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Research Article
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Effect of Silicon and Boron Foliar Applications on Wheat Plants Grown under Saline Soil Conditions
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A.H. Hanafy Ahmed,
E.M. Harb,
M.A. Higazy
and
Sh.H. Morgan
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ABSTRACT
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Two pot experiments were carried out under different
soil salinity levels (0, 2000, 4000 and 6000 ppm), as well as field experiment
under saline soil condition, to investigate the effect of spraying silicon
(0, 250 and 1000 ppm SiO2) and/or boron (0 and 25 ppm B) sprayed
twice (40 and 70 days after sowing) under pots experiment, as well as
spraying silicon (0, 250, 500 and 1000 ppm SiO2) and/or boron
(0, 25 and 50 ppm B) sprayed three successive times (at 35, 60 and 85
days after sowing) under field experiment on growth, yield and chemical
composition of wheat (Triticum aestivum L.) var. Seets 1. Generally,
under pots experiment, both silicon levels either alone or combined with
boron significantly increased shoots height and leaf area as well as grains
yield/plant and weight of 1000. However, under field experiment only the
lowest level of silicon significantly increased all the studied growth
characters, while all levels of silicon significantly increased number
of spikes and grains as well as grains yield when compared with control
non-sprayed plants, however, the lowest level of silicon had the superiority
effect. Both silicon and boron applications correct to some extent the
negative effects of salinity either on growth, yield, nutrients uptake,
free polyamines and endogenous plant hormones (gibberellic acid and cytokinins)
while decreased abscisic acid. However, no constant trend was obtained
for indole-3-acetic acid concentration.
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INTRODUCTION
Soil salinity is a major problem for agriculture throughout the
world. The problem of salinity is of special importance in Egypt for both
the old cultivated area as well as for the newly reclaimed lands. The
major constraints for plant growth and productivity are ion toxicity with
excessive uptake of mainly Cl- and Na+ as well as
nutrients imbalance caused by disturbed uptake or distribution of essential
mineral nutrients (Hu and Schmidhalter, 2005).
Silicon was reported to reduce the hazard effects of various abiotic
and biotic stresses including salt stress, metal toxicity, drought stress,
radiation damage, various pests and diseases caused by both fungi and
bacteria, nutrients imbalance, high temperature and freezing (Ma, 2004).
Usually, plants grown on saline soil suffer from several nutritional disturbances
and nutrient deficiencies and consequently plant growth inhibition. In
this respect, Hanafy Ahmed et al. (2002b) working on wheat plants
grown under different salinity levels (0, 3000 and 6000 ppm) noted that,
growth characters of wheat plants had pronounced increased by adding silicon
under non-saline or saline soil conditions.
Moreover, it`s well known that under salinity conditions boron (B) concentration
was decreased in plants (Holloway and Alston, 1992; Wimmer et al.,
2001). Boron is an important micro nutrient known to exhibit several interactions
with other plant nutrients.
Thus, the aim of the present study was to investigate the effect of Si
and B foliar applications as well as their combination on growth, yield
and chemical composition of wheat, aiming to improve wheat yield under
non-saline or saline soil conditions.
MATERIALS AND METHODS
Pot experiments were carried out in the wire house of the Plant Physiology
Section, Faculty of Agriculture, Cairo University, Giza, Egypt, during
the two successive seasons, 2001-2002 and 2002-2003. Plastic pots (30
cm in diameter) were filled with 10 kg clay loam soil obtained from the
Farm of Faculty of Agriculture, Giza. Field experiment was carried out
in Smosta, Baneswaf governorate during season 2003-2004. Mechanical analysis
for pot experiments were determined according to the methods reported
by Richards (1954). Chemical analyses of soils for pot and field experiments
were determined according to the methods reported by Jackson (1973) (Table
1).
In the two successive seasons, pots were divided into 3 main groups.
