Introduction
High salt concentration in the rooting medium induces
rapid osmotic stress, ion toxicity and ionic imbalance. In addition, enhanced
reactive oxygen species (ROS) production leads to oxidative stress hence,
adversely affecting plant growth and production (Krasensky
and Jonak 2012). Mechanism of osmolyte accumulation and antioxidant defense are biological
markers that restore metabolic homeostasis by maintaining ROS equilibrium via signaling (Turkan and Demiral 2009; Pintó-Marijuan
and Munné-Bosch 2013). Plants under salt stress
sustain their turgor pressure by accumulating compatible solutes or osmolytes
(uncharged, polar organic compounds) which do not restrict cell metabolism even
at higher concentration (Turkan and Demiral 2009; Gupta and Huang 2014;
Al-Farsi et al. 2020). In order to
balance lower water potential due to ions sequestration in vacuole, the
osmolytes like glycinebetaine, free proline, and polyols accumulate in
cytoplasm (Farooq et al. 2020). However, synthesis/accumulation of
organic molecules requires more energy than inorganic ions reducing plant
growth under salt stress (Munns and Gilliham 2015). Excessive soluble salts in
the rooting medium cause osmotic effect and ion toxicity by rendering
plant’s root unfit for water extraction from the soil solution (Muhammad and Hussain 2012; Farooq et
al. 2015) and by accumulating sodium (Na+) and chloride (Cl−)
ions in shoots, respectively (Tavakkoli et
al. 2011; Ahanger et al. 2014) hence, lead to the disturbed
metabolism and oxidative stress (Fayez and Bazaid 2014). Rapid Na+ influx
in plant displaces K+ (essential for binding ribosomes to tRNA,
hence proteins conformation) and Ca2+ ions (Hameed et al.
2014; Shrivastava and Kumar 2015). Sodium (or Cl−) ion
concentration is toxic when surpasses 30 mM
(cytosol) and 100-200 mM
(mitochondria and chloroplast), respectively (Conn and Gilliham
2010; Flowers et al. 2015).
Nutritional imbalance (reduction in potassium, phosphate, calcium
and nitrate availability) as a result of salt (Na+ and Cl−)
accumulation impair plant productivity (Nasri
et al. 2015). Salt-tolerant
species either exclude Na+ and Cl− ions or maintain
their low concentration by sequestering them in the vacuole (e.g., in barley) and increase
concentration of osmotica (organic or inorganic) to regulate osmotic pressure
of the soil and maintain turgor, essential for the growth of plants (Shahbala
2013).
Experiments have been performed to study the impact of salinity
stress on crops and described drastic effects of salt stress
on productivity of glycophytes (Wani et
al. 2013; Rivero et al. 2014;
Zhang et al. 2014; Munns and Gilliham
2015). These studies used diverse methodologies to improve salt tolerance in
plants, especially seed priming with various growth regulators (vitamins and
hormones). Plant growth regulators are key modulators of various progressions
in plants under abiotic stresses (Lalarukh et
al. 2014). Seed priming is a quick and cost effective method in reducing
negative impact of salt stress by improving metabolism, rapid seed germination
and consistency in stand establishment at initial stages of different plant
species (Farooq et al. 2019, 2020).
Alpha-tocopherol acts as an antioxidant (Shao et al. 2008), reduces lipid peroxidation
(Sattler et al. 2006) by detoxifying
ROS (Hincha 2008). Maeda et al.
(2006) reported a crucial role of α-toc in phloem loading. Ludwig (2009)
revealed that α-toc strongly effects nutrient remobilization in Arabidopsis thaliana lines. In leaves of
transgenic alfalfa, increase in α-toc improved protein content and delayed
leaf senescence (Jiang et al. 2016);
whereas overexpression of γ-tocopherol methyltransferase is linked with up-regulation of sugar transport in
transgenic plants (Jin and Daniell 2014). In higher plants
shifts in α-toc levels in stress response, activate signal transduction
pathway (Hyun et al. 2011) and
regulate carbohydrate metabolism (Li et al. 2008).
