Introduction
Soil
salinity is an important abiotic stress and is becoming a major problem
globally because it is encountered in all climates (Evelin et al. 2019). It is estimated that approximately one billion
hectares (ha) across 100 countries are facing salinity problem (FAO 2015).
Furthermore, soil salinity is expanding with an estimated annual addition of
0.3–0.5 million ha of arable land with overall 20% decrease in crop production
(FAO 2015). Therefore, ensuring global food security for mounting population
with decreasing farmlands is the biggest challenge for modern agriculture.
Salinity reduces crop productivity owing to ion toxicity, reduced growth,
osmotic stress, photosynthetic imbalance, mineral deficiencies and combination
of these effects (Rangani et al. 2016). In fact, salt stress may also affect seed
germination, leaf water contents and nutrients uptake and ultimately reduces
yield (Kim et al. 2014). The instant
response of salt stress appears in the form of reduced rate of leaf expansion
due to increased salt concentration. Reduction in leaf area expansion due to
high salinity levels causes about 80% reduction in plant growth while decrease
in stomatal conductance brings about 20% reduction in plant growth (Parida and Das 2005). Salinity also causes oxidative damage
to the plants through overproduction of reactive oxygen species (ROS). However,
plants also
have the ability to counter salinity problem by utilizing different mechanisms
like growth plasticity, turgor maintenance, ion homeostasis, enhanced photosynthesis,
better water use efficiency, scavenging of ROS through antioxidant enzymes and
molecules along with production of phytohormones (Farooq et al. 2015).
Bell pepper (Capsicum
annuum L.) is a member of the Solanaceae family (Penella et al. 2015) and it is traditionally used as
vegetable in the world (Ali et al.
2017). Nutritionally it is rich in antioxidants containing polyphenols such as
vitamin C, complete carotenoids, β-carotene, α-carotene (Nadeem et
al. 2011). It is cultivated in Pakistan to produce 191.8 thousand tonnes of this plant on an area of 66500 hectares (Mehmood et al. 2017). Its production not only
fulfills 88 percent of the country's requirements but also contributes to
foreign exchange income (Zia 2006). Pepper is mostly cultivated in Punjab and
Sindh where high levels of salinity affect its production (Khan 1999).
According to an estimate approximately 21% of irrigated land in Punjab and
Sindh is affected by salinity (Qureshi et
al. 2007). Bell pepper is highly important cash food crop and its improved
quality and production can generate high farm income (Mehmood et al. 2017). Bell pepper has been
classified from moderately sensitive to sensitive under salinity and water
stress conditions (Penella et al. 2015) and it is considered most susceptible to salinity
during its seedling phase (Navarro et al. 2002). Excessive salts in soil
solution or in root zone are transported with in the plant. These salts induce
osmotic stress and ionic imbalances cause various biochemical and morpho-physiological
abnormalities that ultimately leads to plant death (Pessarakli
and Tucker 1988). If salinity problem is not managed properly in pepper it can
become a major limiting factor affecting its production (Villa-Castorena et al.
2003). Therefore, there is a dire need to evaluate the salt tolerance potential
of existing bell pepper genotypes. The comparison among different cultivars
differing in their basic ability to cope with stress is quit useful in the
evaluation of their salt stress tolerance potential. It will not only improve
our understanding of the primary mechanisms responsible for salinity tolerance
but it is also helpful to recognize the best salt tolerant cultivar (Akhtar et al. 2017). Screening of tolerant
crops on the basis of morphological, biochemical, physiological and ionic
responses may help to strengthen the breeding programs by recognizing the
genotypes having higher yield and salt tolerance potential (Ashraf and Foolad 2007). Moreover, it is an easy way to screen salt
tolerant cultivars under controlled conditions compared to field conditions and
it is reported to be used in the screening of different crop cultivars (Akhtar et al. 2003). According to the best of
our knowledge comparative analysis of salt sensitive and salt tolerant bell
pepper genotypes is sparse. Therefore, this study was performed to elucidate
the effects of salt stress on seedling growth, antioxidant activity and ionic
concentration in two bell pepper genotypes viz.,
California wonder (salt tolerant) and Green Beauty (salt sensitive). The
outcomes of this research will help in the identification of novel bell pepper
cultivar with better salt tolerant potential to cultivate on salt affected
soils under agro-climate of Pakistan.
