Salinity stress Categorization of salt affected soils CAUSES OF SALINITY IN SOIL Salinity effects on Plants Injuries due to salt stress different strategies to avoid salt injury salt tolerance salt avoidance salt evasion halophytes non halophytes glycophytes Breeding for salt tolerance

1. SALINITY STRESS
Name - Parul Sharma
PhD Botany

2. Salinity is one of the most serious factors limiting the productivity of agricultural crops, with adverse
effects on germination, plant vigour and crop yield.
High salinity affects plants in several ways: water stress, ion toxicity, nutritional disorders, oxidative
stress, alteration of metabolic processes, membrane disorganization, reduction of cell division and
expansion, genotoxicity.
Together, these effects reduce plant growth, development and survival.
Soil salinity is the salt content in the soil and the process of increasing salt content is known as
salinization.
Salinization can be caused by natural proceses such as minreal weathering or gradual withdrawn of an
ocean. It can also be caused by artificial processes such as irrigation.
Under natural conditions, higher land plants growing near the seashore and estuaries encounter higher
salt concentrations in the soil. The accumulation of salts in the soil from irrigation water is much more
problematic in agriculture. Higher concentrations of sodium, chloride and carbonate ions are
potentially toxic to salt sensitive plants. Presence of high salt concentrations in the soil is a common
and important stress factor in deserts.

3. Categorization of salt affected soils:
1. Saline soil: Soil Structure- Usually good ii) Infiltration rate- High iii) Soil Aeration- Good
c) Colour- Usually white
2. Sodic Soil (Black-alkali soil): i) Soil Structure - very poor (soil is in highly dispersed
condition) ii) Infiltration rate - very poor iii) Soil Aeration - very poor c) Colour- Usually
black
3. Saline-Sodic: i) Soil Structure - good ii) Infiltration rate - good iii) Soil Aeration - good
c) Colour- Usually white
4. Normal: i) Soil Structure - good ii) Infiltration rate - good iii) Soil Aeration - good c)
Colour- Usually white

4. CAUSES OF SALINITY IN SOIL -
Natural causes -
1. The accumulation of salts in the soil can occur through natural processes such as
physical or chemical weathering and transport from parent material, geological deposits or
groundwater.
2. It can also occur due to parent rock constituents, such as carbonate minerals and/or
feldspars or as a result of the one-time submergence of soils under seawater. Sea level rise
also induces seepage into areas lying below sea level.
3. In arid areas, saline soils are formed due to evapotranspiration and lack of rainfall to
flush the soils.
4. Wind in coastal areas can blow moderate amounts of salts inland.

5. Human activities -
1. Human activities can cause salinization through the use of salt-rich irrigation water,
which can be exacerbated by overexploitation of coastal groundwater aquifers causing
seawater intrusion, or due to other inappropriate irrigation practices, and/or poor
drainage conditions. The excessive use of water for irrigation in dry climates, with
heavy soils, causes salt accumulation because they are not washed out by rainfall.
2. The practice of waterlogging without adequate drainage has also become a
serious cause of soil salinization. Waterlogged soils prevent leaching of the salts
imported by the irrigation water.

6. The reactions of plants to salinity depend on specific degree of tolerance against soil
salinity. Plants are classified according to their biomass production under salt stress.
1. Group lA (Eu-halophytes), which show
stimulation of productivity at moderate salinity
(e.g., Suaeda maritima, Atriplex nummularia ).
2. Group lB (Facultative halophytes), showing
slight growth enhancement at low salinity.
These plants tolerate salt but their growth is
retarded. (e.g., Plantago maritima, Aster
tripolium ).
3. Group ll (Non-halophytes), with low salt
tolerance. (e.g., Hordeum sp., Gossypium sp.).
4. Group lll (Halophobic plants), are very salt
sensitive plants that are severely inherited or
killed by low salt concentrations (e.g., Phaseolus
vulgaris, Glycine max).
The growth of different species subjected to salinity relative to that of unsalinized
controls. The curves dividing the regions are based on data for different species.
Plants were grown for 1 to 6 months.

7. Salinity stress involves changes in various physiological and metabolic processes, depending
on severity and duration of the stress, and ultimately inhibits crop production. Initially soil
salinity is known to represses plant growth in the form of osmotic stress which is then
followed by ion toxicity. During the initial phases of salinity stress, water absorption capacity
of root systems decreases and water loss from leaves is accelerated due to osmotic stress of
high salt accumulation in soil and plants, and therefore salinity stress is also considered as
hyperosmotic stress. Osmotic stress in the initial stage of salinity stress causes various
physiological changes, such as interruption of membranes, nutrient imbalance, impairs the
ability to detoxify reactive oxygen species (ROS), differences in the antioxidant enzymes and
decreased photosynthetic activity, and decrease in stomatal aperture. One of the most
detrimental effects of salinity stress is the accumulation of Na+ and Cl− ions in tissues of
plants exposed to soils with high NaCl concentrations. Entry of both Na+ and Cl− into the
cells causes severe ion imbalance and excess uptake might cause significant physiological
disorder.