Sunflower (Helianthus annuus L.) is a short
duration (90–110 days life cycle) oilseed crop cultivated twice in a year,
categorized among moderately salt tolerant crops and can bear 50 mM salt
stress (Moghanibashi et al. 2013;
Kumar et al. 2014). Therefore,
sunflower can be grown in areas where irrigation water is slightly brackish
(Riaz et al. 2012). However, in
presence of (soluble) salts in higher amount in soil can have devastating
effects on sunflower production (Wang et
al. 2017). Previous studies have reported that
α-toc, as an antioxidant plays significant role in abiotic
stress mitigation, in remobilization of
nutrients and modulation of carbohydrate metabolism (Maeda et al.
2006; Ludwig 2009; Farouk 2011; Semida et al. 2016; Hemida et al. 2017). In a previous study, Lalarukh and Shahbaz (2020)
observed that plants raised from seeds primed with α-toc increased growth
and yield related attributes with increase in enzymatic (catalase and
peroxidase), non-enzymatic (total phenolic and ascorbic acid) antioxidants and
reduction in lipid peroxidation in sunflower under salt stress. Lalarukh and
Shahbaz (2018) reported increase in turgor potential, water use efficiency, net
photosynthetic rate and stomatal conductance in leaves of sunflower plants
raised from seeds primed with α-toc (vitamin E) along with increase in
root and shoot (fresh) weight and shoot length. Inversely, little is known about the impact of α-toc seed priming on accumulation of osmolytes and ion
homeostasis under saline condition in sunflower. Therefore,
it is assumed that seed priming with α-toc may improve osmolytes accumulation, ion homeostasis and salt
stress tolerance in sunflower. Thus, the objective of the present research was
to investigate the role of α-toc seed priming in osmolytes accumulation, ion homeostasis and salt stress
alleviation in sunflower.
Materials and Methods
Experimental site
During the years 2015 and 2016, pot experiments were
executed to study the impact of α-toc seed priming on osmotic adjustment
of sunflower under salt stress in ambient environment at the botanic garden,
University of Agriculture (31° 30'N latitude, 73° 10'E longitude and 213m altitude), Faisalabad, Pakistan. The seeds of
sunflower cultivars (FH 572 and FH 621) were obtained from Ayub Agricultural
Research Institute (Oilseed Research Section), Faisalabad, Pakistan.
Experimental treatments
Sunflower
achenes (100) were kept immersed in 100 mL solution of 4 concentrations (0,
100, 200 and 300) mg L-1 of α-toc each for 16 h.
Alpha-tocopherol was dissolved in (2 mL) ethanol (an organic solvent) and then
diluted with distilled water up to the required limit. After drying 10 healthy
sunflower seeds were sown in sand (10 kg) filled plastic pots (24.5 cm in diameter
and 27.94 cm in depth). Plants were supplemented with full strength Hoagland’s
solution (one liter per pot at the vegetative stage and two liter per pot at
the reproductive stage) to fulfill their nutrient demand at two weeks interval.
After thinning (at three leave stage), six sunflower plants were kept in each
pot. Salt stress (
Experimental conditions and design
The design
of the experiment was completely randomized design (CRD) with four
replications. Sixty four plastic pots were used for this experiment and six
plants per pot were maintained till sampling. The experiment was conducted
under ambient environmental conditions with 16.5 to 31.8°C temperature, 66 to 39% relative
humidity, 67.9 to 11.6 mm rainfall and 5.6 to 10.4 h sunshine, from February to
June respectively.
Free
proline
Free
proline was estimated following Bates et al. (1973) method. Fresh leaf
(3rd from the top) 0.5 g was homogenized using mortar and pestle, in
(3% w/v) (10 mL) sulphosalicylic acid and then filtered. Acid ninhydrin (2 mL)
and glacial acetic acid (2 mL) was added to (2 mL) filtrate and heated for 60 min
at 100°C in a water bath. Mixture was cooled by placing test tubes in ice and
vortexed after adding toluene (4 mL) for 15 sec. Quantity of free proline was
determined by measuring absorbance of upper layer formed in the test tube at
520 nm with the help of spectrophotometer (IRMECO
U2020) Germany.