Materials and
Methods
Experimental
details
This pot experiment was conducted under rain out
structure in the GCU Faisalabad, Pakistan. Seeds of two Bell pepper genotypes viz., California Wonder (salt tolerant)
and Green Beauty (salt sensitive) selected on the basis of previous experiments
(Javed 2019) were collected from the Vegetable
Research Institute (AARI), Faisalabad, Pakistan. The experiment was carried out
in sand filled plastic pots (16 cm diameter ×16 cm length). Each pot was
wrapped with plastic bag and filled with 6 kg washed and fine sand. The
Gravimetric method was used to estimate the water holding capacity of the sand
(Bethlahmy 1952). Ten good and healthy seeds of both
bell pepper genotypes were planted in each pot. Seven days after emergence five
healthy seedlings were kept in each pot after thinning. To fulfill the nutrient
requirement of the seedlings Hoagland’s nutrient solution (Hoagland and Arnon 1950) was used. One month after sowing, bell pepper
plants were exposed to different salinity levels (control, 25, 50, 75, 100 and 125 mM
of NaCl). In splits, NaCl concentrations were given by raising 25 mM with an interval of two days until the
necessary concentration was reached. For proper maintenance of salinity levels
regular evaluation of salt levels were conducted by using an EC meter. The
experiment was laid out in a completely randomized design with factorial
arrangement having three replications.
Sampling and measurement of
seedling growth related traits
Growth parameters including seedling shoot length (SL),
root length (RL), seedling fresh (SFW) and dry weights (SDW), root dry weight
(RDW), antioxidant activity, proline content and Sodium (Na+) and
Potassium (K+) ion concentrations were determined 50 days after
sowing.
Three seedlings from each pot
were uprooted, cleaned and washed with distilled water to remove any sand or
dirt particle. Seedlings were put in filter paper to remove any drop of water
present on the surface of leaf or shoot. The seedlings were then cut into shoot
and root parts and shoot and root lengths were measured separately with the
help of a meter rod. Average data from each pot was recorded. Then SFW of three
plants from each pot was recorded by using the digital balance and average was
noted. After measuring the fresh weight, the three plants from each pot were
taken into paper bags and then put in oven (Memmert-110, Schawabach)
and dried out at 70οC for seven days to
get constant weight. Then SDW and RDW were recorded on digital balance and
average dry weight of each replicate was noted.
Catalase (CAT) and peroxidase
(POD) measurement
The activities of catalase (CAT) and peroxidase (POD)
were calculated with some alteration by the Chance and Maehly
(1955) procedure. The CAT reaction solution (3 mL) consisted of an enzyme
extract of 50 mM phosphate buffer (pH
7.0), 5.9 mM H2O2
and 0.1 mL. Changes in reaction solution absorbance
were recorded at 240 nm for every 20s. As an absorbance change of 0.01 units
per min, one-unit CAT activity was specified. It consisted of 50 mM phosphate buffer (pH 5.0), 20 mM guaiacol, 40 mM H2O2 and 0.1 mL enzyme extract. After every
20 s, variations in reaction solution absorption were calculated at 470 nm. As
an absorbance change of 0.01 units per min, one-unit POD activity was assigned.
Based on the protein content, the activity of each enzyme was articulated.
Free proline determination
To determine the free proline
content in leaves Bates et al. (1973) method was followed. Fresh leaf
sample of 0.5 g was thoroughly homogenized and mixed in 10 mL of 3%
sulfosalicylic acid, then after filtration, two mL filtered sample was taken in
a test tube and reacted to two mL ninhydrin solution. After adding two
milliliters of glacial acetic acid in test tube, this sample mixture was heated
at 100°C for one hour. After heating this sample was extracted with 4 mL
toluene solution. The chromophore containing toluene was aspirated from the
aqueous phase and absorbance was recorded on spectrophotometer at 520 nm.