8. Salinity effects on Plants -
Under natural conditions terrestrial plants encounter higher concentration of salts.
A much more extensive problem in agriculture is the accumulation of salts from
irrigation water. Evaporation and transpiration remove pure water from the soil,
and this water loss concentrates solutes in the soil. When irrigation water contain a
high concentration of solutes and when there is no opportunity to flush out
accumulated salt to a drainage system, salt can quickly reach levels that are
injurious to salt sensitive species. Salinity in soil affects plants growth to various
levels.

9. 1 High salt concentration in soil solution reduces the ability of plant to acquire
water, which is referred to as the osmotic for water deficit effect of salinity. The
osmotic effect of salinity induces metabolic changes in the plant identical to those
caused by water stress induced wilting.
2. Salinity stress reduces plant growth due to specific ion toxicity and nutritional
imbalances. Salinity effects on plant growth reduction is divided into two phases. The
first phase is very rapid growth reduction due to development of water deficit. the
second phase is due to the accumulation of salts in the shoot at toxic level and is very
slow.
3. Salinity affects photosynthesis by decreasing carbon dioxide availability as a result of
diffusion limitation and A reduction of the contents of photosynthetic pigments. Salt
accumulation in spinach inhibits photosynthesis by decreasing stomatal and mesophilic
conductance to carbon dioxide and reducing chlorophyll content which can affect light
absorbance.

10. In radish about 80% of the gross deduction at high salinity could be attributed to reduction
of leaf area expansion and hence to A reduction of light interception. The remaining 20% of
salinity effect on growth was most likely explained by a decrease in stomatal conductance.
4. Salt accumulation in the root zone causes the development of osmotic stress and
distrupt cell ion homeostasis by inducing both the inhibition in uptake of essential elements
such as potassium calcium and nitrate and the accumulation of an Na+ and Cl-.
5. Accumulation of injurious ions may inhibit photosynthesis and protein synthesis in
activate enzymes and damage chloroplast and other organelles. these effects are more
important in older leaves as they have been transpiling the longest so they accumulate more
ions.
6. A decrease in plant biomass, leaf area and growth has been observed in different
vegetable crops under salt stress.

11. 7. Plant deficiencies of several nutrients and nutritional imbalances may be caused by
higher concentration of Na+ and Cl- in the soil solution derived from ion competition
(Na+/ Ca2+, Na+/K+, Ca2+/Mg2+ and Cl-/NO3-) in plant tissues. Calcium deficiency
symptoms are common when the the sodium calcium desho is higher in soil water.
8. Visible symptoms of salt injury in plant growth appear progressively. The first signs of salt
stress are wilting, yellow leaves, and stunted growth. In a second phase the damage
manifests as chlorosis of green parts, leaf tip burning, and necrosis of leaves and the
the oldest leaves display scroaching.
9. Salt stress decreases marketable yield due to decreased productivity and an increased
and marketable yield of fruits roots tubers and leaves without commercial value.

12. A schematic view on
salinity effects on plants
(osmotic effect and ionic
effect) and a general plant
response to salinity effects
leading to signalling,
changes in gene
expression, changes in
protein expression.

13. Injuries due to salt stress -
1. High osmotic pressure of the soil solution/Osmotic stress: due to high conc. of salt
resulted in decrease in osmotic potential leading to development of more (-) water
potential, so to maintain downhill gradient from soil to the plant, more –ve water
potential is to be created in the leaves. This requires synthesis of organic solutes which is
an energy dependent process. So a lot of energy of the plant is utilized to maintain this
downhill gradient.
It can increase the osmotic potential and hence decrease water availability; Because of
high O.P plant root find difficulty in absorbing high quantity of water and it is due to
presence of soluble salts in soil.
Due to high salt conc. plants have to spent more energy to absorb water and smaller
quantity of energy is left for growth in function, seriously affected in cell elongation,
leaves become deep green colour, cell becomes flaccid and loss turgidity of the cell.

