Stress and strain terminology, Nature of stress injury,

resistance and causes of stress

Vajinder Pal KalraPunjab Agricultural University Ludhiana

Stress Stress

• Mechanical force per unit area applied to an object

• Biological stress is not easily defined but it implies

adverse effects on an organism

• Like all other living organisms, the plants are subjected

to various environmental stresses such as water deficit

and drought, cold, heat, salinity and air pollution etc.

Stress is any change in environmental conditions that might reduce or adversely change plant’s growth and development (Levitt, 1972)

Adverse force or influence that tends to inhibit normal systems from functioning (Jones, 1989)

Any situation where the external constraints limit the rate of dry-matter production of all or part of the vegetation below its ‘genetic potential’ (Grime, 1979)

Therefore, most practical definition of a biological stress is an adverse force or a condition, which inhibits the normal functioning and well being of a biological system such as plants

Strain

• In response to the applied stress, an object undergoes a change in the dimension, which is known as strain

• Strain can be elastic or plastic depending on the degree and lasting time of stress

– Elastic strain: recoverable/reversible, temporary

– Plastic strain: non-recoverable/irreversible, permanent

According to Newton's law of motion, a force is always accompanied by a counterforce, for an action there is always equal and opposite reaction. Stress is the action and whereas strain is the reaction

Difference Between Plant Stress and Strain

• The adverse reaction of plants to environmental conditions that are unfavorable to growth, development and productivity is called as plant stress

• Any environmental factor capable of inducing a potentially injurious strain in plant

• Strain is the biological changes in plants under the influence of plant stress

Stress and strain terminology

Stress: An external factor acting adversely on an organism

Strain: Any physical or chemical change produced by a stress

Elastic strain: A reversible physical or chemical change produced by a stress

Plastic strain: An irreversible physical or chemical change produced by a stress

Stressor/Stress factor: Any factor that causes injury or stress stimulus

Stress response: Stress stimulus with ensuing state of adaptation

Eustress: It is an activating, stimulating stress that increase the physiological activity of a plant and thus a positive element for plant development.

Distress: It is a severe and a real stress that causes damage and thus has a negative effect on the plant and its development

Zero stress: The stress that is just insufficient to produce a plastic strain

Stress resistance: Ability of the plant to survive under adverse environmental condition is termed as stress resistance (adaptation, avoidance and tolerance)

Elastic resistance: Ability of the plant to prevent reversible or elastic strain (physical or chemical change) when exposed to a specific stress

• Plastic resistance: Ability of the plant to reduce or prevent irreversible or plastic strain

• Adaptation refers to heritable modifications in structure or function that increase the fitness of the organism in the stressful environment. It is also called protection. e.g. CAM plants to desert

• Acclimation refers to non-heritable physiological modifications that occur over the life of an individual. These modifications are induced by gradual exposure to the stress. The process of acclimation is known as hardening

Resistance adaptation/Plastic adaptation: Adaptation leading to increased plastic resistance which prevent injury by a stress (Precht et al, 1955 ) or it is a measure of tolerance of elastic strain

Capacity adaptation/Elastic adaptation: Measure of avoidance of elastic strain

Damage/Stress injury: It is the result of too high a stress which can not be compensated

Dehydration : The loss of water from a cell. Plant cells dehydrate during drought or water deficit

Desiccation : The extreme form of dehydration. Denotes the process whereby all free water is lost from the protoplasm

Homoiohydry : Water economy strategy whereby plants strive to maintain a high water potential under water limiting conditions Homoiohydric plants possess drought avoidance

Poikilohydry : Water economy strategy whereby plants lack the ability to control water loss to the environment. Poikilohydric plants must be drought tolerant

Poikilotherms: Plants that tend to assume the temp. of their environment i.e they must develop temp. tolerance