Toluene was used as a blank.
Sodium (Na+)
and potassium (K+) ions
determination
Method of
Allen et al. (1986) for ion determination in leaf and roots was used.
A mixture of 14 g of LiSO4.2H2O, Se (0.42 g), H2O2
(350 mL) and conc. H2SO4 (420 mL) was prepared. Dry leaf
material (0.1 g) was digested separately from each template in a blend of
digestion (2 mL). All flasks with leaf samples and mixture of digestion were
heated at 200°C on a hot plate. Each
digested sample was diluted to 50 mL and used to estimate the Na+,
and K+ ion concentration using flame photometer (Jenway, PFP-7).
Statistical analyses
Fisher’s analysis of variance technique using
Statistix-8.1 software was used to statistically analyze the recorded data.
Comparison among significant means was executed by employing least significant
difference (LSD) test at 5% probability level (Steel et al. 1997).
Microsoft excel program was used to calculate regression correlation among
different traits.
Results
Growth traits
Seedling shoot fresh weight
showed significant (P ≤ 0.05) individual as well as interactive effect of
salt stress levels and bell pepper genotypes (Table 1). As the salinity levels increased from
0 to 125 mM, a linear decrease in
shoot fresh weight of both genotypes was observed. The highest reduction in SFW
was noted at 125 mM salinity level in
salt sensitive genotype Green Beauty (Table 1). Imposition of salinity stress
reduced the SFW by 11% (25 mM), 17%
(50 mM), 27% (75 mM), 45% (100 mM) and 53% (125 mM) in
California Wonder compared to control plants (Table 1). While salt sensitive
genotype Green Beauty showed 15, 21, 36, 48 and 59% of reduction against 25,
50, 75, 100 and 125 mM salinity
levels as compared to plants grown under normal conditions, respectively (Table
1). Nevertheless, both genotypes
exhibited decrease in shoot fresh weight but the magnitude of decrease was
higher in salt sensitive genotype as compared to salt tolerant genotype at all
salinity stress levels.
A
significant (P ≤ 0.05) individual effect of bell pepper genotypes and salt
stress levels and their interaction was noted regarding shoot and root dry
weights (Table 1). Both seedling shoot and root dry weights linearly decreased
with increasing salt stress levels. Maximum reduction in shoot and root dry
weights was recorded at 125 mM salinity
level in sensitive genotype Green Beauty. Salt tolerant genotype California
Wonder exhibited a reduction of (5, 24, 31, 40 and 49%), while sensitive
genotype Green Beauty revealed 12, 28, 38, 47 and 67% reduction in SDW at 25,
50, 75, 100 and 125 mM salinity
levels respectively as compared to non-stressed plants (Table 1). Similarly at 25, 50, 75, 100 and 125 mM salinity levels, RDW was decreased by
9, 28, 43, 57 and 67% in California Wonder, while this reduction was 19, 37,
53, 66 and 89% in genotype Green Beauty, respectively against normally grown
plants (0 mM) (Table 1). In fact, at all salt stress levels, salt
tolerant genotype California Wonder showed higher values of shoot and root dry
weights when compared to salt sensitive genotype Green Beauty. Seedling
shoot and root lengths depicted an individual significance of bell pepper
genotypes and salt stress levels, as well as a significant (P ≤ 0.05) interaction between the two factors. Salt
tolerant genotype California Wonder at all salinity levels (25, 50, 75, 100 and
125 mM) revealed a significant
decrease in SL (8, 16, 21, 27 and 32%) and RL (2, 12, 19, 31 and 40%)
respectively, as compared to control plants (0 mM) (Table 1). Likewise salt sensitive genotype Green beauty
indicated a decline of 10, 19, 25, 31 and 41% in SL, while RL was reduced by 8,
18, 28, 37 and 52% at 25, 50, 75, 100 and 125 mM salinity levels respectively against control plants (Table 1).
This reduction in shoot length and root length parameters was higher in Green
beauty in comparison to California Wonder at all stress levels.
