2. Introduction
Environmental stresses represent the most limiting
factors for agricultural productivity. Crop yields fall because
plants usually grow under environmental stress. Such
stress is both biotic and abiotic.
Biotic (Pathogens, Herbivores )
Abiotic stress (Heat, Cold, Drought, Salt, Wind, Oxidative,
Anaerobic, Heavy metals, Wounding, Nutrient deprivation,
Excessive light )
3. Agricultural productivity is severely affected by soil salinity
because salt levels that are harmful to plant growth affect
large terrestrial areas of the world
Salt-affected soils covers over 400 million hectares, which
is over 6% of the world land area (FAO report)
Current 230 million ha of irrigated land, 45 million ha are salt-
affected (19.5 %) and of the 1,500 million ha under
dryland agriculture, 32 million are salt-affected to varying
degrees (2.1 %)
In Tamil Nadu, about around 7.0 million ha of cultivable land is
reeling under salt stress
4. State Area under saline/alkaline soils (million ha)
Uttar pradesh 1.280
Gujarat 1.200
West bengal 0.840
Rajasthan 0.720
Punjab 0.680
Maharashtra 0.528
Haryana 0.520
Karnataka 0.400
Orissa 0.400
Madhya pradesh 0.240
Andhra pradesh 0.024
Delhi 0.016
Kerela 0.016
Tamil nadu 6.872 (or about 7 million hectares)
The extent of saline and alkali soils in India
5. Salinity Stress
This is due to the following soil characteristics:
i. High soil pH (less than 8.5)
ii. High electrical conductivity - EC value (more than 4.0 dS m-1)
(deciSeimens per meter)
i. Low ESP - Exchangeable Sodium Percentage (less than 15.0)
ii. Predominantly due to excessive content of chlorides and
sulphates of sodium
Types of salt stress
6. Following soil features are found to cause the sodicity
stress in plants
i. High soil pH (more than 8.5)
ii. Low EC value (less than 1.0 dS m-1)
iii. High Exchangeable Sodium Percentage (more than 15.0)
iv. Predominantly due to excessive content of carbonate and
bicarbonates of sodium
Sodicity Stress
7. Classes of saline and alkali soils
Type of salinity/
Property
SALINE SOILS SALINE-ALKALI
SOILS
NON SALINE
ALKALI OR
SODIC SOILS
pH < 8.5 < 8.5 > 8.5
EC
(mmhos/cm)
> 4.0 > 4.0 < 4.0
ESP < 15 > 15 > 15
Other properties white
encrustation on
the surface
- surface soil is
discolored and
black
8. Based on the reaction of plants to salt stress, the mesophytes
are classified into following categories:
Halophytes:
These plants can grow in saline environment and are found to
be tolerant to salt stress. Depending upon the degree of
tolerance, there are two types of halophytes.
1. Euhalophytes
They are extremely tolerant to salt stress (eg. Atriplex).
2. Oligohalophytes (Facultative halophytes)
These plants are moderately tolerant to salty environment (eg.
salt tolerant crop species)
Classification of plants under salt stress
9. Glycophytes
These plants can not grow in the presence of salts in the
growing medium. They are susceptible to salt stress.
Almost all cultivated salt-sensitive crop species come
under this category only.
10. Besides, depending upon the nature of accumulation of sodium
content, the plants are classified as:
Sodium accumulator: These plants will absorb more of
sodium form the external medium and accumulate the excess
content in the stem portions without being translocated to other
parts of the plant.
Sodium avoider: Under this category, the plants will avoid the
uptake of sodium form the soil preferably due to excessive
uptake of potassium, calcium etc.
11. 1. Primary effects
2. Secondary effects
1. Primary Effects:
These effects are again sub-divided into:
Direct Effects:
This is mainly due to accumulation of specific ions. As a result,
osmotic pressure is increased; availability of physiological
water is reduced causing "Physiological Drought".
.
Salt stress effects on plants
12. Indirect Effects:
This is mainly due to disturbances in the metabolic activities
of the plants causing inhibition in growth and development
of plants. There is a greater penetration and accumulation of
larger quantities of Na+ in the form of Cl-, CO3
- or HCO3
- etc.
13. Secondary Effects
Causing nutrient deficiency
Excessive accumulation of Na+ causes deficiencies of certain
essential elements such as N, P, K, Ca, Mg, Zn, Fe etc. thus causing
derangement of metabolism in the plants
14. Salinity inhibit plant growth for two reasons
Presence of salt in the soil solution reduces the ability of the
plant to take up water, and this leads to reductions in the
growth rate (osmotic or water-deficit effect of salinity )
Excessive amounts of salt enter the plant in the transpiration
stream there will be injury to cells in the transpiring leaves and
this may cause further reductions in growth (salt-specific or
ion-excess effect of salinity )
15. Variation in Salt Tolerance between Species
Number of agricultural horticultural crops affected by salinity
Degree of responses vary (depending on salt tolerance
mechanism)
Examples:
Wheat is one of the more salt-tolerant crop species, and many
cultivars that have been selected for yield in water-limited
conditions do not suffer a 50% reduction in biomass until
salinities reach 15 dS/m (approximately 150 mM NaCl)
Rice is more salt-sensitive, and many cultivars suffer a 50%
reduction in growth at half this concentration of salts
16. All living organisms in dry and saline environments is to
maintain water content this is achieved by solute accumulation
which lowers solute potential
The solutes accumulated in the cytoplasm must be non-toxic
(compatible) with respect to metabolic processes (they should not
interfere with protein structure or function when present at high
concentration)
This osmotic adjustment is a fundamental adaptive response
of plant cells to salinity.