Glycinebetaine
Amount of glycinebetaine produced in
leaves was determined by using Grieve and Grattan (1983) method. Freshly
sampled 3rd leaf (0.5 g) from the top was ground using distilled H2O
and centrifuged (12000 x g)
for 10 min. One mL sulphuric acid (2 N) was added to the supernatant (1 mL)
extracted in a test-tube. From the above blend (0.5 mL) extract was pipetted
out in another test tube and was kept for 90 min in ice after adding periodide
solution (0.2 mL). Distilled H2O (1.4 mL) and 1, 2-dichloromethane
(chilled 6 mL) was supplemented in the mixture. Lower layer absorbance was
recorded with the help of spectrophotometer (IRMECO
U2020) Germany, at 365 nm soon after formation of two distinct layers.
Total free amino acids
Fresh leaves were sampled and homogenized for amino
acids determination in phosphate buffer (7.0 pH)
following Hamilton and VanSlyke (1943) technique. To 1 mL of extract 10%
pyridine (1 mL) and 2 % ninhydrin (1 mL) was added. After heating the mixture
on (boiling) water bath for half an hour, distilled water was added to maintain
volume up to 50 mL. Optical density of the mixture was recorded at 570 nm using
a spectrophotometer. Readings were calibrated with the help of standard curve
developed by using amino acid leucine.
Total
solvable sugars
Yoshida et al.
(1976) protocol was used to determine the amount of total (soluble) sugars. In
a test tube, 0.1 mL ethanolic aliquot was taken, 3 mL anthrone reagent (freshly prepared) was added, mixed and
vortexed. Mixture was heated for 15 min at 95°C, cooled at room temperature and absorbance at 625 nm was recorded using
spectrophotometer (IRMECO U2020) Germany.
Ionorganic
Ions
Ionic
concentration (Na+, Ca2+ and K+) of shoot and
root were determined using Allen et al. (1985) procedure. In a digestion
flask, 0.1 g oven dried (ground) shoot/root and H2SO4 (2
mL) was added and kept for 24 h at room temperature. Flasks were heated to 200°C
and H2O2 (1 mL) was added to the mixture upon cooling.
Volume of the colorless mixture was maintained to 50 mL with distilled water
and filtered. Amount of Ca2+, Na+ and K+ in
roots/shoots was determined with the help of Flame photometer (Sherwood Model
410, Cambridge, U.K.).
Statistical analysis
Snedecor
and Cochran (1980) method was used to determine the analysis of variance data
for various attributes using COSTAT computer program and mean values were
compared. Tukey’s test was used for mean separation with 5% level of
significance.
Results
Osmolytes accumulation
Seed
priming with α-toc showed non-considerable influence on free proline.
Sunflower cultivars showed similar (non-significant) response in case of free
proline. However, free proline increased (P
≤ 0.05) considerably in (both) sunflower cultivars under saline
condition. Remarkably significant (P
≤ 0.01) interaction in between salt stress and α-toc revealed that
seed priming with 100 and 200 mg L-1 α-toc levels amplified
free proline production in FH-572 (11.11%) and FH-621 (65.41%) cultivars
respectively, in saline condition compared to hydro-primed seeds. Strong
interaction (P ≤ 0.05) was also
observed in between cultivars and α-toc (Table 1; Fig. 1).
Plants raised from seeds primed with α-toc
exhibited remarkable rise (P ≤
0.01) in glycinebetaine (GB). Overall production of GB was greater (P < 0.001) in FH-572 than FH-621
cultivar. Salinity had non-significant influence on GB production.
Significant (P ≤ 0.01)
interaction among α-toc, salt stress and cultivars indicated that under
salt stress, seeds primed with α-toc levels, 300 and 100 mg L-1
effectively enhanced production of GB in FH-572 (53.25%) and FH-621 (75%) cultivars, respectively under saline condition compared
to hydro-primed seeds (Table 1; Fig. 1).
Seed priming with α-toc considerably increased (P < 0.001) total free amino acids.
Overall production of total free amino acids was greater (P < 0.001) in FH-621 than FH-572 cultivar. Under salt stress,
considerable rise (P ≤ 0.05) in
(total) free amino acids in FH-621 cultivar was observed (Table 1; Fig. 1).
Substantially higher (P < 0.001)
interaction between salt stress and cultivars showed rise in total free amino
acids in FH-621 and descend in FH-572 under salt stress. Significant (P ≤ 0.05) interaction in between
salt stress and α-toc revealed that seed priming with 200 mg L-1 α-toc
level enhanced total free amino acids production in FH-572 (75%) and FH-621
(31.25%) cultivars, respectively in saline condition compared to hydro-primed
seeds.