Anti-oxidative defense system
Table 1: Effect
of different salinity stress levels on growth traits of two bell pepper
genotypes
|
Salinity stress levels |
Bell pepper genotypes |
Shoot fresh weight (g/plant) |
Shoot dry weight (g/plant) |
Root dry weight (g/plant) |
Shoot length (cm) |
Root length (cm) |
|
Control (0 mM) |
California Wonder |
7.37 ± 0.89a |
3.58 ± 1.06a |
2.91 ± 0.37a |
8.61 ± 0.72a |
6.33 ± 0.89a |
|
Green Beauty |
7.16 ± 0.69a |
3.45 ± 0.33a |
2.89 ± 0.47a |
8.46 ± 0.94a |
6.29 ± 0.29a |
|
|
25 mM |
California Wonder |
6.56 ± 0.84b |
3.41 ± 0.29a |
2.66 ± 0.77b |
7.92 ± 0.77b |
6.23 ± 1.06a |
|
Green Beauty |
6.10 ± 1.02c |
3.06 ± 0.69b |
2.34 ± 1.02c |
7.62 ± 1.06c |
5.79 ± 0.33b |
|
|
50 mM |
California Wonder |
6.11 ± 0.77c |
2.73 ± 0.76c |
2.10 ± 0.94d |
7.25 ± 0.77d |
5.56 ± 1.02b |
|
Green Beauty |
5.61 ± 0.20d |
2.51 ± 1.06cd |
1.82 ± 0.89e |
6.89 ± 1.07e |
5.17 ± 0.36c |
|
|
75 mM |
California Wonder |
5.39 ± 0.69d |
2.47 ± 0.20d |
1.65 ± 0.77e |
6.81 ± 0.94e |
5.11 ± 0.84c |
|
Green Beauty |
4.56 ± 0.33e |
2.15 ± 0.36e |
1.35 ± 0.69f |
6.34 ± 0.20f |
4.54 ± 0.29d |
|
|
100 mM |
California Wonder |
4.06 ± 0.84f |
2.13 ± 0.29e |
1.25 ± 0.29f |
6.24 ± 0.94f |
4.37 ± 1.02d |
|
Green Beauty |
3.71 ± 0.77g |
1.85 ± 0.33f |
0.98 ± 0.37g |
5.81 ± 0.36g |
3.93 ± 1.09e |
|
|
125 mM |
California Wonder |
3.44 ± 1.07h |
1.81 ± 0.20f |
0.96 ± 0.14g |
5.72 ± 0.84g |
3.82 ± 0.69e |
|
Green Beauty |
2.92 ± 0.77i |
1.13 ± 1.02g |
0.56 ± 0.17h |
4.98 ± 1.06h |
3.02 ± 0.77f |
|
|
LSD values at 5% |
0.26 |
0.23 |
0.16 |
0.29 |
0.29 |
|
Means ± SE values in each coulmn with
different letters indicate that treatments are statistically different at P < 0.05
Table 2: Effect
of different salinity stress levels on ionic and biochemical traits of two bell
pepper genotypes
|
Salinity stress levels |
Bell pepper genotypes |
Catalase (Units g-1
FW) |
Peroxidase (Units g-1
FW) |
Free proline (μmol g-1
FW) |
Leaf Na+ (mg g-1
DW) |
Root Na+ (mg g-1
DW) |
Leaf K+ (mg g-1
DW) |
Root K+ (mg g-1
DW) |
|
Control (0 mM) |
California Wonder |
312 ± 1h |
335 ± 0.7fg |
1.51 ± 1fg |
1.68 ± 0.3f |
0.86 ± 0.4g |
21 ± 0.9a |
20 ± 0.8a |
|
Green Beauty |
307 ± 0.6h |
327 ± 0.9g |
1.44 ± 0.9g |
1.72 ± 0.9f |
0.76 ± 0.3g |
21 ± 0.7ab |
20 ± 1a |
|
|
25 mM |
California Wonder |
347 ± 1f |
372 ± 0.6e |
1.76 ± 0.4e |
1.92 ± 0.7e |
1.11 ± 0.3f |
20 ± 0.9b |
17 ± 0.3b |
|
Green Beauty |
332 ± 0.3g |
347 ± 0.6f |
1.62 ± 0.5f |
2.16 ± 1d |
0.90 ± 0.2g |
17 ±.0.7c |
19 ± 0.9a |
|
|
50 mM |
California Wonder |
367 ± 0.7de |
384 ± 1e |
1.94 ± 0.3d |
2.05 ± 0.9d |
1.54 ± 0.4d |
17 ± 0.3c |
14 ± 0.6c |
|
Green Beauty |
353 ± 1ef |
378 ± 0.3e |
