Investigating physiological and biochemical

 
Genotypes 
MDA (nmol g
-1
 FW) 
Control 
NaCl
(100 mM) 
NaCl
(200 mM) 
S-907 
5.11±0.16 
8.40±0.21
11.70±0.15
C-99-3-115 
5.45±0.12 
8.50±0.17
11.23±0.15
637-72 
4.82±0.13 
6.99±0.26
8.39±0.21
NO-303 
4.58±0.13 
6.37±0.19
8.30±0.17
Each value is an average of 3 replications ± SE 
3.3.4. Discussion 
The genetic variations between crop plants provide a precious tool in the 
selection of genotypes with desirable characters (Misra and Dwivedi, 2004). The 
present study clearly demonstrates some physiological and biochemical traits of salt 
tolerance in linseed genotypes subjected to different levels of salinity. It is obvious 
that salt stress reduced plant growth by affecting almost all the physiological 
processes, like photosynthetic activities, stomatal conductance, carbon and nitrogen 
assimilation, and ROS scavenging antioxidant enzymes.
In general, an appropriate growth strategy is key to fitness in a competitive 
situation, so too in linseed genotypes, their growth strategy is critical to survive (Paz 
and Marinez-Ramos, 2003; Du and Huang, 2008). Salt induced effects on relative 
water contents (RWC) has been used as one of the imperative water relation attributes 
for assessing the degree of salt tolerance in different crops like pea (Noreen and 
Ashraf, 2009), Safflower (Siddiqui and Ashraf, 2008) and hot pepper (Zaif et al., 
2009). In the present study, salinity caused significant reduction in RWC of all the 
four linseed genotypes, however, both salt tolerant and sensitive genotypes did not 
differ significantly in their RWC. Decrease in RWC under increasing levels of salinity 

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119 
in linseed genotypes was due to the decreased water potential of growth medium due 
to the high salt concentration as speculated by Sairam et al. (2002); Khan et al. (2007); 
and Siddiqui and Ashraf (2008).
Salt stress resulted in increased concentration of toxic ions (Na
+
and Cl
-
) which 
caused injury to cell membrane and hence reduced the membrane permeability. In 
addition, production of ROS under salt stress also caused significant reduction in 
membrane permeability and hence increases electrolyte leakage (EL). Thus EL is used 
as a criterion for salt tolerance to assess the membrane permeability of plants under 
stress conditions. Salt stress significantly increased the EL in linseed genotypes but 
there was no significant difference between salt tolerant and sensitive genotypes in 
this aspect. Thus in linseed EL may not be used as salt tolerant trait. 
One of the most notable effects of salt stress is the alteration of photosynthetic 
pigment biosynthesis (Maxwell and Johnson, 2000). The decrease in chlorophyll 
contents under salt stress is a commonly reported phenomenon. In various studies, the 
chlorophyll contents were used as a sensitive indicator of the cellular metabolic state 
(Chutipaijit et al., 2011). It is obvious from the results that salinity stress lead to the 
reduction in chlorophyll contents (Chlorophyll ‘a’ and ‘b’) in linseed genotypes. In 
linseed genotypes, chlorophyll ‘a’ was affected less (89-97% of respective control) 
than chlorophyll ‘b’ (48-71% of respective control) under salt stress. Similar results 
were found in Oryza sativa where reduction of chlorophyll ‘a’, and ‘b’ contents was 
observed after NaCl treatment (200 mM NaCl for 14 days) where reduction of 
chlorophyll ‘b’ contents (41%) was more than the chlorophyll ‘a’ contents (33%) 
(Amirjani, 2011). In another study, Saha et al. (2010) observed a linear decrease in 
the levels of total chlorophyll, chlorophyll ‘a’, chlorophyll ‘b’, carotenoids and 
xanthophylls as well as the intensity of chlorophyll fluorescence in Vigna radiata 
under increasing concentrations of NaCl treatments. The results revealed that 
chlorophyll ‘b’ was affected more than chlorophyll ‘a’ in all the genotypes but this 
reduction was more in salt sensitive genotypes than salt tolerant genotypes. The 
decrease in chlorophyll ‘a’ and ‘b’ in linseed genotypes might occur due to salt 

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120 
induced acceleration of chlorophyll enzymes degradation (Hernandez and Almansa, 
2002) and/or disorder of chloroplast structure and associated proteins (Cha-um and 
kirdmanee, 2009). The decrease in chlorophyll contents in plants grown under NaCl 
stress may be the consequence of the activation of chlorophyllase (Reddy and Vora, 
1986), the enzyme that degrades chlorophyll, which is activated by various stresses.
It is well established that photosynthetic capacity in crop plants is vital for final 
biological yield. The photosynthetic capacity of plants is reduced by the harmful 
effects of salinity on different photosynthesis related traits especially photosynthetic 
rate and stomatal conductance (Fisarakis et al., 2001; Sudhir and Murthy, 2004). In 
the present study, photosynthetic rate (57-65% of respective control) and stomatal 
conductance (30-48% of respective control) were significantly reduced due to 
increased concentration of NaCl. The reduction in photosynthetic rates in linseed 
under salt stress might be due to the reduction in water potential and high 
concentrations of Na
+
and/or Cl
-
which are accumulated in chloroplasts and hence 
affects carbon metabolism or photophosphorylation as reported by Sudhir and Murthy 
(2004). Some other factors that reduce photosynthetic rates under salt stress are; 
enhanced senescence, changes in enzyme activity, induced modifications in 
cytoplasmic structure and negative feedback by reduced sink activity (Iyengar and 
Reddy, 1996).
The reduction in stomatal conductance which results in restricting the availability 
of CO
2
for carboxylation reactions is also a factor that reduces photosynthesis under 
salt stress (Brugnoli and Bjorkman, 1992). In addition, stomatal closure minimizes 
loss of water through transpiration and this affects light-harvesting and 
energy-conversion systems thus leading to alteration in chloroplast activity (Iyengar 
and Reddy, 1996). Rubisco, the key enzyme that determines the Pn in plants, is 
regulated by a number of factors, including CO
2
concentration (Hopkins, 1999). 
Salinity stress may have decreased CO
2
availability by inducing stomatal closure 
(Bethke and Drew, 1992); therefore, partly inhibiting rubisco activity (Soussi et al., 

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121 
1999) and, consequently, the Pn. Moreover, the decrease in CA activity and lowered 
quantity of chlorophyll pigment may be the other reasons that the P
n
decreased. 
To sustain the carboxylation reaction of photosynthesis, carbonic anhydrase (CA) 
enzyme rapidly converts atmospheric CO
2
to HCO
3
-
and is considered the first step in 
photosynthesis. In the current study, activity of CA enzyme was significantly reduced 
(56% of respective control) in linseed genotypes under the increased levels of salinity. 
CA catalyzes the reversible inter-conversion of CO
2
and HCO
3
-
in plants, whose level 
is regulated by photon flux density, CO
2
concentration, and availability of zinc 
(Tiwari et al., 2005). Salinity stress is reported to cause stomatal closure, thereby 
decreasing CO
2