14. 2. Specific-ionic effects: Different ion toxicity varies from plant to plant however
accumulation of Na+, Cl- and SO42- are highly injurious.
Under normal condition Na+/K+ ratio will be low. But under salinity high Na+/K+ ratio
resulted in reduction in enzyme activity, reduced protein synthesis, inhibition in
photosynthesis. However, in case of photosynthesis electron transport chain (ETC) is less
sensitive to salt stress but carbon metabolism and photophosphorylation is high.
Phophate, Fe, Zn and Mn become unavailable to the plant at high pH value and soil structure
tends to become water unstable bringing about conditions of low water permeability and poor
aeration.
3. Nutritional imbalance :In antagonistic, there is competition between the absorption and
excess of Na will be antagonistic to k absorption and K plays important role in different
physiological function. So high conc. of Na will reduce the K uptake. In synergistic effect:
the presence of ions will promote the uptake of other ions as in case of high Ca, absorption
of K increases. In case of neutral, there is no effect of ion on the absorption of other ions.

15. Plants uses different strategies to avoid salt injury
1. Salt tolerance-
Salt tolerance is obtained only in those plants where protoplasm can endure high salt content
without apparent damages and function normally. It varies among different organs of the
same plant, having tissues and among different stages of development of a plant. Tolerance to
salt stress is the ability to tolerate toxic as well as osmotic effects of salt ions in the cytoplasm.
Plant cells are capable of sensing high salinity and ion specific signals of salt stress. It is
presumed that Na+ can be sensed either before or after entering the cell. Presence of
membrane receptors and membrane proteins are probably responsible for sensing extra cell
To counter balance or to prevent sodium accumulation into cytoplasm, three possible
tolerance mechanisms have been implicated, such as:

16. (i) Reducing Na+ entry into cells,
(ii) Active Na+ efflux from the cell and
(iii) Active transport of Na+ into the vacuoular and intracellular Na+, respectively.
Transportation of sodium into the vacuole requires input of energy and is executed against
concentration gradient. This process is accomplished by coupling transport protein to a
proton pump. The electrochemical gradients of protons are generated by the vacuolar H+-
translocating enzyme like H+ ATPase. Consequently, H+ ion moves in opposite direction
through tonoplast membrane.
In Arabidopsis, Na+ efflux is carried out by the plasma membrane Na+/H+ antiporter is
encoded by the SOS1 (salt overly sensor) gene. Salt stressed plants have been found to exhibit
SOS1 activity in almost all tissues.SOS1 facilitates long distance transport of Na+ and it is
significant in the removal of sodium through transpiration and the vacuolar sequestration of Na+
in leaves.

17. In the proposed model of signal
transduction under salt stress
condition, high level of Na+ increases
calcium in cytoplasm that might act as
a key component of Na+ stress
signalling. SOS3 is a calcium-binding
protein capable of sensing cytosolic
calcium and activate SOS2 (protein
kinase). This protein kinase in turn
phosphorylates and activates plasma
membrane Na+/H+ antiporter SOS1.
The carboxyl-terminal regulatory
domain of SOS2 interact with SOS3,
mediated by 21 amino-acid sequences,
FISL, motif. Finally SOS1 mRNA is
stabilized and accumulates under salt
stress.

18. 2. Salt avoidance - this is usually accomplished by growth and reproduction in specific
seasons during the year, by growing roots into non saline regions or by limiting germination.
3. Salt evasion- this is achieved by accumulation of solutes in specific cells of the plant or by
secretion of of excess of salts from the plant. In some helophytes incrustation of salt can be
observed on the surface of the leaves as a result of excretion of excess salt e.g., Tarmarix
pentandra. In Atriplex spongiosa special salt glands are found on the surface of leaves. The
ions are transported to these glands bear crystallization of salt occures.

19. 4. Restriction of uptake of salts-
the restriction of uptake can be achieved by inhibition of root uptake which is found in
mangroves. Strategies have evolved to restrict salt transport into sensitive organs or tissues. In
various species of fabiaceae, plants sequester salt ions which move with the transpiration
stream and prevent them from reaching sensitive parts of plant.
5. Dilution of salt by Succulence -
Certain plants develop fleshy and thick succulent organs under salt stress. succulents results
from increased water uptake of the tissue and this may help to dilute the salt ions. However
the dilution capacity of tissue is limited and this strategy can help plants to cope with low
levels of salinity.
6. ROS Detoxification - The pathway of reactive oxygen species detoxification plays an
important role in scavenging toxic radicals overexpression of enzymatic genes that led to
NaCl tolerance.

