Agriculture Reference
In-Depth Information
Sensing
Ionic homeostasis
Cell death
Osmotic adjustment
Salt stress
sensitive
germplasm
Signal
transduction
Photosynthesis &
photorespiration
Speci c gene
expression
ROS homeostasis
Retarded plant
growth
Energy metabolism
RNA
processing
Acclimatization
to salt stress
Leaf senescence
Salt stress
tolerance
germplasm
Cell wall & other
organelle functions
Salt response
protein
synthesis
Other metabolism
Figure 2.1 Effect of salt stress and responses of plant.
The harmful impacts of salinity include:
• reduced agricultural production;
• lower economic returns due to higher costs of cultiva-
tion, reclamation, management, etc.;
• soil erosion due to high dispersibility of soil;
• ecological imbalance due to halophytes and marine
life forms invading as fresh waters become brackish;
• adverse impact on human health due to toxic effects
of elements such as B, F and Se.
Most crops are sensitive to salt stress at all stages of
plant development, from seed germination through veg-
etative and reproductive growth. Thus, soil salinity is a
major factor limiting sustainable agriculture (Hu &
Schmidhalter, 2002). Salt stress instigates molecular
damage in plant cells that primarily causes hyperos-
motic stress, homeostasis disruption and ionic toxicity
(Figure  2.1). Osmotic stress inhibits a plant's ability to
take up water, and this leads to what is termed 'water
deficit' or 'physiological drought' or 'cellular dehydra-
tion' (Munns, 2005). Water deficit causes a reduction in
the rate of photosynthesis, and exposure of chloroplasts
to excess excitation energy limits carbon dioxide fixa-
tion, leading to the generation of active/reactive oxygen
species (A/ROS). Electrolytes of mineral salts, but espe-
cially Na + and Cl , can enter plant cells through the
transpiration stream and accumulate in cellular and
extracellular compartments. Although Na + represents
the major ion causing toxicity related to high salinity,
some plant species are also sensitive to chloride, the
major anion found in saline soils. Eventually excess
build-up of internal concentrations of Na + and Cl injures
cells and affects growth (Chinnusamy et al., 2005).
Salt stress also causes secondary stresses like oxidative
stress, which accelerates the production of A/ROS and
subsequently alters the balance between the formation
and removal of such species (Zhu, 2002). Many plants
under salt stress also suffer with other kinds of secondary
stresses like mechanical stress and nutritional imbal-
ance. The elevated levels of Na + , a major ion in saline
environments, can induce deficiency of the essential
element K + , impose toxic effects by perturbing K + -
dependent processes, and induce deleterious changes in
protein conformation (Mahajan & Tuteja, 2005). During
the onset and development of salt stress, all major
processes are affected, including photosynthesis, protein
synthesis, energy production and lipid metabolism. The
effects of mild cases of salt stress are primarily limited to
plant growth, development and crop productivity, but
in extreme cases, salt stress can lead to plant death
(Parida & Das, 2005).
2.3 Responses of plants to salinity
Plants are traditionally classified as halophytes or non-
halophytes (glycophytes) referring to their capacity to
grow on highly saline environments (Flowers et  al.,
1977). The exact definition of halophytes is still disputable.
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