. Which of stresses result in water potential declination and how to do them?

​