Types of Stress

Stress response flow chart

Preferable

Types of Stress/Strain Injury

Primary StressPrimary Stress

Elastic strainElastic strain Secondary stressSecondary stress

Direct plastic strain

Direct plastic strain

Indirect plastic strain

Indirect plastic strain

Elastic and plastic strains

Elastic and plastic strains

(1) Primary direct injury

(1) Primary direct injury

(2) Primary indirect injury(2) Primary

indirect injury(3) Secondary stress injury

(3) Secondary stress injury

Primary StressPrimary Stress

Elastic strainElastic strain Secondary stressSecondary stress

Direct plastic strain

Direct plastic strain

Indirect plastic strain

Indirect plastic strain

Stress resistance and tolerance Stress resistance and tolerance

STRESS RESISTANCE

(1)Stress avoidance (excluded from tissue)

Stress tolerance

(survival of internal stress)

(2)Avoidance of elastic strain

Tolerance of elastic strain

(3)Avoidance of plastic strain

(4)Tolerance of plastic strain

(Reparability)

Four possible mechanisms of stress resistance

Plants respond to stress in different ways

Avoidance

Plants avoid the injury of stress by building up a barrier to prevent stress factors entering the plant– alfalfa survive dry habitats by sending down deep

root systems that penetrate the water table

– Halophytes secrete the salts out from the leaf thus reduce salt content in the leaf

(A) Aeluropus lagopoides(Halophytes) plants growing in natural habitat(B) Photograph showing secretion of ions from leaf sheath and leaf surfaces(C) Salt crystals on the adaxial leaf surface (D) Crystal count on leaves of A. lagopoides in NaCl stress Sanadhya (2015)

AoB PLANTS

Avoidance mechanismsS.no. Primary Stress Avoidance mechanism

1 Chilling temperature None

2 Freezing temperature None

3 Heat High transpiration rate

4 Water a) Water conservation b) Rapid water absorption

5 Visible and IR radiation High reflection, transmission and absorption

6 UV radiation High reflection, transmission and absorption

7 Ionizing radiation High proportion of non-living:living mass

8 Salt Exclusion, excretion and dilution

9 SO2 Exclusion, precipitation and vaporization

10 Ion Stomatal closure

11 O3, PAN, etc None known

12 Pressure, EMF None possible

Escape

Plant avoid the injury of stress by regulating its life cycle to avoid meeting with stress. This is not the kind of resistance

– some short-lived, desert ephemeral plants germinate, grow and flower very quickly following seasonal rains. They thus complete their life cycle during a period of adequate moisture and form dormant seeds before the onset of dry season

Diawara (1997)Msc Thesis, Kansas State University

Tolerance

• Plants adapt to the stress environment by regulating their metabolism and repair the damage caused by stress

– Highly salt tolerant halophytes survive salty habitat by many strategies such as high ROS scavenging ability, high osmotic adjustment ability, stress proteins and so on

Environmental Stresses to Which Plants may be Subjected

High Temperature (Heat) Low Temperature (Chilling, Freezing) Water Deficits (Drought, Low water potential) Salinity Excess water (Flooding, Anoxia) Chemical (Heavy metals, Air Pollutants) Radiation (Visible, Ultraviolet) Pathogens Competition

Drought/water stressSoil drought: no rain for long time and no available water in the soilAir drought: RH < 20% in atmosphere, transpiration>>water absorption

If longer, soil drought occursAgricultural drought: Moisture in the soil is not sufficient to meet the ET needs of the crop

Symptoms

Stunting, red color in base ， small cell and leaf area ， leaf yellowish and abscissionYoung leaves or/and reproductive organs wilt to death

Stress in plants

Fig. In vitro activities of key enzymes of C metabolism; Rubisco, G3PDH, Ru5Pkin and FruBPase in well watered (open bars) and drought stressed ‐ ‐grapevine (closed bars) in the middle of the summer in Évora, Portugal.

