salinity

1. PRIMARY AND SECONDARY EFFECTS
OF SALINITY ON PLANTS

2. PRIMARY EFFECTS
OSMOTIC EFFECT
• Salt's negative effects on plant growth have initially been associated with
the osmotic stress component caused by decreases in soil water potential. By
decreasing the water potential of the soil solution, plant access to soil water is
decreased. As the soil dries, the concentration of salt in the soil solution increases,
further decreasing the osmotic potential. This leads to decreased water uptake by
roots and lose of plant turgidity. Cell membrane pulls away from cell wall due to
shrinkage of cytoplasm by plasmolysis. Leaf cell expansion and overall growth of
plant also declines sharply.

3. G = θ (P-Y)
G= volumetric growth
Θ= cell wall extensibility
P= turgor Pressure
Y= threshold Pressure

4. • The difference in water potential gradient between soil and plant roots
determines the rate of water uptake.
Δ Ψ = Ψroot – Ψsoil
Due to osmosis, water naturally moves from an area containing less salt (higher
water potential) to an area containing more salt (low water potential). Low soil
water potential due to salinity causes difficulty in uptake of water by roots.

5. PRIMARY EFFECTS
ION TOXICITY
• With increasing salt concentration in soil, the uptake of Na+ and Cl- ions by
roots increases sharply causes ion toxicity. This increased intake of Na+ and Cl-
ions interferes with the uptake of some essential plant nutrients leading to
nutritional imbalance.
• Enzyme inhibition: Na+ ion concentration greater than 0.4 M causes
disturbance in electrostatic forces thus disturbing the enzyme structure.
Disrupted structure of enzymes leads to inactivity hindering essential pathways
i.e. glycolytic pathway. Na+ ion start to inhibit function of most enzymes even
at 100 mM. the concentration at which Cl- ion is toxic is even less. Even K+ ion
may inhibit enzymatic activity at 100-200 mM.

6. • Na+ replaces Ca2+ from cell membrane bridges affecting membrane
functionality. Furthermore, the third and final stage of the respiration, the ETC
occurs in the inner mitochondrial membrane also affects.

7. SECONDARY EFFECTS
IONIC IMBALANCE
• Salinity may cause nutrient deficiencies or imbalances, due to the competition of
Na+ and Cl– with nutrients such as K+, Ca2+, and NO.
• Na+ induced K+ and Ca2+ deficiency: Na+ ions interfere with the uptake of K+ and
Ca2+ and sometimes may exclude them out of the cell thus causing deficiency.
Deficiency of Ca2+ in the cell may affect its role as secondary messenger.

8. CA2+ BRIDGES IN PLANT CELL WALL

9. • Nitrogen and Phosphorus deficiency by Cl- and SO4
-2: High concentration of Cl-
and SO4-2 in soil cause reduced uptake of nitrogen. Moreover, The key enzyme,
nitrate reductase is very sensitive to NaCl. Application of N fertilizers may
the effect. Uptake of Phosphorus in the form of H2 PO4 and HPO4 and its
utilization efficiency also reduce drastically. Uptake of some micronutrients such
as Fe, Zn, and Cu increases in salinity which are harmful in high concentration.

10. SECONDARY EFFECTS
OXIDATIVE STRESS
• Reactive oxygen species (ROS) including hydrogen peroxide (H2O2), superoxide
anions (O2•-), hydroxyl radical (OH•) and singlet oxygen (1O2) are by-products
of physiological metabolisms. ROS are significantly accumulated under salinity
stress conditions, which cause oxidative damage and eventually resulting in cell
death. ROS causes damage to cell membrane, enzymes, organelles and even
DNA.

12. PRIMARY AND SECONDARY EFFECTS OF SALINITY ON PLANTS

13. Salinity
Tolerance

15. CROP CATEGORIES TO SALT STRESS
Based on the responses to high concentration of salts, plants can be divided into
two broad groups.
1)Halophytes
2) Glycophytes
• Halophyte (salt tolerant crops) They are native to saline soils.
• Glycophytes (Literally "sweet plants") Non-halophytes.
They are sensitive plants and unable to grow under saline conditions. Most of the
cultivated crop species belong to glycophytes.