Table 1: Mean squares from analyses of variance of data for
organic osmolytes in sunflower grown from seeds primed with α-tocopherol (16 h) under salt
stress and non-stress conditions
|
Source of variations |
df |
Free proline |
Glycinebetaine |
Free amino acid |
Total soluble sugars |
|
Cultivars (Cvs) |
1 |
4.165ns |
9592.64*** |
0.020*** |
0.007* |
|
Salinity (S) |
1 |
180.79* |
397.84ns |
0.002* |
0.206*** |
|
α-tocopherol
(α-toc ) |
3 |
76.84ns |
892.03** |
0.007*** |
0.006* |
|
Cvs × S |
1 |
75.77 |
551.4ns |
0.014*** |
0.002ns |
|
Cvs × α-toc |
3 |
116.77* |
251.4ns |
0.0007ns |
0.0001ns |
|
S × α-toc
|
3 |
185.57** |
223.4ns |
0.002* |
0.007** |
|
Cvs × S ×
α-toc |
3 |
77.43ns |
841.9** |
0.001ns |
0.001ns |
|
Error |
48 |
35.35 |
151.989 |
0.0005 |
0.002 |
* = P ≤ 0.05, ** = P ≤ 0.01, *** = P < 0.001, ns = non-significant, df = degrees of freedom
%201672-1680_files/image002.jpg)
Fig. 1: Osmolytes accumulation in sunflower
upraised from seeds treated with α-tocopherol
(16 h) under salt stress and non-stress regimes
Plants raised from α-toc
primed seeds showed significant increase (P
≤ 0.05) in (total) soluble sugars. Overall production of (total) soluble
sugars was more (P ≤ 0.05) in
FH-621 compared to FH-572 cultivar. Salt stress greatly enhanced (P < 0.001) production of (total)
soluble sugars in both sunflower cultivars (Table 1; Fig. 1). Considerable
interaction amongst α-toc and salt stress (P ≤ 0.01) indicated that α-toc level, 200 mg L-1
played influential role in increasing total soluble sugars under salt stress in
FH-572 (30%) and FH-621 (25%) than hydro-primed seeds.
Ion accumulation
Plants
raised from α-toc primed seeds exhibited
substantial decrease (P ≤ 0.01)
in sodium (Na+) concentration of shoot. Salinity stress imposition
caused significant increase (P <
0.001) in shoot Na+ concentration of FH-572 contrary to FH-621
cultivar which showed reduction in shoot Na+ concentration compared
to non-stressed plants. Overall, decrease in Na+ concentration of
shoot was more pronounced (P ≤
0.01) in FH-621 cultivar in salt stress compared to control. Plants raised from
α-toc (200 mg L-1) primed seeds exhibited substantial decrease
in Na+ concentration of shoot in FH-572 (5.71%) and FH-621 (25.15%)
under saline condition than hydro-primed. Interactions between and among all
three factors (salinity, α-toc and cultivars) were highly significant
(Table 2; Fig. 2).
The
effect of α-toc seed priming on potassium (K+)
concentration of the sunflower shoot was not significant. On the whole,
accumulation of K+ concentration in the shoot of FH-621 was higher (P < 0.001) compared to FH-572
cultivar. Potassium concentration increased substantially (P ≤ 0.01) in the shoots of both sunflower cultivars in salt
stress. Significant interaction (P
≤ 0.05) in between cultivars and salt stress showed more increase in
shoot K+ of FH-572 under salt stress compared to FH-621 cultivar.
Substantial (P ≤ 0.05)
interaction also occurs in between α-toc and cultivars. Plants raised from
α-toc (level, 300 mg L-1) primed seeds showed more increase in
shoot K+ of FH-572 (67.8%) than FH-621 (1%) cultivar upon seed
priming with 300 mg L-1 α-toc level, under saline condition
than hydro-primed seeds (Table 2; Fig. 2).