1.78 ± 0.9e |
2.14 ± 0.7d |
1.26 ± 0.2e |
14 ± 0.6d |
16 ± 1b |
|
|
75 mM |
California Wonder |
388 ± 0.3c |
427 ± 0.6c |
2.16 ± 0.8c |
2.07 ± 0.4d |
1.96 ± 0.4c |
13 ± 0.6d |
10 ± 1e |
|
Green Beauty |
373 ± 0.9d |
403 ± 0.6d |
1.98 ± 0.9d |
2.33 ± 0.9c |
1.63 ± 0.7d |
10 ± 0.9e |
12 ± 1d |
|
|
100 mM |
California Wonder |
411 ± 0.4b |
454 ± 0.6b |
2.37 ± 1b |
2.39 ± 0.6c |
2.22 ± 0.5b |
9 ± 1f |
8 ± 1f |
|
Green Beauty |
391 ± 0.9c |
438 ± 1c |
2.17 ± 1c |
2.59 ± 0.7b |
1.95 ± 0.3c |
7 ± 0.6g |
10 ± 1e |
|
|
125 mM |
California Wonder |
457 ± 1a |
492 ± 0.5a |
2.73 ± 0.9a |
2.66 ± 0.40b |
2.71 ± 0.4a |
7 ± 01g |
5 ± 0.8g |
|
Green Beauty |
418 ± 0.5b |
454 ± 1b |
2.32 ± 0.8b |
2.85 ± 1a |
2.25 ± 0.8b |
4 ± 1h |
7 ± 0.6f |
LSD values at 5% 13.77 13.04
0.13 0.14 0.11 1.17
1.29
Means ±
SE values in each coulmn with different letters indicate that treatments are statistically different at P < 0.05
Data regarding CAT and POD
enzyme activity indicated a significant (P ≤
0.05) individual as well as an
interactive effect of bell pepper genotypes and salt stress levels (Table 2). A
linear increase in CAT and POD enzyme activity was noted in both bell pepper
genotypes with increasing salt stress levels from 0 to 125 mM (Table 2). An increase of 11, 17, 24,
32 and 46% in CAT activity, while 11, 15, 28, 35 and 47% in escalation in POD
activity was noted at 25, 50, 75, 100 and 125 mM salinity levels respectively against
normally treated plants (0mM) in
genotype California Wonder (Table 2). Corresponding to 25, 50, 75, 100
and 125 mM salt stress levels
genotype Green Beauty showed an increase of 8, 15, 22, 27 and 36% in CAT
activity, while POD activity was enhanced by 6, 15, 23, 34 and 39%
respectively, as compared to non-stressed plants (Table 2). Although CAT and POD levels increased in
both genotypes at all stress levels but this increase was more pronounced in
salt tolerant genotype. Regarding leaf free proline
content of bell pepper genotypes a highly significant (P ≤ 0.05) individual as well as interactive
effect has been found between genotypes and salt stress levels (Table 2). Leaf free
proline content increased with increasing salt stress
levels from 0 to 125 mM salt stress
level. California Wonder (Salt tolerant genotype) exhibited 17, 29, 44, 58 and
81% increase, while salt sensitive genotype Green Beauty showed 15, 27, 41, 55
and 65% increase in free proline contents against 25,
50, 75, 100 and 125 mM salinity
levels respectively when compared to control (Table 2). Salt tolerant genotype California Wonder showed better leaf free proline contents at all stress levels as compared to salt
sensitive genotype Green Beauty.