16. PHYSIOLOGICAL ASPECTS OF
SALINITY TOLERANCE
THE MECHANISMS OF SALINITY TOLERANCE FALL INTO THREE CATEGORIES.

17. 1. TOLERANCE TO OSMOTIC STRESS.
i. The osmotic stress immediately reduces cell expansion in root tips and
young leaves, and causes stomatal closure.
ii. A reduced response to the osmotic stress would result in greater leaf
growth and stomatal conductance, but the resulting increased leaf area
would benefit only plants that have sufficient soil water.
iii. Greater leaf area expansion would be beneficial only when a supply of
water is ensured such as in irrigated food production systems, but
could be undesirable in water limited systems, and cause the soil water
to be used up before the grain is fully matured.

18. 2. AVOIDANCE
Avoidance is the process of keeping the salt ions away from the parts of the plant where they are
harmful.
i. Salt exclusion: The ability to exclude salts occurs through filtration at the surface of the root.
Root membranes prevent salt from entering while allowing the water to pass through. The red
mangrove is an example of a salt-excluding species.
ii. Salt excretion/extrusion: Salt excreters remove salt through glands or bladders or cuticle located
on each leaf.
 Salt bladders - e.g.) Atriplex ,
 Salt glands - active process, selective for sodium and chloride(e.g.) Black and white mangroves
Secretion through cuticle.
 Tamarix Salt glands- dump sites for the excess salt absorbed in water from the soil; help plants adapt
to life in saline environments.

19. iii. Salt Dilution: By dilution of ions in the tissue of the plant by maintaining
succulence. Plants achieve this by increasing their storage volume by
developing thick, fleshy, succulent structures Succulence is mainly a result of
vacuoles of mesophyll cells filling with water and increasing in size. This
mechanism is limited by the dilution capacity of plant tissues.
iv. Compartmentation: of ions Organ level - high salts only in roots compared to
shoots especially leaves.

20. 3. TISSUE TOLERANCE
i. Tolerance of tissue to accumulated Na+, or in some species, to Cl-.
ii. Tolerance requires compartmentalization of Na+ and Cl at the cellular and
intracellular level to avoid toxic concentrations within the cytoplasm,
especially in mesophyll cells in the leaf.
iii. Toxicity occurs with time, Na+ increases to high concentrations in the older
leaves.

21. COMPARTMENTALIZATION

22. • HKT1 (High affinity K transport)- potassium transporter activated in salinity stress to
avoid potassium deficiency and to reduce Na+ accumulation in leaves by both
removing Na+ from the xylem sap.
• SOS 1- salt overly sensitive channels exclude Na+ out of the cell at the expense of
H+
• NHX ((Na+)/H+ exchangers)- antiporter channels that transport Na+ into the
vacuole at the expense of H+
• Energy Providing Molecules: vacuolar H+-ATPase or V-ATPase, vacuolar H+-
translocating pyrophosphatase or V-PPase)
• Activating molecules: (SOS3, SOS2)

25. SUMMARY POINTS
1. Plant responses to salinity occur in two phases: a rapid, osmotic phase that
inhibits growth of young leaves, and a slower, ionic phase that accelerates
senescence of mature leaves.
2. Plant adaptations to salinity are of three distinct types: osmotic stress tolerance;
Na+ exclusion; and tissue tolerance, i.e., tolerance of tissue to accumulated Na+,
and possibly Cl -.
3. The salt overly sensitive (SOS) signal transduction pathway is clearly important
in salinity tolerance, although the mechanism of action at the whole plant level
remains to be established.

26. SUMMARY POINTS
4. Osmotic tolerance and tissue tolerance both increase the ability to maintain
growth for a given accumulation of Na+ in the leaf tissue. Increased osmotic
tolerance is evident mainly by the increased ability to continue production of new
leaves, whereas tissue tolerance is evident primarily by the increased survival of
older leaves.
5. Na+ compartmentation and compatible solute synthesis are important
processes for tissue tolerance. Mechanisms of osmotic tolerance remain unknown.
6. To benefit more from the new genomics approaches, molecular studies with
plants grown in physiologically realistic conditions are needed.

27. VIDEO LINK
• https://www.youtube.com/watch?v=2ZRAUO51Wf8