Table 2: Mean squares from analyses of variance of data for
inorganic ions of sunflower grown from
seeds primed with α-tocopherol (16 h) under salt stress and non-stress
conditions
|
Source of variations |
df |
Shoot Na+ |
Shoot K+ |
Shoot K+/Na+ |
Shoot Ca2+ |
Root Na+ |
Root K+ |
Root K+/Na+ |
Root Ca2+ |
|
Cultivars (Cvs) |
1 |
150.10** |
1980.25*** |
0.434*** |
115.562* |
42.25ns |
210.25*** |
0.041*** |
10.560*** |
|
Salinity (S) |
1 |
297.50*** |
900.00** |
0.277*** |
76.562* |
6.25ns |
182.25*** |
0.046*** |
10.560*** |
|
α-tocopherol
(α-toc) |
3 |
97.56** |
33.50ns |
0.021ns |
11.395ns |
40.42ns |
18.25ns |
0.002ns |
0.468ns |
|
Cvs × S |
1 |
637.60*** |
462.25* |
0.003ns |
52.562ns |
600.25*** |
6.25ns |
0.002ns |
0.062ns |
|
Cvs × α-toc |
3 |
94.06** |
294.42* |
0.065** |
15.562ns |
33.75ns |
2.92ns |
0.002ns |
0.718ns |
|
S × α-toc
|
3 |
253.60*** |
85.17ns |
0.041* |
9.562ns |
55.75* |
8.92ns |
0.004* |
0.573ns |
|
Cvs × S ×
α-toc |
3 |
162.90*** |
117.42ns |
0.039* |
43.229ns |
57.75* |
22.25* |
0.003ns |
1.135ns |
|
Error |
48 |
17.65 |
75.79 |
0.011 |
16.729 |
14.58 |
6.58 |
0.001 |
0.554 |
* = P ≤ 0.05, ** = P ≤ 0.01, *** = P < 0.001, ns = non-significant, df = degrees of freedom
%201672-1680_files/image004.jpg)
Fig. 2: Ionic concentration in sunflower upraised from seeds primed with
α-tocopherol (16 h) under salt stress and
non-stress regimes
Seed priming with α-toc
showed non-significant influence on potassium to sodium (K+/Na+)
ratio in shoots of sunflower. Overall, K+/Na+ ratio was considerably
higher (P < 0.001) in shoots of
FH-621 cultivar compared to FH-572. Salt stress substantially increased (P < 0.001) K+/Na+ ratio
in both sunflower cultivars (Table 2; Fig. 2). Interaction between α-toc
and cultivars (P ≤ 0.01) showed
that in salt stress, seed priming with 300 mg L-1 α-toc caused substantial increase in K+/Na+
ratio in shoots of FH-572 (73.53%) whereas, priming with 200 mg L-1
α-toc increased K+/Na+ ratio in shoots of FH-621
cultivar to only 0.47% than hydro-primed seeds. Significant interactions
between α-toc and salinity (P
≤ 0.05) and amongst all three factors (P ≤ 0.05) (α-toc, salt stress and cultivars)
were also observed.
Seed priming with α-toc had no significant effect
on calcium (Ca2+) concentration in sunflower shoot. On the whole,
the amount of Ca2+ concentration in the shoot of FH-621 cultivar was
higher (P ≤ 0.05) compared to
FH-572 cultivar. Imposition of salt stress significantly reduced Ca2+
concentration (P ≤ 0.05) in the
shoot of FH-572 cultivar whereas; it triggers the amount of Ca2+
concentration in the shoot of cv. FH-621 (Table 2; Fig. 2).
All three factors, seed priming with α-toc,
cultivars and salt stress had non-significant effect on Na+
concentration of the root (Table 2; Fig. 2). However, substantial interaction (P < 0.001) between salt stress and
cultivars showed increase and considerable reduction in Na+ concentration
of root in FH-572 and FH-621 cultivars respectively, in salt stress.
Significant interaction (P ≤
0.05) in between salinity and α-toc and amongst all three factors
(α-toc, salt stress and cultivars) (P
≤ 0.05) were observed. Plants raised from α-toc (200 mg L-1)
primed seeds exhibited substantial decrease in Na+ concentration of
root in FH-572 (5.34%) and FH-621 (8%) under saline condition than
hydro-primed.
Seed priming with α-toc had no considerable effect
on potassium (K+) concentration of the root. On the whole, amount of
K+ retained in roots of FH-621 was significantly higher (P < 0.001) compared to FH-572
cultivar. Salt stress significantly reduced (P < 0.001) K+ concentration in roots of both
sunflower cultivars. Significant (P
≤ 0.05) interaction amongst α-toc, salt stress and cultivars was
observed. Plants raised from α-toc (100 mg L-1) primed seeds
exhibited minimum decrease in K+ concentration of root in FH-572
(38%) whereas, FH-621 showed no decrease in K+ concentration of root
in FH-621 upon seed priming with 300 mg L-1 α-toc, under saline
condition than hydro-primed seeds (Table 2; Fig. 2).