Ionic compositions and balance
Table 3: Correlation
among growth traits, antioxidants and ions of bell pepper genotypes under
salinity stress
|
Crop traits |
CAT |
Leaf
K+ |
Root
K+ |
Leaf
Na+ |
Root
Na+ |
POD |
Proline |
RL |
SDW |
SFW |
|
Leaf
K+ |
-0.55* |
|
|
|
|
|
|
|
|
|
|
Root
K+ |
-0.74* |
0.25* |
|
|
|
|
|
|
|
|
|
Leaf
Na+ |
0.73*
|
-0.70*
|
-0.51* |
|
|
|
|
|
|
|
|
Root
Na+ |
0.90**
|
-0.39*
|
-0.68* |
0.69* |
|
|
|
|
|
|
|
POD |
0.93**
|
-0.36*
|
-0.70* |
0.57* |
0.89** |
|
|
|
|
|
|
Proline |
0.97**
|
-0.56*
|
-0.71* |
0.74* |
0.89** |
0.93** |
|
|
|
|
|
RL |
-0.69* |
0.43* |
0.49*
|
-0.59*
|
-0.69*
|
-0.68*
|
-0.71* |
|
|
|
|
SDW |
-0.43* |
0.14* |
0.13*
|
-0.36*
|
-0.38*
|
-0.42*
|
-0.42* |
0.31* |
|
|
|
SFW |
-0.73** |
0.41* |
0.32*
|
-0.71*
|
-0.72*
|
-0.71*
|
-0.73* |
0.50* |
0.71* |
|
|
SL |
-0.76* |
0.54* |
0.63*
|
-0.71*
|
-0.74*
|
-0.66*
|
-0.76* |
0.65* |
0.22* |
0.47* |
*, ** significant at 0.05 and
0.01% probability levels, respectively
CAT: Catalase, POD: Peroxidase, RL: Root length, SL: Shoot length, SDW: Shoot
dry weight, SFW: Shoot fresh weight
A highly significant (P ≤ 0.05) individual as well as interactive effect of bell pepper genotypes and
salt stress levels has been noted regarding Na+ ion concentration in
leaf and roots. Na+ ion concentration increased with increasing the
level of salt stress in leaves and roots of both genotypes. Salt tolerant
genotype California Wonder showed 14, 22, 23, 42 and 58% increase in leaf Na+
contents, while salt sensitive genotype Green Beauty revealed 26, 24, 35, 51
and 66% rise in leaf Na+ contents at 25, 50, 75, 100 and 125
mM salinity levels respectively in
comparison to control plants (Table 2). In case of root Na+ contents California Wonder indicated 30, 79, 122, 159 and
215% increase, while Green Beauty displayed 18, 67, 115, 158 and 197% rise at 25,
50, 75, 100 and 125 mM salinity
levels respectively against control plants (Table 2). Hence, more roots and less leaf Na+ contents were noted in
California Wonder as compared to Green Beauty at all salt stress levels.
Data pertaining to the effect
of different bell pepper genotypes and salt stress levels on K+ ion concentration in leaf and
roots indicated a significant (P ≤ 0.05) individual as well as interactive effect of
two factors. A decline in the K+
ion concentration in leaves and roots has been noted in both bell pepper
genotypes with the increase of salt stress in both leaves and roots. In case of
California Wonder, leaf K+ contents revealed 6, 21, 39, 57 and 68%
decline, while genotype Green Beauty showed 18, 36, 51, 67 and 80% reduction in
leaf K+ contents with respect to 25, 50, 75, 100 and 125 mM salinity levels respectively as
compared to normally grown plants (Table 2). Regarding root K+ contents, California Wonder
showed 11, 30, 37, 62 and 75 % decrease and genotype Green Beauty indicated 6,
20, 39, 51 and 63% decline at 25, 50, 75, 100 and 125 mM salinity levels respectively when
compared to control plants (0 mM)
(Table 2).
