synthesized under stress are late embryogenesis abundant (LEA), dehydrins, desiccation stress proteins, protease, etc. (Lisar et al. 2012).
When there is flooding plants undergo hypoxia, a stage when oxygen concentration is less. During flooding or excessive irrigation water logging occurs in soil, thus
limiting the oxygen availability to roots. When water logging is for a prolonged
period damage occurs depending on the extent of adaptability and soil conditions.
The damage also depends on the type, developmental stage, genotype of plants,
extent of severity, and duration of water stress (Fukao and Bailey-Serres 2004;
Mariani and Ferrante 2017).
1.2.4 Salinity Stress
Salinity stress is another major problem of world, which decreases crop productivity.
High concentration of salt accumulates in the soil either due to improper irrigation or
poor drainage. Soil salinity is measured by accumulation of soluble salts such as Cl
À ,
SO 4
2À
, Na
+
, Ca
2+ , etc., and it harms the plant growth and development through
water stress and cytotoxicity (Isayenkov and Maathuis 2019). Soil salinity may be
categorized into primary (natural salinity; for example, marshes, salt lake, tidal
swamps, etc.) or secondary (due to anthropogenic activities). High saline condition
decreases the water potential and causes osmotic stress. Osmotically active
compounds or osmolytes (Glycine-Betaine) also known as compatible metabolites
accumulate in cells to lower the osmotic potential (Rasool et al. 2013). Salinity is
also many times accompanied by the formation of ROS, polyamines (putrescene),
and proline. Saline soils also limit the uptake of nutrients by root from soil (Sharma
et al. 2016). During high salinity stress, the photosynthetic activities gets affected as
thylakoid membranes of chloroplast get damaged, reduction of photosynthetic
pigments takes place, further inactivation of electron transport chain, photophosphorylation occurs (Srivastava et al. 2019). Also, the turgor pressure of leaves
changes reducing the stomatal conductance and rate of transpiration (Srivastava
et al. 2019). ABA accumulates in guard cells, excessive production of hydrogen
peroxide occurs which leads to triggering of oxidative stress by influencing the
production of reactive oxygen species (Foyer and Noctor 2005). It has been reported
that legumes and cereal crops are very sensitive to salinity. Salinity affects nitrogen
fixation as it suppresses the formation of root nodules (Ramana et al. 2012).
Nutritional imbalance enhances vegetative growth over reproductive growth, thus
decreasing crop yield (Munns and Tester 2008). It also causes chlorosis and senescence, DNA, RNA content decreases, mitotic and enzymatic activity slows down
(Ali et al. 2004).
The physiological drought condition in plants can be overcome by the application
of certain soil microorganisms such as arbuscular mycorrhizal fungi (AMF). Symbiotic association established between AMF and plant roots, mainly in saline
environments, could withstand plants under excessive saline conditions as the
AMF hyphae run several metres away from the stressed area or from the zone of
nutrient depletion, thereby increasing root surface area and facilitating nutrient
8
P. Baweja and G. Kumar
When there is flooding plants undergo hypoxia, a stage when oxygen concentration is less. During flooding or excessive irrigation water logging occurs in soil, thus
limiting the oxygen availability to roots. When water logging is for a prolonged
period damage occurs depending on the extent of adaptability and soil conditions.
The damage also depends on the type, developmental stage, genotype of plants,
extent of severity, and duration of water stress (Fukao and Bailey-Serres 2004;
Mariani and Ferrante 2017).
1.2.4 Salinity Stress
Salinity stress is another major problem of world, which decreases crop productivity.
High concentration of salt accumulates in the soil either due to improper irrigation or
poor drainage. Soil salinity is measured by accumulation of soluble salts such as Cl
À ,
SO 4
2À
, Na
+
, Ca
2+ , etc., and it harms the plant growth and development through
water stress and cytotoxicity (Isayenkov and Maathuis 2019). Soil salinity may be
categorized into primary (natural salinity; for example, marshes, salt lake, tidal
swamps, etc.) or secondary (due to anthropogenic activities). High saline condition
decreases the water potential and causes osmotic stress. Osmotically active
compounds or osmolytes (Glycine-Betaine) also known as compatible metabolites
accumulate in cells to lower the osmotic potential (Rasool et al. 2013). Salinity is
also many times accompanied by the formation of ROS, polyamines (putrescene),
and proline. Saline soils also limit the uptake of nutrients by root from soil (Sharma
et al. 2016). During high salinity stress, the photosynthetic activities gets affected as
thylakoid membranes of chloroplast get damaged, reduction of photosynthetic
pigments takes place, further inactivation of electron transport chain, photophosphorylation occurs (Srivastava et al. 2019). Also, the turgor pressure of leaves
changes reducing the stomatal conductance and rate of transpiration (Srivastava
et al. 2019). ABA accumulates in guard cells, excessive production of hydrogen
peroxide occurs which leads to triggering of oxidative stress by influencing the
production of reactive oxygen species (Foyer and Noctor 2005). It has been reported
that legumes and cereal crops are very sensitive to salinity. Salinity affects nitrogen
fixation as it suppresses the formation of root nodules (Ramana et al. 2012).
Nutritional imbalance enhances vegetative growth over reproductive growth, thus
decreasing crop yield (Munns and Tester 2008). It also causes chlorosis and senescence, DNA, RNA content decreases, mitotic and enzymatic activity slows down
(Ali et al. 2004).
The physiological drought condition in plants can be overcome by the application
of certain soil microorganisms such as arbuscular mycorrhizal fungi (AMF). Symbiotic association established between AMF and plant roots, mainly in saline
environments, could withstand plants under excessive saline conditions as the
AMF hyphae run several metres away from the stressed area or from the zone of
nutrient depletion, thereby increasing root surface area and facilitating nutrient
8
P. Baweja and G. Kumar