The first main group was sprayed with fresh water (control A) and the
other two main groups were sprayed with silicon as sodium meta silicate
(Na2SiO3.5H2O) at concentration 250 or
1000 ppm SiO2, respectively. Each main group was divided into
two sub-groups. The first sub-group was sprayed with boron as boric acid
(H3BO3) at concentration 25 ppm B, while the second
sub-group was sprayed with water. Each sub-group of pots was divided into
4 sub-sub-groups of salinity levels (0, 2000, 4000 and 6000 ppm). Soil
salination was made by adding mixture of sodium chloride, calcium chloride
and magnesium sulphate at the ratio of 1:1:0.5 by weight. Each pot received
2.2 g of calcium superphosphate (15.5% P2O5) and
1.1 g potassium sulphate (48% K2O) before sowing and 3.0 g
ammonium nitrate (33.5% N) added two weeks after sowing. In each pot,
10 grains of wheat (Triticum aestivum L.) var. Seets 1 were sown
on the 30th Nov. 2001 in the first season and on the 29th Nov. 2002 in
the second one. The seedlings were thinned to 6 plants per pot two weeks
later after sowing. Seeds were weekly irrigated with fresh water keeping
it not more than field capacity. Different foliar treatments were applied
at 40 and 70 days after sowing.
Field affected with salinity (4400 ppm) was parted into 3 main plots.
The first main plot sprayed with water whereas the second and third main
plots were sprayed with boric acid at concentrations 25 and 50 ppm B,
respectively. Each main plot was splited into 4 sub plots. The first sub
plot was sprayed with fresh water (control A) and the other 3 sub plots
were sprayed with silicon at the rates of 250, 500 and 1000 ppm SiO2,
respectively. Fertilization was carried out according to recommendation
of Ministry of Agriculture; calcium superphosphate (15.5% P2O5)
and potassium sulphate (48% K2O) were added before planting
at the rate of 150 and 50 kg/fed., respectively. Ammonium nitrate (33.5%
N) were added before the first irrigation at the rate of 300 kg/fed. Wheat
grains were sown on 24th Nov. 2003. Foliar application treatments were
applied at three successive times 35, 60 and 85 days after sowing.
In both successive seasons of pots and in field experiment, one plant
sample was taken 90 days after sowing. The following measurements were
recorded: shoot height (cm), number of tillers/plant, leaf number, leaf
area (cm2) and shoot dry weight (g). At harvest (160 days after
sowing), yield and yield components were estimated as follows: number
of spikes and grains per plant, grains as yield g/plant in pots and ardab/fed
in field, straw yield as g/plant in pots and ton/fed in field as well
as weight of 1000 grains (g).
Chemical Analysis
Determinations of N, P, K, Ca, Na and Mg as well as proline were carried
out on the ground dry material. Nitrogen concentration was determined
by Nessler method according to AOAC (1960). Phosphorus was estimated colorimeterically
using the chlorostannous reduced molybdophosphoric blue color method according
to Jackson (1973). Potassium and sodium were determined using the flame
photometer (CORNING M 410). Calcium and magnesium were determined using
the atomic absorption spectrophotometer (GBC, 932 AA). Free proline concentration
was measured colorimeterically using ninhydrin reagent (Bates et al.,
1973). Total chlorophyll in fresh leaves were extracted with dimethyl
formamide and calculated as described by Nornai (1982).
Ethanol extract of shoots were used for the determination of total sugars,
total free amino acids and total soluble phenols. Total sugars were determined
by using the phenol sulphuric acid reagent (Dubois et al., 1956).
Total free amino acids were determined by using ninhydrin reagent (Moore
and Stein, 1954). Total soluble phenols were determined by using the Folin-Denis
colorimetric method (Swain and Hillis, 1959).
Free polyamines as putrescine (Put) was only determined in second season
of pot experiment as well as in field experiment by dansyl chloride reagent
and quantified with spectrophotofluroimeter according to Galston (1983).
Extraction of plant hormones was only done in second season of pot experiment,
according to Sadeghian (1971). The alkaline fraction was used for the
determination of cytokinins according to El-Ghamrawy and Neumann (1977)
while the acidic fraction was used for determination of gibberellic acid,
abscisic acid and indole-3-acetic acid according to Vogel (1975) using
Gas-Liquid Chromatography (GLC).
Data of pot experiments were statistically analyzed by using three factorial
completely randomized design, while data of field experiment were statistically
analyzed using a split plot design which boron foliar application was
the main plot and silicon foliar application was the sub-plot. The means
were compared using the least significant difference test (LSD) at 5%
level (Snedecor and Cochran, 1980).
RESULTS AND DISCUSSION
Growth Characters and Yield Components
Concerning the effect of salinity (Table 2, 3,
5 and 6) with increasing levels there
was a significant and gradual decreases in all of the studied growth characters
and yield components in the two successive seasons of pot experiment.