Correlation coefficients
Correlation
coefficients between bell pepper genotype seedling traits, antioxidant
activity, free proline and ion (Na+ and K+) accumulation
are presented in Table 3. Correlation coefficients were highly significant for
most of the ionic and biochemical attributes studied in this experiment. Among
the positive and highly significant correlations were, POD, CAT, free proline, Na+ with POD, free proline, Na+ with
K+ correlated highly significant and negative with POD, CAT, free proline and Na+, SL with RL, SDW with RDW and SFW
with SDW (Table 3).
Discussion
Crop growth is influenced by change in salt level in growth medium and
measuring crop differential response determines the salt tolerance level that
assists in the varietal selection. In this study, the contrasting varietal
response of pepper was measured in terms of growth, ionic composition and
defense system (Tables 1 and 2). Growth traits like dry and fresh weights of
shoot and root and their respective lengths were decreased with increasing salt stress depicting the negative correlation between growth and salt stress in
both varieties. Actually, high salt concentration reduces the water
potential of the growth medium thus, causing substantial decline in cell turgor
to prevent cell elongation and cell division, hence minimizing the plant growth
(Shahid et al. 2012). Similar growth
trend has been reported by Ambede et al. (2012) in groundnut (Vigna subterranea)
when subjected to supra-optimal saline environment. Moreover, higher
accumulation of Na+ ions in leaves minimizes the photosynthetic
capacity by decreasing the activity of rubisco and NADPH enzymes involved in
photosynthesis, which might have caused the biomass production (Ashraf and Foolad 2007). Comparing performance of both pepper
varieties lesser growth decline in California
wonder could have been due to their better maintenance of cell turgor (Ashraf
and Foolad 2007). Maintenance of better cell turgor
by California Wonder can be correlated with higher K+ uptake and free
proline accumulation and chlorophyll content in its
leaves under salinity stress which is an important stress tolerance strategy
(Huang et al. 2013; Kholghi et al. 2018).
Furthermore, California Wonder mediated the oxidative
stress by employing antioxidant defense system to scavenge reactive oxygen
species. Catalase and peroxidase were produced in its leaves more as compared
to sensitive variety in the presence of salt stress (Table 2). These higher
activities might decompose H2O2 into water thereby
minimizing the damaging effects of H2O2 from the stressed
cells and improves the oxidative capacity of plants enabling them to better
tolerate the salt stress (Parida et al. 2004; Elgawad et al. 2016). So the California
Wonder with defense system depicted higher salt tolerance through up-regulation
of CAT and POD activities under salt stress. The susceptibility or tolerance of
a plant genotype depends on the relationship between ROS and antioxidant enzyme
production under stress (Mittler 2002; Farooq et al. 2017).
It is a well-established fact that salt stress uplifts
the Na+ concentration in different plant parts and declines the
concentration of cations like Ca2+ and K+ (Husain et
al. 2012, 2013). In the present investigation, salt tolerant genotype
California Wonder showed higher leaf K+/ Na+ ratio
compared to sensitive genotype. Maintenance of K+ ions uptake and
preventing K+ ion efflux from leaf cells, while avoiding Na+
and favoring efflux of Na+ ions from leaf cells are potential
strategies undertaken by plants to achieve desirable K+/ Na+
ratio in the cytosol to tolerate the salt stress (Wakeel et al. 2011; Hussain et al.
2013). Moreover, salt tolerant plants may retain more Na+ ions in
the roots and limit their transport to the upper parts (leaves). It is an
adaptation strategy to cope with salinity stress while salt sensitive plants
lack this kind of adaptations (Shahid et
al. 2012). In current research, overall the Green beauty could not compete
with California wonder because of reduced K uptake, weaker defense system and
growth traits and finally declared to be sensitive variety.
Conclusion
The genotype California wonder showed better growth
performance by maintaining higher K+/Na+ ratio, enhanced
anti-oxidative activity and greater free proline
accumulation in the presence of slat stress as compared to Green beauty and
proved to be salt tolerant.
Acknowledgements
The data presented here is a part of PhD thesis at GC University Faisalabad and authors highly
acknowledge the support of HEC-NRPU 5635. Thanks are due to Dr. Abid Ali (UAF)
for his comments and suggestion on earlier draft of this manuscript.
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