This process is necessary for their survival and growth under
saline conditions
17. A positive turgor is indispensable for expansion growth of
cells and stomatal openings in plants
A decrease in water potential due to soil salinity causes
osmotic stress that leads to turgor loss
Plants have evolved an osmotic adjustment (active solute
accumulation) mechanism that maintains water uptake and turgor
under osmotic stress conditions
Sodium Compartmentation
18. Osmotic adjustment, plants use inorganic K+ and/or
synthesize organic compatible solutes such as proline, betaine,
polyols and soluble sugars
Salinization causes relatively little change in vacuolar and
cytoplasmic K+ in leaves. Na+ and C1- accumulated mainly in the
vacuole and Betaine mainly in the cytoplasm (chloroplasts and
cytosol)
19. The vacuole occupies about 90% of the cell volume and the
chloroplast and cytosol about 5% each. Hence osmotic adjustment
of the bulk of the cell water is achieved with Na+ and C1- which are
readily available from the salinized growing medium but are toxic
to metabolism
Glycine-betaine or other osmoprotectants which are non-toxic
but energetically expensive is useful for osmotic adjustment only
in the crucial metabolic compartments.
20. Vacuolar sequestration of Na is an important and cost-effective
strategy for osmotic adjustment that also reduces the Na
concentration in the cytosol
Na sequestration into the vacuole depends on expression and
activity of Na/H antiporters as well as on H –ATPase
These phosphatases generate the necessary proton gradient
required for activity of Na/H antiporters
Sequestration of Sodium
22. Growth inhibition
In most of the mesophytes, salt level of more than 50mM
often causes decline in the growth of plants
Both root and shoot growth is affected
Biomass production is decreased by 50% than the normal
environment
This reduction is mainly due to several physiological
aberrations induced by the salt stress
Physiological and Biochemical Changes
23. Pigments
There was a reduction in the leaf chlorophyll content from 8.2
to 32.8 per cent
Hill reaction of photosynthesis is affected severely in salt
stress leading to decline in the production of energy rich ATP
molecules and thus there was a close correlation between salt
tolerance and the rate of Hill reaction in the chloroplast
Carotene contents increased and neoxanthine content
decreased in salt resistant varieties, while in the susceptible
varieties, neoxanthine content increased but the carotene
content did not change
However, adverse effect of salinity was expressed mainly
through its effect on photosynthesis rather than decrease in the
chlorophyll level.
24. Inhibition of nitrogen assimilation is reported in several
cases under salt stress
Inhibition of nitrate reductase (NRase), the enzyme reducing
nitrate to nitrite, is the most sensitive reaction in nitrate
assimilation process as reported in beans and spinach when
grown under salt affected soils
Other enzymes involved in ammonium assimilation pathway
are also inhibited by salt stress
Nitrogen and Protein metabolism
25. Protein content decreased due to salinity stress. Soluble
protein content is also found to be decreasing in salt
stressed plants
A large number of protein revealed to show up / down
regulation due to salt stress. The proteins, which are up-
graded by the stress condition, are refereed as "Stress
Proteins“
26. Reduced uptake of K+ and other monovalent cations has been
reported under salt stress and also inhibition of mineral
transport from root to shoot is common
This may be due to the competitive effects of Na+ and Cl- with
other essential elements. Contents of P, Mg, Ca, Zn, B, Fe and
Cu are also found decreasing due to salt stress
Mineral uptake and transport
27. Enzyme activities
The activities of free radical scavenging enzymes such as
catalase, peroxidase and Super Oxide Dismutase (SOD) are
known to increase in the tolerant genotypes under salt
stress. This helps in the removal of excessive accumulation
of free radicals thus averting the membrane damage due to
the stress.
28. There is a five-fold reduction in the content of IAA
observed under salinity stress in plants
Similarly, the levels of gibberellins and cytokinins are also
found to decrease due to salinity stress
Whereas ABA and ethylene increase under the salt stress,
which will induce early on-set of senescence (yellowing due
to ageing) in crops thus reducing yielding ability of crops
Growth Regulators
29. Compatible Osmolytes
osmotic adjustment may be energetically more favorable than biosynthesis of
organic osmolyte under osmotic stresses, many plants accumulate organic
osmolytes to tolerate osmotic stresses
STRESS
Physiological and
developmental
response
Stress
recognition
Signal
transduction
Gene expression
Altered cell
metabolism
30. These osmolytes include proline, betaine, polyols, sugar
alcohols, and soluble sugars
The range of compounds participating in osmotic adjustment is
usually qualitatively the same as that accounting for solute
potential in nonstressed tissue
Glycine betaine and trehalose act as osmoprotectants by
stabilizing quaternary structures of proteins and highly ordered
states of membranes
Proline serves as a storage sink for carbon and nitrogen and a
free-radical scavenger. It also stabilizes subcellular structures
(membranes and proteins), and buffers cellular redox potential
under stress
31. The physiological changes of the adaptation process are
accompanied by increases or decreases in a relatively small set of
cellular proteins, some of which have been purified and
characterized
The most extensively studied protein of this class has been
referred to as osmotin (26 kDa)
rab 21 gene from rice which encode a gly-thr rich protein. rab
21 is not an integral membrane protein
Sal T mRNA accumulates very rapidly in sheaths and roots of
rice
Stress-induced proteins
32. Conclusion
Understanding the biosynthesis, transport, roles of osmoprotectants
during stress and the signaling events that regulate stress-induced
accumulation is vital in developing plants for stress-tolerance
Vacuole occupies Na+ and Cl- about 90% of the cell volume and the
chloroplast and cytosol about 5% each and are toxic to metabolism
Glycinebetaine or other osmoprotectants which are non-toxic but
energetically expensive is useful for osmotic adjustment only in the
crucial metabolic compartments
It is important to use Molecular approaches to study the available
transgenic plants to identify the metabolic pathways influenced due to
changes in osmoprotectant level