Plants grown from seeds primed with α-toc had
non-significant influence on potassium/sodium (K+/Na+)
ratio of roots in sunflower. Overall increase in K+/Na+
ratio of roots in FH-621 was much higher (P
< 0.001) compared to FH-572 cultivar. Sunflower cultivars showed substantial
reduction (P < 0.001) in K+/Na+
ratio of roots in salt stress. However, significant (P ≤ 0.05) interaction between α-toc and salt stress
showed that seed priming with 200 mg L-1 α-toc increased K+/Na+
ratio in roots of FH-621 to 4.76% whereas all three levels (100, 200 and 300 mg
L-1) of α-toc showed decrease in K+/Na+
ratio in roots of FH-572 than hydro-primed seeds under saline condition (Table
2; Fig. 2).
Seed priming with α-toc had no remarkable influence
on calcium (Ca2+) concentration of the roots in
sunflower. Overall increase in the Ca2+ concentration of the roots
was more pronounced (P < 0.001) in
FH-621 compared to FH-572 cultivar. Salt stress considerably increased (P < 0.001) Ca2+ concentration
in the roots of both sunflower cultivars (Table 2; Fig. 2).
Discussion
In the
present investigation, increase in the amount of free proline in both sunflower
cultivars in salt stress was similar to the previous findings on sunflower, Vicia faba and cotton (Rady et al. 2011; Orabi
and Abdelhamid 2016; Hussien et al. 2015). Accumulation of free proline under abiotic stress is
a common aspect in plants and protects the plants from adversities of salt
stress (Saxena et al. 2013; Bose et al. 2014). Proline improves salinity
tolerance in plants by accelerating (enzymatic) antioxidants activities (Hoque et al. 2008), photosynthetic rate
(Ben-Ahmed et al. 2010), maintaining
plant water relation (Deivanai et al.
2011) and detoxifying ROS (Matysik et al.
2002). Proline is an osmolyte which protects complex II in (mitochondrial)
electron transport chain and also PS I and II from hydroxyl radical and singlet
oxygen (Szabados and Savoure 2010). Results from previous studies on sunflower
(Rady et al. 2011), Vicia faba (Semida et al. 2014) and onion (Semida et
al. 2016) have shown increase in free proline in response to α-toc
exogenous application however, in the present study plants grown from seeds
treated with α-toc had no influence on the tissue accumulation of free
proline.
Results
from the present study revealed an increase in GB amount in both sunflower
cultivars grown from α-toc primed seeds. Glycine betaine is an osmolyte
which helps in salt stress mitigation by stabilizing proteins and shields
photosynthetic machinery from ROS injury (Makela et al. 2000; Cha-Um and Kirdmanee 2010; Yildiztugay et al. 2013).
Accumulation of free proline and GB help in maintaining turgor pressure
essential for plants elongation by regulating osmotic potential under salt
stress (Hajlaoui et al. 2010; Munns
and Gilliham 2015). Fitzgerald et al.
(2009) reported that GB protects membranes, photosynthetic apparatus and PS-II (oxygen
evolving complex) even in small concentration. Hassine et al. (2008) revealed that GB and free proline levels increased in
(salt tolerant) Atriplex halimus L.
upon exposure to 160 mM salt stress.
Plants grown from seeds primed with α-toc showed remarkable increase in
(total) free amino acids. Stress induced increase in free amino acids was more
distinct in FH-621 cultivar. Sadak et al.
(2010) and Rady et al. (2011)
reported the same in sunflower. Accumulation of amino acids, lower osmotic
potential and also act as osmoprotants in plants.
Although both sunflower cultivars showed increase in
total (soluble) sugars under salt stress but seed priming with α-toc (vitamin E) was found quite effective in producing more
sugars especially in sunflower cv. FH-621 under salt stress, hence increased salt
tolerance in sunflower. Likewise, Sadak et
al. (2010) observed increase in total carbohydrates upon
exogenous application of α-toc and reported that sugars and protein
accumulation delayed leaf senescence in sunflower. Sadak and Dawood (2014)
reported α-toc and ascorbic acid mediated increase in total soluble sugars
and proteins.
