Investigating physiological and biochemical

partial pressure (Bethke and Drew, 1992). The fall in CO
2
levels in 
NaCl grown plants seems to be the cause of the decrease in CA activity. 
NR (nitrate reductase) activity is determined by several external and internal 
factors. Campbell (1999) has highlighted at least 4, which include (a) the availability 
of substrate (NO
3
-
) at the level of cytoplasm; (b) the level of functional NR; (c) the 
activity level of functional NR; (d) the overall metabolic state of the plant. Salinity 
was found to affect nitrate uptake in at least 2 ways: by direct competition of chloride 
with nitrate, and at the membrane level and/or membrane proteins by changing 
plasmalemma integrity (Cramer et al., 1985). This may have led to restricted nitrate 
influx, thus decreasing substrate availability. Since nitrate (substrate) is a key 
regulator of NR (Solomonson and Barber, 1990), the activity of NR decreased in 
response to saline stress. Moreover, the degradation/inactivation, and reduction in 
gene expression and NR-protein synthesis in response to NaCl stress (Ferrario et al., 
1998) may be another cause of lower NR activity. Thus, nitrate reductase (NR) 
enzyme was significantly decreased (63-69% of respective control) with the 
increasing salinity in linseed genotypes. 
Water potential, solute potential and turgor potential are inter-related in plant 
cells and are markedly affected when plants are exposed to salt stress. As a general 
principle, when plants experience high osmotic stress because of a low external water 
potential, a lowering of the solute potential (more negative) is stimulated, a process 

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referred to as osmotic adjustment. Osmotic adjustment under salt stress is due to 
uptake of ions from the external medium and/or accumulation of organic osmotica 
(Hernandez & Almansa 2002; Taiz & Zeiger 2002; Chaparzadeh et al., 2003). We 
did not find significant interaction between saltinity and genotypes regarding leaf 
osmotic potential, however, genotypes differed significantly in their leaf osmotic 
potential.
Proline accumulation in salt stressed plants is a key defense response to sustain the 
osmotic pressure in a cell, which is reported in salt tolerant and salt sensitive 
genotypes of many crops (Kumar et al., 2003; Misra and Gupta, 2006; Koca et al., 
2007). Similarly, glycine betaine (GB) accumulates in response to stress in many 
crops, including spinach, barley, tomato, potato, rice, carrot and sorghum (Mohanty et 
al., 2002; Yang et al., 2003). This organic compound is mainly localized in 
chloroplasts and plays a vital role in chloroplast adjustment and protection of 
thylakoid membranes, thereby maintaining photosynthetic efficiency (Genard et al., 
1991). Murata et al. (1992) reported that GB protects the photosystem II (PSII) 
complex by stabilizing the association of the extrinsic PSII complex proteins under 
salt stress. Salinity stress increased (120-130% of respective control) proline contents 
in linseed but the concentration of glycine betaine increased (140% of respective 
control) markedly under same conditions. In some plant species, proline plays a major 
role in osmotic adjustment such as in potato (Claussen, 2005). But in linseed, GB 
played significant role compared to proline in osmotic adjustment in leaves. Cha-um 
et al. (2006) investigated that high level of glycine betain in salt-tolerant lines of rice 
(Oryza sativa L. spp. indica) played a significant role as a salt defensive response 
mechanism in terms of chlorophyll pigment stabilization and water oxidation in PSII, 
resulting in high net photosynthetic rate (NPR) and growth efficiency. A positive 
correlation between GB and SOD activity was observed in linseed genotypes which 
might indicate its role in up-regulating the SOD activity. These antioxidant enzymes 
activity depends on osmolyte concentration in cell, as these enzyme need availability 
of in vivo melieu for maximal catalytic activity (Burg and Ferraris, 2008; Sahu et al., 

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