Investigating physiological and biochemical

 
Genotypes 
Stomatal conductance (mmol m
-2
 s
-1
) 
Photosynthetic rate (µmol CO
2
 m
-2
 s
-1
) 
Control 
NaCl
(100 mM) 
NaCl
(200 mM) 
Control 
NaCl
(100 mM) 
NaCl
(200 mM) 
S-907 
1.67±0.18 
1.24±0.19
0.50±0.15 
16.33±0.20 
11.67±0.35 
9.33±0.24
C-99-3-115 
1.77±0.15 
1.33±0.12 
0.57±0.09 
16.50±0.32 
11.93±0.26 
9.83±0.29
637-72 
1.93±0.18 
1.57±0.12 
0.93±0.15 
16.83±0.22 
12.70±0.21 
10.93±0.32
NO-303 
1.85±0.13 
1.63±0.09 
0.83±0.12 
16.70±0.26 
12.50±0.29 
10.87±0.43
Each value is an average of 3 replications ± SE 

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107 
3.3.3.4 Carbonic anhydrase (CA) and nitrate reductase (NR)
 
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 while nitrate reductase (NR) is one of the most important enzymes in 
the assimilation of exogenous nitrate, the predominant form of nitrogen available to 
green plants growing in soil. Activity of this enzyme in plants gives a good estimate 
of the nitrogen status of the plant and is very often correlated with growth and yield.
CA and NR activity of all the genotypes significantly decreased by the increased 
levels of salinity (NaCl). At control, activity of CA had no difference and genotypes 
showed almost equal activity of CA but significant difference was observed among 
genotypes in their NR activity (Table 3.3.4). A progressive decrease in the activity of 
CA and NR was observed in genotypes and CA activity of salt sensitive genotypes 
decreased from 341.07 to 265.20 µmol CO
2
kg
-1
LFW s
-1 
in S-907 and from 341.60 to 
265.43 µmol CO
2
kg
-1
LFW s
-1 
in NO-303 genotype at 100 mM NaCl. Similarly, NR 
activity decreased in salt tolerant genotype NO-303 from 0.58 to 0.48 µmol NO
2
h
-1
g
-1
LFW s
-1 
and from 0.41 to 0.33 µmol NO
2
h
-1
g
-1
LFW s
-1
in salt sensitive genotype 
C-993-115 at same level of salt stress. At 200 mM NaCl, the maximum activity of CA 
was measured in NO-303 (191.50 µmol CO
2
kg
-1
LFW s
-1
) followed by 637-72 
(191.47 µmol CO
2
kg
-1
LFW s
-1
) while the minimum activity was noted in S-907 
(191.25) µmol CO
2
kg
-1
LFW s
-1
) and C-99-3-115 (191.27 µmol CO
2
kg
-1
LFW s
-1
) 
(Table 4). Activity of NR differed greatly among genotypes and the maximum activity 
occurred in genotype NO-303 (0.48 µmol NO
2
h
-1
g
-1
LFW s
-1
) and 637-72 (0.44 µmol 
NO
2
h
-1
g
-1
LFW s
-1
) when compared with C-99-3-115 (0.33 µmol NO
2
h
-1
g
-1
LFW s
-1
) 
and S-907 (0.35 µmol NO
2
h
-1
g
-1
LFW s
-1
) genotypes at 100 mM NaCl. When 
compared with percent of control, genotypes were in the order as 637-72 > NO-303 > 
C-99-3-115 = S-907 regarding their CA concentration at 100 mM NaCl levels while 
at 200 mM NaCl, 56% activity of CA was observed in all genotypes of their 
respective control (Table 3.3.4). A different type of order was observed in genotypes 
regarding the activity of NR at 100 mM NaCl while at 200 mM, the order of 
genotypes was as 637-72 (69%) = NO-303 (69%) > S-907 (64%) > C-99-3-115 
(63%). 

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108 
Table 3.3.4. Activities of carbonic anhydrase and nitrate reductase enzymes in the leaves of linseed genotypes at different salinity
levels