20. 7. Osmotic adjustment -
In order to counterbalance low water potentials of saline soils, some plants use a controlled
accumulation of salt ions. This is osmotic adjustment on the whole plant level. In cells, salt ions
are compartmentalized and sequestered in vacuoles to avoid toxic effects in the cytosol. the
osmotic balance between vacuole and cytosol is maintained by accumulation of compatible
organic solutes in the cytoplasm. These include polyols (e.g., sorbitol or mannitol), amino acids
and amides (proline), quaternary ammonium compounds (betaine) and soluble carbohydrates
(sugars). Besides osmotic adjustment, some compatible solutes, like betaine or proline were
shown to have stabilizing effects on enzymes under salt stress.The accumulation of salt ions
into the vacuoles results in low osmotic potentials in the vacuole. To prevent dehydration of
the cytosol, its osmotic potential must be adjusted to the level of the vacuole. This can be
achieved by accumulation of osmotically active, organic solutes in the cytosol, which do not
interfere with the physiological processes. Such substances are called compatible solutes.

21. Type types of plants on the basis of salinity tolerance -
On the basis of ability of plants to tolerate salt, they are broadly divided into
halophytes and non halophytes or glycophytes.
A. HALOPHYTES -
Plants which grow and complete their life cycle in the habitats with a high salt
content are called halophytes or salt plants. These plants grow in in saline soils
where the concentration of simple or inorganic salts is so great that only specially
adapted plants can grow. These plants can grow in water of high salinity coming
into contact with saline water through its roots.

22. Classification the halophytes on the basis of plant – soil relationship, salt resistance
mechanism and internal salt relationship :
I. Euhalophytes-
1.Salt requiring halophytes
A. Obligatory halophytes : Plants survive only in saline environment e.g. Salicornia spp.,
Aphanothece halophytica (Blue green alga).
B. Preferential halophytes-: Plant whose growth and development improved in the
presence of salt, e.g. Suaeda sp., Aster sp.
2. Salt-resisting halophytes:
A. Salt-enduring : Plant enduring a high protoplasmic salt content. e.g., Suaeda monoica.

23. B. Salt excluding halophytes : Plant accumulating salts in special hair e.g. Atriplex spp.
plant secreting salts from their shoot (Tamarix sp.); plants re-transporting salts from the shoot
into root.
C. Salt evading halophytes: Plant evading salt uptake (Rhizophora); evading salt transport
into the leaves (Prosopis foxta).
II. Pseudo halophytes: Plants behave like halophytes, e.g. ephemerals.
Halophytes can be clearly distinguished on the basis of salt composition in their ash
into the following:
1. Sulphate halophytes e.g., Salsola rigida
2. Chloride halophytes e.g., Salicornia europaea
3. Alkaline halophytes e.g., Suaeda microphylla

24. The ecological conditions which are essential for the development of mangrove
vegetation or halophytes are:
(a) Shallow water with thick mud,
(b) Water logged saline soil or sandy or loose soil or heavy clays containing large amount or
organic matter,
(c) High rainfall, and
(d) High humidity in the atmosphere.

25. Characters of halophytes -
A majority of halophytes in the tropical and subtropical region are shurbs but a few of them
are herbaceous. In temperate zones the halophytic vegetation is purely herbaceous.
Roots - In halophytes, in addition to normal roots, many stilt or prop roots develop from
the aerial branches of stem for efficient anchorage in muddy or loose sandy soil. These roots
grow downward and enter the deep and tough strata of the soil. The soil in coastal region is
poorly aerated and it contains very small percentage of oxygen because of water logging.
Under such conditions, the roots of halophytes do not get sufficient aerahon. In order to
compensate this lack of soil aeration, the hydro halophytes develop special type of negatively
geotropic roots, called pneumatophores.They usually develop from the underground roots
and project in the air well above the surface of mud and water.

26. Stem - Stems in several halophytes develop succulence e.g., Salicornia herbacia. Succulence
is induced only after the accumulation of free ions in an organ increases above a critical
level. Salinity inhibits the cell division and stimulates cell elongation. Such effects cause
decrease in the cell number and increase in cell size, so typical of succulents. The succulence
is directly correlated with salt tolerance of plants and the degree of their development can
serve as an indicator of the ability of plants to survive in highly saline habitats.

27. Leaves: The leaves in most of the halophytes are thick, entire, succulent, generally small-
sized, and are often glassy in appearance. Some species are aphyllous. Leaves of submerged
marine halophytes are thin and have very poorly developed vascular system and frequently
green epidermis. They are adapted to absorb water and nutrients from the medium directly.
Fruits, Seeds and their dispersal: The fruits and seeds are generally light in weight. Fruit
walls have a number of air chambers and the fruits, seeds, and seedlings which can float on
the water surface for pretty long time are dispersed to distant places by water current. All
littoral halophytes show very wide areas of distribution. This is because of medium and
uniform temperature. The air containing spaces in the integument or in other parts helps
fruits, seeds and seedling of mangroves in floating.
Osmotic potential : The salts which are absorbed by halophytes accumulate in transpiring
organs. The presence of salt in cell vacuoles of these organs causes a decrease in osmotic
potential and will not affect the hydration of the protoplasm as by water stress.

