Osmoregulation allows organisms to maintain homeostasis by regulating water and electrolyte balance. It works through osmosis, where water moves across semipermeable membranes to areas of higher solute concentration. Organisms excrete excess water and solutes. Plants regulate osmoregulation through structures like stomata and use vacuoles and specialized adaptations depending on their habitat. Abiotic stress like drought, salinity, temperature extremes, and heavy metals can damage plants, but plants have developed tolerance mechanisms like avoidance, escape, and acclimation to survive environmental stresses.

1. Osmoregulation
“Osmoregulation is the process by which an organism regulates the
water and electrolytic balance in its body to maintain homeostasis.

2. How Osmoregulation Works
• Osmosis is the movement of solvent molecules through a
semipermeable membrane into an area that has a higher solute
concentration.
• In an organism, the solvent is water and the solute particles are
mainly dissolved salts and other ions. To maintain the water and
electrolyte balance, organisms excrete excess water, solute molecules,
and wastes

3. Osmoregulation in plants
• Higher plants use the stomata on the underside of leaves to control water loss. Plant cells
rely on vacuoles to regulate cytoplasm osmolarity.
• Plants that live in hydrated soil (mesophytes) easily compensate for water lost from
transpiration by absorbing more water. The leaves and stem of the plants may be
protected from excessive water loss by a waxy outer coating called the cuticle.
• Plants that live in dry habitats (xerophytes) store water in vacuoles, have thick cuticles,
and may have structural modifications (i.e., needle-shaped leaves, protected stomata) to
protect against water loss.
• Plants that live in salty environments (halophytes) have to regulate not only water
intake/loss but also the effect on osmotic pressure by salt. Some species store salts in
their roots so the low water potential will draw the solvent in via osmosis. Salt may be
excreted onto leaves to trap water molecules for absorption by leaf cells.
• Plants that live in water or damp environments (hydrophytes) can absorb water across
their entire surface.

4. Plant respond to stress in several different ways
• Plant stress can be divided into two primary categories. Abiotic stress
is a physical (e.g., light, temperature) or chemical strain that the
environment may impose on a plant.
• Biotic stress: is stress that occurs as a result of damage done to plant
by other living organism. Such as bacteria, virus, fungi, beneficial and
harmful insects and cultivated plant.
• Abiotic stress: is defined as the negative impact of non-living factor
on the living organism in a specific environment.

5. The effect of environmental stress on plant
survival
• 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 the dry season.
• In a similar manner, many arctic annuals rapidly complete their life cycle during the
short arctic summer and survive over winter in the form of seeds. Because
ephemeral plants never really experience the stress of drought or low temperature,
these plants survive the environmental stress by stress avoidance . Avoidance
mechanisms reduce the impact of a stress, even though the stress is present in the
environment.
• Many plants have the capacity to tolerate a particular stress and hence are
considered to be stress resistant. Stress resistance requires that the organism exhibit
the capacity to adjust or to acclimate to the stress

6. Adaptation and phenotypic plasticity
• Plants have various mechanisms that allow them to survive and often prosper in the
complex environments in which they live.
• Adaptation to the environment is characterized by genetic changes in the entire
population that have been fixed by natural selection over many generations.
• In contrast, individual plants can also respond to changes in the environment, by directly
altering their physiology or morphology to allow them to better survive the new
environment. These responses require no new genetic modifications, and if the response
of an individual improves with repeated exposure to the new environmental condition
then the response is one of acclimation. Such responses are often referred to as
phenotypic plasticity, and represent nonpermanent changes in the physiology or
morphology of the individual that can be reversed if the prevailing environmental
conditions change

7. Temperature stress
• Mesophytic plants (terrestrial plants adapted to temperate environments
that are neither excessively wet nor dry) have a relatively narrow
temperature range of about 10°C for optimal growth and development.
• Outside of this range, varying amounts of damage occur, depending on the
magnitude and duration of the temperature fluctuation.
• Temperature stress can result in damaged membranes and enzymes
• Temperature stress can inhibit photosynthesis
• Freezing temperatures cause ice crystal formation and dehydration

8. • Soil mineral content can result in plant stress in various ways
• Several anomalies associated with the elemental composition of soils can result in
plant stress, including high concentrations of salts (e.g., Na+ and Cl-) and toxic ions
(e.g., As and Cd), and low concentrations of essential mineral nutrients, such as
Ca2+, Mg2+, N, and P.
• The term salinity is used to describe excessive accumulation of salt in the soil
solution. Salinity stress has two components: nonspecific osmotic stress that causes
water deficits, and specific ion effects resulting from the accumulation of toxic ions,
which disturb nutrient acquisition and result in cytotoxicity.
• Salt-tolerant plants genetically adapted to salinity are termed halophytes, while less
salt-tolerant plants that are not adapted to salinity are termed glycophytes.

9. • Soil salinity occurs naturally and as the result of improper water
management practices
• Saline soils are often associated with high concentrations of NaCl, but in
some areas Ca2+, Mg2+, and SO4- are also present in high
concentrations in saline soils.
• High Na+ concentrations that occur in sodic soils (soils in which Na+
occupies 10% of the cation exchange capacity) not only injure plants but
also degrade the soil structure, decreasing porosity and water
permeability. Salt incursion into the soil solution causes water deficits in
leaves and inhibits plant growth and metabolism.