In the present study, sunflower plants raised from
α-toc (200 mg L-1) primed seeds have shown considerable
reduction in Na+ concentration of shoot in cv. FH-621. However,
differential response of both sunflower cultivars i.e., decrease and increase in Na+ concentration of
shoots in cvs. FH-621 and FH-572 respectively under salt stress have shown that
cv. FH-621 is a salt tolerant variety. Haleem and Mohammed (2007) and Cuin et al. (2009) reported that Na+
ion not only competes antagonistically with K+ ion but also reduces
its uptake and Ca2+ ion concentration in the plants. Previous
studies have shown that increased Na+ accumulation reduced the
uptake of essential nutrients (especially K+) in roots and shoots of
sunflower and mungbean (Shahbaz et al.
2011; Kanwal et al. 2013). Inhibition
in cell elongation and division, metabolic dysfunction, membrane disruption and
inhibition of enzymes activities are all attributed to increased Na+
toxicity in salt sensitive plant species (Kassem 2006). Results from the
present investigation have shown the accumulation of K+ ions in
shoots of both sunflower cultivars but was more pronounced in cv. FH-572 under
salt stress. Ashraf and Tufail (1995) have reported that tolerant accessions of
sunflower in comparison with salt sensitive ones deposit more K+, K+/Na+
ratio and less Cl- ion in leaves under salt stress. However, seeds
priming with α-toc had no remarkable impact on K+ and Ca2+
ions concentration and K+/Na+ ratio in the shoots,
in the present study.
In the current investigation, shoot K+/Na+
ratio though increased in both sunflower cultivars but was considerably more
significant in cv. FH-621 in salt stress. More K+ ions accumulation
protect the plants from Na+ ion toxicity and maintain water
potential lower to accomplish osmotic adjustment. Previously it is shown that
salt tolerant genotypes retained higher K+/Na+ ratio
while sequestering Na+ ions in the vacuole (Rahnama et al. 2011). Results from this research
revealed considerable increase and decrease in Ca2+ concentration of
shoot in cvs. FH-621 and FH-572 respectively, under salt
stress. Calcium, being a sec messenger, plays significant role in stress
related signal transduction pathways. A previous study on sunflower has revealed
that seed soaking with α-toc and nicotinamide enhanced K+, Mg2+
and Ca2+ and reduced Na+ accumulation (Rady et al. 2011). In the current study,
neither α-toc seed priming nor salt stress had any substantial influence
on Na+ concentration of root. However, K+ ions and K+/Na+
ratio decreased in roots of sunflower under saline condition. Increase in Ca2+
accumulation in roots of both sunflower cultivars under salt stress revealed
its importance as sec messenger in stress related signal transduction pathways
and also as membrane stabilizer. Similar to our study a previous study has
reported increase in Ca2+ accumulation in the root of mungbean under
salt stress (Kanwal et al. 2013).
However, in the present study seed priming with α-toc showed no
considerable impact on root ionic accumulation of sunflower.
Conclusion
Among
organic solutes seed priming with α-toc improved accumulation of
glycinebetaine, amino acids and soluble sugars in the leaves of sunflower by
regulating proteins and carbohydrates metabolism. Seed priming with α-toc
had non-significant influence on ion homeostasis however, it caused significant
reduction in Na+ ion concentration of the shoot by the mechanism needed
further investigation. Sunflower cv. FH-572 showed overall more accumulation of
glycinebetaine but FH-621 cultivar a potentially high yielding variety by its
inherent ability of maintaining overall, high K+, K+/Na+
ratio and Ca2+ ions in shoot and root was proved to be more salt
tolerant. In most of the studied parameters seed priming with medium
concentration of α-toc (200 mg L-1) was found effective in
alleviating the negative impact of salt stress.
Acknowledgement
The data of
this manuscript are portion of PhD dissertation of Irfana Lalarukh
(scholar) at Botany Department, University of Agriculture, Faisalabad, Pakistan.
Author Contributions
I.L performed research, collected data
and wrote manuscript whereas, M.S. checked and supervized the whole work.
Authors approved the final manuscript version.
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