28. Anatomical features:
1. A thick cuticle is present even in the young
stem.
2. Epidermal cells are thick and filled with tannin
or oils.
3. Cortex is fleshy, several cells thick and in old
stems it may become lacunar.
4. Leaves may be dorsiventral or isobilateral. They
develop protected stomata which are not deeply
sunken. Highly developed air chambers are
continuous with the stomata of leaves and with
the cortex and primary phloem of the stems.
5. Stems in the succulent plants possess thin-walled
water storing parenchyma cells in them.

29. Salt tolerance in halophytes -
1. Research efforts during the last two decades have led to the identification of essential
features of the cellular basis of slat tolerance in halophytes. For e.g. Members of the
Chenopodiaceae at least, the ability to withstand high external salt concentrations
appears to depend on osmotic adjustment, most commonly with Na+ and Cl-. As
these are toxic, their concentration in the cytoplasm is maintained substantially lower
than in the vacuole. Osmotic adjustment in the cytoplasm is achieved with compatible
solutes such as glycine betaine and proline.
2. It has been concluded that transport and regulation of Na+ and Cl- ions is of primary
importance in the physiology of salt tolerance of halophytes.
3. Although halophytes are tolerant to salinity but it has been observed that at
germination stage they are less tolerant as compared to later growth stages.
4. Under natural conditions, seeds of halophytes may remain dormant till the onset of
rains.

30. B. GLYCOPHYTES/NON HALOPHYTES -
Glycophytes can be defined as any plant that will only grow healthily in soils with a low
content of sodium salts or plant whose growth is inhibited by saline soil.
These plants evolved by adaptation under natural selective pressures in ecosystems with low
soil sodium levels and which maintains low sodium levels in its aboveground tissues,
especially in its leaves.
The glycophytes can be distinguished as plants of non-saline environments, normally having
high K/Na ratio in their leaves, from halophytes, the plants of saline environments, in which
K/Na ratio tends to be lower and the overall ion concentration higher.

31. Breeding for salt tolerance -
The success of the crop-breeding program largely depends on the availability of natural
genetic variation among the germplasm resources. Large number of cultivated and wild
germplasm in major crops, preserved in the Consultative Group on International Agricultural
Research (CGIAR) institutions and national centers, provide unique resources for systematic
screening for discovery of novel variability to improve adaptation of crop plants in saline
environments.
Several parameters for salinity tolerance are studied by growing the germplasm in a variety of
culture techniques such as hydroponics, pot culture, and fi eld screening. Plant materials are
evaluated from germinating seeds through seedlings up to mature plants.

32. Mutation breeding –
Creation of novel and useful genetic variation in important agronomic traits is the most
important prerequisite for a crop-breeding program. Mutagenic agents, such as X-rays,
gamma rays, fast neutrons, and chemical mutagens such as ethyl methane sulfonate have been
used to induce mutations in seeds to generate genetic variation for crop improvement. It
offers the possibility of inducing desired characters that either cannot be found in nature or
have been lost during evolution.
The mutants with abiotic stress tolerance can be selected by plant breeders to develop
salinitytolerant crop plants. The purpose of induced mutations is to enhance the mutation
frequency rate in order to select appropriate variants for salinity tolerance.
Some outstanding examples of mutant rice cultivars are VND 95-20 and VND 99-3 which
have long grains with excellent grain quality and wide adaptation to acid sulphate soil and
salinity. These mutants not only increased biodiversity, but also provided valuable breeding
material for crop improvement.

34. Tissue culture approach -
Plant tissue culture techniques provide a promising and feasible approach to develop salt-
tolerant crop plants. Haploid culture, double haploidy, somaclonal variation, and in vitro-
induced mutagenesis has been used to create variability to improve salinity tolerance in crop
plants.
Cell and tissue culture techniques have been used to obtain salttolerant plants through in
vitro culture approaches: selection of mutant cell lines from cultured cells and subsequent
plant regeneration.
Tissue culture has been used as a breeding tool for rapid screening of genetic materials for
salt tolerance in wheat.
The in vitro selected tolerant genotypes showed signifi cantly better performance for
biomass production under high salinity condition than fi eld-derived tolerant genotypes.

35. An in vitro procedure for generation of salinity tolerant crop plants.

36. THANK YOU