M. M. CHAVES et al Ann Bot 2002,89:907-916

Membrane damage

Senescence, bio-membrane changes

Change in states, such as hexagonal phase and become leaked

Metabolic disorder

• Redistribution of water among organs

• Photosynthesis decreases, while respiration rises leads to

Starvation to death

Decrease in nuclear acids and proteins

• Protease activity↑， free amino acids↑， RNAase activity↑， RNA

hydrolysis ,DNA content falls down

Damages caused by stress

Pro accumulation：• Pro from protein hydrolysis; synthesis↑,oxidation↓

Pro function：• Detoxification of NH3; bound water ↑

Changes in plant hormones:

• Promoters↓， inhibitors↑， esp. ABA↑

Poisonous agents accumulation:

• NH3 and amines↑

Morphologically

• Increase in water absorption and transportation

• Declination of transpiration

• Developed root system and higher ratio of root to shoot

• Thick leaf , smaller leaf area and thick cuticle

• Developed bundle and veins

• smaller and more stomata

Physiologically

• Increase in ABA accumulation leads to Stomatal closure

• Rapid accumulation of Pro, glycinebetaine, dehydrin, osmotins, ions etc. to Increase in capacity of resistance to dehydration of cytoplasm

Mechanisms of resistance to drought

Selection of cultivars with high resistance to drought ， high yield

and quality

drought hardening by Seed priming special technology to control

seed water absorption and re-drying slowly

Suitable fertilizer application by application of more P and K to

plants

Chemical regents application like Soaking in 0.25% CaCl2 or

0.05%ZnSO4 solution

Application of plant substance:ABA, CCC etc

Methods to increase the resistance in Agriculture

Moisture injury is caused when soil space is filled with water and without air

flooding injury: Whole plant or part of shoot is submerged in water

Injures of flood to plant Flood is actual deficiency in O2

Anything that increases soluble O2, will decrease the injury

Anything that decreases soluble O2, will increase the injury

Such as slowly streaming water causes less damage than static water

flood/excess water

Injury in morphology and anatomy by O2 deficiency

Growth decreases leaf yellowish (nutrition deficiency ） root darkness （ low Eh ） epinasty （ Eth) air root(IAA, Eth) stem hollow (tissue degradation caused by Eth ）

Injury in metabolism by O2 deficiency

photosynthesis decreases leads to stomatal block and inhibition of CO2 entrance

Anaerobic respiration increses toxicants: alcohol acetaldehyde ， NH3 ， lactate , H2S

Nutrition disorder

absorption ↓ ， Soil N, P, K, Ca loss but H2S, Fe, Mn ↑ ， microelements poison

Changes in plant hormones IAA and CTK ↓. ACC synthesis in root and release of Eth in shoot

Mechanical damage and infection by harmful organism

Resistance is different in plants hydrophytes>landplants ， rice>rape>barley;

O.sativa>O.japonica ， and in growth stages : seedling >other stages

Tolerance in tissues ： Well-developed aerenchyma

Tolerance in metabolism ： mitochondria well develops in anaerobic conditions, succinic acid dehydrogenase↑ ， tolerance to ethanol ; PPP instead of EMP ， Glutamate dehydrogenase ↑

Mechanism of resistance to flood

Strategies of adaptation to excess water stresses in the form of submergence or waterlogging in rice plants

Nishiuchi et al (2012)Rice J

Kennedy et al (1980)Plant cell and Environment

☼ Each plant has its unique set of temperature requirement for growth and development. There are two types of limits of temperature-upper limit and lower limit.