These results were in agree with that reported by Hu and Schmidhalter
(1997), Hanafy Ahmed et al. (2002b) and Iqbal (2005) working on
wheat. However, with soil salinity level 2000 ppm a reverse trend was
recorded on leaf number and shoot dry weight in both seasons as well as
on number of grains/plant in the first season of pot experiment.
The reduction in plant growth and yield due to salinity might be attributed
to the inhibiting effects of salinity on many metabolic processes including
protein, nucleic acid and polyamine synthesis (Mittle and Dubey, 1991;
Reggiani et al., 1994), activity of mitochondria and chloroplasts
(Udovenko and Tsibkovaskayai, 1983; Singh and Dubey, 1995), decreasing
transpiration, stomatal conductance and photosynthesis (Sharma, 1995),
restricts the absorption of water by plant roots and water use efficiency
(Mansour, 1994), the toxic effects of certain ions present in soil solution
(Maas and Nieman, 1978) and/or imbalance in phytohormone levels through
its effect on either the biosynthesis or the destruction of the plant
hormones (Nesiem and Ghallab, 1999).
Table 2: |
Shoot height (cm), leaf area (cm2) and leaf
number of wheat plants sprayed by different levels of silicon and
boron grown under different levels of soil salinity in pot experiment
during the two successive seasons 2001-2002 and 2002-2003 |
 |
S: Soil salinity levels, A: Silicon treatment (Si 1
= 250 ppm SiO2 and Si 2 = 1000 ppm SiO2) and
B: Boron treatment (0B = Tap water and 1B = 25 ppm boron), NS: Non
Significant |
The reduction in grain yield/plant at soil salinity levels 2000, 4000
and 6000 ppm reached 5.6, 30.3 and 42.9%, respectively when compared with
control non-stressed plants in the first season. Meanwhile, the corresponding
values in the second season were 17.7, 31.4 and 53.5%, respectively. In
this respect, Hanafy Ahmed et al. (2002b) found in wheat a reduction
on the most growth characters and yield components [number and/or weight
of grains, weight of spikes as well as grain filling (weight of 1000 grain)]
due to salinity.
Table 3: |
Number of tillers and shoot dry weight (g) of wheat
plants sprayed by different levels of silicon and boron grown under
different levels of soil salinity in pot experiment during two successive
seasons (2001-2002 and 2002-2003) |
 |
S: Soil salinity levels, A: Silicon treatment (Si 1
= 250 ppm SiO2 and Si 2 = 1000 ppm SiO2), B:
Boron treatment (0B = Tap water and 1B = 25 ppm boron) NS: Non Significant |
Concerning the effect of silicon, in pots experiments, data presented
in Table 2 and 3 revealed that, both
levels of silicon either alone or combined with boron under both non-saline
and saline soil conditions significantly increased shoots height and leaf
area. However, no constant trend could be detected on leaf number, number
of tillers and shoot dry weight, where they significantly increased in
the first season at the lower level of silicon, while a reverse trend
was recorded in the second season. These findings are in contrast with
those previously stated by Hanafy Ahmed et al. (2002b) on wheat
plants. They reported that, the highest rate of silicon application (1000
ppm SiO2) resulted in pronounced increase on all of the studied
growth characters and all yield components of wheat, especially at the
highest level of soil salinity (6000 ppm), but not at the lower rate of
silicon (500 ppm SiO2).
The lowest level of silicon (250 ppm) significantly increased all the
studied growth characters when compared with control non-sprayed plants
(Table 4). Similar results were recorded by Trivedi et
al. (2004).
Table 4: |
Growth characters and yield components of wheat plants
sprayed by different levels of silicon and boron grown under saline
field (4400 ppm) |
 |
b: Silicon treatment, Si 1 = 250 ppm SiO2,
Si 2 = 500 ppm SiO2, Si 3 = 1000 ppm SiO2, (a)
Boron treatment, 0B = Tap water, 1B = 25 ppm boron, 2B = 50 ppm boron,
NS: Non Significant |
It could be suggested that, silicon application might exert their favorable
effect to counteract the detrimental effects of salinity when the plants
would show obvious stunting. In this connection, evidence has been provided
that silicon not only contributes to cell wall rigidity and strengthening
but might also increase cell wall elasticity during extension growth (Marschner,
1995). Moreover, the enhancement effect on shoot height of wheat plants
supplied with Si might be induced through its role in both cell division
and cell expansion by their effect on RNA and DNA synthesis. In this respect,
Aleshin (1988) mentioned that nucleic acid preparations from rice tissues
contain large amounts of Si. Moreover, silicon increased the thickness
of the culm wall and the size of the vascular bundles (Shimoyama,
1958) preventing lodging in rice, thereby enhancing the
strength of the stems.