10. Adaptation in plants against
abiotic stress
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11. What’s in name?
 Stress: Factors of environment interfering the complete expression
of genotypic potential.
 Abiotic stress: The negative impact of non-living factors on the
living organisms in a specific environment.
 Abiotic stress factors or stressors are naturally occurring, often
intangible factors
 The four major abiotic stresses: drought , salinity, temperature and
heavy metals, cause drastic yield reduction in most crops.
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12. Environmental conditions that can cause stress
 Water-logging & drought
 Excessive soil salinity
 High or low temperatures
 Ozone
 Low oxygen
 Phytotoxic compounds
 Inadequate mineral in the soil
 Too much or too little light
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13. PLANT RESPONSE TO STRESS
Stresses trigger a wide range of plant
responses:
Changes in growth rates and crop yields
Cellular metabolism
Altered gene expression
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14. PLANT RESPONSE TO STRESS
Stress
Ozone
Extreme temperature
Flooding
Drought
Salt
Response
Physiological &
developmental event
Altered cellular
metabolism
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15. Resistance or sensitivity of plants to stress
depends on
Stress characters
• Severity
• Duration
• No of exposure
• Continuation of
stress
Plant Character
• Organs or tissues
in question
• Stage of
Development
• Genotype
Response and result
• ResistenceSurvival
and growth
• Susceptibility Death
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16. Stress resistance mechanisms
Avoidance
- prevents exposure to stress
Tolerance
- permit the plant to withstand stress
Acclimation
- alter their physiology in response to stress
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17. I. DROUGHT STRESS
 Drought:
Moisture scarcity which restricts the full expression of
genetic yield potential of a plant.
 Mechanisms of drought resistance:
a) Drought escape: mature early
b) Drought avoidance: Maintain water balance
c) Drought tolerance: higher yield even under low water potential
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18. Morphological features providing drought
resistance
 Leaf rolling, folding, shedding, leaf reflectance
 Reduced leaf area; narrow leaf, change in leaf angle
 Hairiness
 Color of leaves
 Wax coating
 Root systems
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19. Physiological response to drought
Reduced transpiration and reduced respiration
losses
Photosynthetic efficiency is reduced due to
chloroplast damage
Stomatal behavior
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20. Biochemical response to drought
Accumulation of compatible solutes
Increase in ABA and Ethylene
Protein synthesis
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21. Osmotic adjustment
In response to dehydration or osmotic stress a series
of compatible solutes/ osmolytes are accumulated for
osmotic adjustment, water retention and free radical
scavenging.
The cell actively accumulates solutes and as a result
the solute potential drops, promoting the flow of
water into the cell.
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22. Few osmolytes:
D-Pinitol
Osmotin
Mannitol
Glycine betaine
Proline
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23. II. SALT TOLERANCE
Salt tolerance: Ability of plants to prevent ,reduce or
overcome injurious effects of soluble salts present in their
root zone
Salinity can be overcome by
1)Soil reclamation: costly ,time consuming & short lived
2) Resistant varieties: less costly, more effective, long lasting
but require longer period to develop.
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24. Symptoms of plant to salt stress
Retardation of growth
Necrosis
Leaf abscission
Loss of turgor
Ultimate death of plant
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25. Mechanism of salt tolerance
1. Salt tolerance:
By accumulating salt, generally in their
cells or glands & roots.
Halophytes show tolerance by ion
accumulation mechanism
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26. 2. Salt avoidance:
By maintaining their cell salt concentration
unchanged either by water absorption (e.g. Rice,
chenopodiaceae) or by salt exclusion (e.g. tomato,
Soya bean, citrus, wheat grass)
Glycophytes (nonhalophytes) owe their resistance
primarily to avoidance e.g. barley
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27. III. COLD TOLERANCE
Chilling: When temp remain above freezing i.e. >0°C to <
10-15°C.
Freezing: When temp remains below freezing i.e. <0°C.
a) Chilling resistance
Chilling sensitive plants are typically tropical plants.
Temperate plants generally tolerate chilling injury.
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28. Effects of chilling on plants
ABA accumulation
Locked open stomata
Poor seed set/ seed formation.
Pollen sterility
Wilting, Chlorosis, necrosis
Stunted growth
Poor seedling establishment
Reduced germination
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29. At subcellular level:
Toxicity due to H2O2 formation
Reduced photosynthesis
Poor chlorophyll synthesis
Reduces membrane stability
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30. b) Freezing resistance
Dormant state is conducive to freezing resistance, while
resistance is rare in actively growing tissue.
As water in plants cool below 0°C, it may either
1) freeze i.e. form ice.
2) super cool with out forming ice.
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31. Effects of freezing stress
1. Ice formation :
Intercellular ice formation:
Intracellular ice formation:
It is most lethal may be due to physical disruption of sub
cellular structure by ice crystals.
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32. 2. Membrane disruption:
Freezing causes disruption and alter the semi
permeable properties of plasma membrane
Loss of solutes from the cells occur
Cells remain plasmolyzed even after thawing
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33. Super cooling:
This is regarded as imp mechanism of freezing
avoidance.
It is possible because internal ice-nucleators are
absent.
In plants water may cool down to -1 to -15°C
In plants cooling of water below 0°C with out ice
crystal formation is called super cooling
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34. IV. OXIDATIVE STRESS
 Results from conditions promoting the formation of active oxygen species that damage or
kill cells
Environmental factors that cause oxidative stress:
 Intense light that stimulate photoinhibition
 UV light
 wounding
 heat and cold stress
 drought
 heavy metals
 oxidant forming herbicides e.g. Paraquat dichloride
 Air pollution (increased amounts of ozone or sulfur dioxide)
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35. Reactive oxygen species (ROS)
 Formed during certain redox reactions and during incomplete reduction of oxygen or
oxidation of water by the mitochondrial or chloroplast electron transfer chain.
 e.g. Singlet oxygen, hydrogen peroxide, superoxide anion, hydroxyl and perhydroxyl
radicals
The negative effects of ozone on plants
 Reduced crop yield
 Accelerated senescence
 Reduced growth of shoots and roots
 Leaf injury
 Decreased rates of photosynthesis
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