☼ Except in the relatively stable climates vary depending on the environment. Temperature may be of two kinds-

-High Temperature

-Low Temperature

Temperature stress

On close inspection, leaves display minute pores (right). When the temperature in the leaf rises above 1 or 2 degrees Centigrade, the pores open and gasses begin flowing from the colder region in the leaf to the warmer region. Oxygen is thus taken inside the plant. The highest rate has been measured in the Amazon lily (below left), at 30 liters (8 gallons) of gas an hour.1. Pore (stoma)

Continuous…

• The effects of Low Temperature included the following –

A.Chilling StressB.Freezing Stress

Chilling Stress

› Chilling injury refers to an injury that is caused by a temperature drop to below 15°C but above the freezing point

› Plasma membrane is the most common site for chilling injury

› The consequences of this change may lead to cell leakage or disruption and loss of cell liquid

Chilling affects on Plants

• Chilling injury causes several physiological dysfunctions to the plant including-

Disruption of the conversion of starch to sugars

Decreased carbon dioxide exchange

Reduction in net photosynthesis

The destruction/degradation of chlorophyll

Methods to increase the resistance chilling stress

• The resistant cultivars

• Low temperature hardening

• Chemical control: ABA ,CCC ， PP330 ， Amo-1618

• Others: PK application, keep warm with artificial things

Freezing Stress

• It freezes at about -2°C, depriving the plant of its source of water. It occurs by rapid freezing of cells to a very low temperature

• Freezing injury in plants can be from two sources:

1. Freezing of soil water2. Freezing of the fluids within the plant

Mechanism of freezing stress 2 types: (intercellular and intracellular freezing)

Intercellular freezing

Freezing

Intercellular freezing occurs when temperature falls gradually

ice

Intracellular Freezing often occurs when temperature falls suddenly

Ice results in the direct injury in cytoplasm, bio-membrane and organelle, and damages to cell compartmentation and metabolic disorder

Much more serious damage is caused by Intracellular freezing than by Intercellular freezing

Damage of protein: Sulfhydryl group Damage of bio-membrane:Electric conductivity↑ ， cell

material leakage↑ ， photochemical activity and ATP production ↓, while photoinhibition ↑

• Change in state of lipid and protein denaturation

• Cold-favored plants: some alga ， bacteria and fungi ， meets heat injury at 15 － 20 ℃

• Temperature-mediate plant: most of crops at 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℃

High temperature stress and heat resistance of plants

Thermal image of a tomato plant (Solanum lycopersicum). The image compares a living, transpiring leaf (top) to a dead leaf (bottom). The cooling effect of transpiration is seen as darker colours in the living leaf compared to the dead leaf

Bakanae (2013)

Indirect damage Starvation and respiration is much larger than

photosynthesis. Poisoning: Ethanol or acetaldehyde, free radicals deficiency of biotins: Biotins， Vitamins damage of nuclear acids and proteins

Direct damage Protein denaturation Configuration damage The degree in denaturation is positively related to water

content in plant tissue. Dry seed is able to resist 70－ 80℃

Reasons for heat injury

High stability of protein under heat stress

Lower water content

High contents of saturated fatty acid.

High contents of organic acid: CAM-extremely heat-resistance

having a great number of organic acid.

Lessen or protect them from NH3 poison

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 they protect or repair

proteins, nuclear acids and biomembrane from heat injury

Mechanism of heat resistance

Salt stress Salt stress occurs due to excess salt accumulation in the soil. As a

result, water potential of soil solution decreases and therefore exosmosis occurs. This leads to physiological drought causing wilting of plants

Classification of plants: They are halophytes and glycophytes Halophytes: plants that grow under high salt concentrations and

further divided into two types based on extreme of tolerance

• Euhalophytes: can tolerate extreme salt stress

• Oligohalophytes: can tolerate moderate salt stress Glycophytes

• Glycophytes are the plants that cannot grow under high salt concentration

Effect of salt stress on plant growth and yield

Delays seed germination due to the reduced activity of the

enzyme, α-amylase

Significant reduction in root emergence, root growth and root

length at emergence stage

Salt injury is more severe only at high temperature and low

humidity at vegetative stage

Salinity affects panicle initiation, spikelet formation,

fertilization and pollen grain germination at reproductive

stage

Salinity drastically declines photosynthetic process

Padilla et al (2014)Cultivos Tropicales 35:62-66

Mechanism of salt tolerance

Some plants are able to maintain high water potential by reducing the transpiration rate

