Investigating physiological and biochemical

2.2. Soil Salinity and plant growth 
Salt tress adversely affects the growth and development of crop plants. High 
salinity may cause the adverse effects on membrane integrity, enzyme activity, 
nutrient uptake and photosynthesis. The production of ROS during salt stress is one of 
the important reasons of this damage. So broadly salt stress causes four types of 
injuries on plants such as: 
2.2.1. Osmotic or water-deficit effect 
Salts affect the ability of plants to take up water and hence indirectly reduce plant 
growth. The plants which cannot regulate osmotic stress are unable to maintain 
pressure potential and ultimately close their stomata and reduce photosynthesis. This 
loss of pressure potential severely affects cell division and elongation and inhibits 
plant growth during salt stress (Qin et al., 2010). Many comprehensive reviews 
(Hasegawa et al., 2000; Munns, 2005; Munns and Tester, 2008) documented adverse 
effects of salinity in terms of water-deficit or osmotic stress at cell level.
Salinity induced water deficit reduces root and shoot cell expansion. This 
reduction occurs due to osmotic-induced production of abscisic acid (ABA), which 
promotes root growth and inhibits shoot growth (Munns and Tester, 2008). Thus root 
growth is affected less than shoot growth under salinity induced osmotic stress 
(Munns, 2011). Carbon partitioning from the shoot to the root facilitates root growth 
into soil regions where water is available (Saab et al., 1990; Munns and Tester, 2008; 
Yamazaki et al., 2012). Plants, particularly in response to moderate water deficit, can 
osmotically adjust and re-establish cell turgor (Greenway and Munns, 1983; Binzel et 

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33 
al., 1988; Munns and Tester, 2008; Shalaba and Mackay, 2011). Cells of glycophytes 
and most other halophytes exhibit higher cell growth yield thresholds and reduced 
extensibility at low water potentials that restrict cell expansion (Matthews et al., 1984; 
Munns and Tester, 2008; Tardieu et al., 2011; Shalaba and Mackay, 2011).
In glycophytes, leaf succulency (thickening of leaf tissues and increase in the 
volume f leaf sap) is a typical adaptive response under saline conditions and may be 
achieved by increasing the size of mesophyll cells and the relative size of their 
vacuoles (Gorham et al., 1985; Longstreth and Nobel, 1979). The number of spongy 
cell layers may also be increased under salt stress (Longstreth and Nobel, 1979). Root 
suberisation increases with salinity (Steudle, 2000); this may be functionally 
important to prevent apoplastic flow of toxic Na
+
ions and to increase water retention 
in the roots of glycophytes. Some halophytes (e.g. Mesembryanthemum crystallinum) 
have made this trait constitutive, by developing an extra endodermis layer (Inan et al., 
2004). Some halophytes have evolved unique adaptations such as salt glands and 
bladders, succulence, life cycle avoidance and salt induced facultative metabolism to 
cope with salinity (Flowers et al., 1986, 2010; Bohnert et al., 1995; Shabala and 
MacKay, 2011). In halophytes cell turgor is maintained by storage of Na
+
and Cl
-
in 
vacuoles, with the solute potential of the cytosol adjusted by accumulation of K
+
and 
organic solutes (Storey and Wyn Jones, 1979; Storey, 1995; Glenn et al., 1999). 
According to Glenn et al. (1999), Na
+
, K
+
and Cl
-
contribute to maintain 80–95% of 
the cell sap osmotic pressure in both halophyte grasses and dicots. This results in the 
hyper accumulation (>10% of dry weight each) of Na
+
and Cl
-
in their shoots (Grattan 
et al., 2008), largely in vacuoles (Flowers and Colmer, 2008). At the same time, 
halophytes maintain cytoplasmic K
+
concentrations similar to those of glycophytes 
(Flowers and Colmer, 2008) and thus have a high vacuole/cytosol Na
+
ratio and a 
high cytosol/vacuole K
+
ratio (Ye and Zhao, 2003).
In halophytes, presence of salt bladders/glands is the most remarkable feature 
(Shalaba and Mackay, 2011). Glands extrude salts on a regular basis, while bladders 
release salts only when rupture, after a prolonged period of accumulation (Tester and 

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34 
Davenport, 2003). Epidermal bladder cells are also thought to be storage sites for 
excess Na
+
, Cl
-
and K
+
(Adams et al., 1998; Agarie et al., 2007) and may play an 
important role to reduce water loss and prevent UV damage (Shalaba and Mackay, 
2011). In halophytes cell turgor is maintained by storage of Na
+
and Cl
-
in vacuoles, 
with the solute potential of the cytosol adjusted by accumulation of K
+
and organic 
solutes (Storey, 1995; Glenn et al., 1999). According to Glenn et al. (1999), Na
+
, K
+
and Cl
-
contribute to maintain 80–95% of the cell sap osmotic pressure in both 
halophyte grasses and dicots. Thus halophytes accumulate excessive amount (>10% 
of dry weight each) of Na
+
and Cl
-
in their shoots (Grattan et al., 2008), largely in 
vacuoles. At the same time, halophytes maintain cytoplasmic K
+
concentrations 
similar to those of glycophytes (Flowersand Colmer, 2008) and thus have a high 
vacuole/cytosol Na
+
ratio and a high cytosol/vacuole K
+
ratio (Ye and Zhao, 2003). 
Those halophytes, exhibiting enhanced fresh and dry weight gains at high NaCl 
concentrations, are potential genetic resources for osmotic tolerance determinants that 
would facilitate growth and yield stability under water deficit (Flowers et al., 1986; 
Greenway and Munns, 1983; Munns and Tester, 2008).