Salts are accumulated in stem and older leaves in which metabolic processes take place in a slower rate

Na+ (sodium ion) toxicity is avoided by accumulating high amount of K+ ions

Accumulation of toxic ions in the vacuole but not in the cytoplasm. Accumulation of proline and abscissic acid which are associated

with tolerance of the plants to salt

Relative salt tolerant crops Tolerant crops: Cotton, sugar cane, barley Semi tolerant crops: Rice, maize, wheat, oats, sunflower, soybean Sensitive crops: Cow pea, beans, groundnut and grams

Halophytes that can Accumulate metals

Atriplex Halimus typically accumulates low concentrations of Pb and Cd, but has high biomass, making it a viable candidate for use in phytoextraction in arid, saline soils

(Manousaki and Kalergarakis 2009)Environmental Research 326-32.The halophyte Tamarix aphylla

(mangrove) tree excretes metals like Cd, Li, Mg,and Ca onto the leaf surface from its salt glands along with Na-Cl. Even in soils with high concentrations of Cd(16 ppm), the concentration of Cd in mangroves remains below toxic levels, suggesting that excretion may play a rolein Cd tolerance.

(Hagemeyer and Waisel 1988)Physiologia Plantarum

Mitigation of Salt Stress

Seed hardening with NaCl (10 mM concentration) Application of gypsum @ 50% Gypsum Requirement (GR) Incorporation of daincha (6.25 t/ha) in soil before planting Foliar spray of 0.5 ppm brassinolode for increasing

photosynthetic activity Foliar spray of 2% DAP + 1% KCl (MOP) during critical stages Spray of 100 ppm salicylic acid Spray of 40 ppm of NAA for arresting pre-mature fall of flowers /

buds / fruits Extra dose of nitrogen (25%) in excess of the recommended Split application of N and K fertilizers Seed treatment + soil application + foliar spray of Pink Pigmented

Facultative Methnaotrops (PPFM) @ 106 as a source of cytokinins.

Biotic stress

Stress that occurs as a result of damage done to plants by other living organisms, such as bacteria, viruses (although they are not considered to be living organisms, also cause biotic stress to plants), fungi, parasites, beneficial and harmful insects, weeds, and cultivated or native plant

For example, browning of leaves on an oak tree caused by drought stress may appear similar to leaf browning caused by oak wilt, a serious vascular disease, or the browning cause by anthracnose a fairly minor leaf disease

It is a major focus of agricultural research, due to the vast economic losses caused by biotic stress to crops

Resistance and biotic stress Plant defenses Physical barriers: cuticle, thorns, cell walls Constitutively produced chemicals (e.g., phytoalexins) and

proteins (e.g., Ricin) Induced responses (Plant Defense Response)

How does the plant recognize and defend itself against pathogens?

• Plant disease has an underlying genetic basis

• Pathogens may be more or less potentially infectious to a host

– virulent on susceptible hosts

– avirulent on non-susceptible hosts

• Pathogens carry avirulence (avr) genes and hosts carry

resistance (R) genes

• The normal presence of both prevents pathogens from attacking

the plant

• Infection occurs when pathogen lacks avr genes or plant is

homozygous recessive for resistance genes (rr)

Potential damage by biotic stress

Fungi cause more diseases in plants than any other biotic stress

factor

Over 8,000 fungal species are known to cause plant disease

Not many plant pathogenic viruses exist, but they are serious

enough to cause nearly as much crop damage worldwide as fungi

Microorganisms can cause plant wilt, leaf spots, root rot, or seed

damage

Insects can cause severe physical damage to plants, including to

the leaves, stem, bark, and flowers. Insects can also act as a vector

of viruses and bacteria from infected plants to healthy plants

Stress only poses a problem to people or the environment if they are not prepared for it. There can be steps taken by humans to lessen the effects. Plants have the ability to adapt to stress over time. This is natures way of taking care of itself and keeping everything in balance.