Moreover, Ma and Takahashi (2002) and Ma (2004) reported that, Si could
alleviate the effects of abiotic stresses including salt stress,
metal toxicity and nutrient imbalance. These beneficial effects
are mostly expressed through Si deposition in the leaves, stems
and hulls. Therefore, Si is characterized by wide effects associated
with the greater Si accumulation in the shoots.
Hanafy Ahmed et al. (2002b) and Trivedi et al. (2004)
mentioned that, adding silicon under both non-saline and saline conditions
reversing the salt inhibition on growth mainly by reducing the sodium
uptake into the shoot. Moreover, Liang et al. (1996) concluded
that silicon could increase photosynthesis and decrease the permeability
of plasma membranes of leaves of salt stressed barley. They also mentioned
that silicon could inhibit the uptake of Na and increase the uptake of
K, thus mitigating salt toxicity to the plants and improving the vegetative
growth of salt stressed
barley. Moreover, Marschner (1995) noted that, silicon could stimulate
growth and yield under both non-saline and saline soil by several indirect
actions. These include decreasing mutual shading through improving leaf
erectness. Ma and Takahashi (1993) mentioned that the maintenance of erect
leaves as a result of silicate application can easy account for 10% increase
in the photosynthesis of the canopy and consequently increase in both
growth and yield.
In addition, Agrie et al. (1992) working on rice, reported
that silicon reduced transpiration and increased water use efficiency
in leaves, which in turn reduced the decline in photosynthesis
and chlorophyll destruction in older leaves. Also, Liang et al.
(1996) pointed out that silicon treatment increased CO2 assimilation
of barley leaves.
Concerning the effect of silicon on yield components of pot experiments
the two successive seasons, silicon decreased spikes number/plant, straw
yield and grains number/plant when compared with control non-sprayed plants
(Table 5 and 6). While, grains yield/plant
and weight of 1000 grain were significantly increased by both silicon
treatments either alone or combined with boron under saline soil conditions
in the two successive seasons, with some exceptions, but under non-saline
soil the lower level of silicon combined with boron have the superiority
effect comparing with control non-sprayed plants under the same saline
soil condition. Concerning the effect of silicon on yield components under
field conditions, all measured yield components significantly increased
by all levels of silicon with some exceptions, as compared with control
non-sprayed plants (Table 4). However, the lowest level
of silicon had the superiority effect in all yield components, except
of straw yield which was higher by the middle level of silicon.
In this respect, Pandey and Yadav (1999) reported that spraying silicon
(50 or 100 ppm Si) increased grain yield/plant of wheat. They referred
that to an increase in plant water status, chlorophyll content, biological
yield and harvest index, coupled with reduced values of water potential,
increase in dry matter accumulation, dry matter production rate, leaf
area/plant at the flowering stage, productive tillers, grains and grain
yield/main spike and per plant and transpiration rate coupled with a decrease
in stomatal conductance. Moreover, Ma et al. (1989) working on
rice mentioned that during the reproductive stage, silicon is preferentially
transported into the flag leaf and interruption of silicon supply at this
stage is detrimental for spikelet fertility.
Concerning the effects of boron foliar application treatment, in pots
experiments, the two successive seasons, boron increased most of the studied
growth characters, but these increases did not reach a significant level
when compared with non-sprayed control plants with some exceptions (Table
2 and 3). Moreover, the shoot height and dry weight
was significantly increased by boron application (Table 4).
Similar results were recorded by Gunes et al. (2003) and Mete et
al. (2005) on wheat. Meanwhile, number of leaves, leaves area and
number or tillers did not affected with both boron levels, except the
lower level of boron which significantly increased the number or tillers
as compared with control non-sprayed plants with some exceptions. Similar
results were recorded by Grieve and Poss (2000) on wheat. In this respect,
several authors noted that, it`s well known that under salinity conditions
boron concentration in the plants was decreased (Holloway and Alston,
1992; Wimmer et al., 2001). Moreover, changes in wall properties
that resulted from decreased borate cross-linking of pectin might lead
to many of the symptoms associated with boron deficiency, which almost
associated with salinity in plants (O'Neill et al., 2005).
Concerning the effects of boron on yield components of pot experiments,
the two successive seasons all measured yield components significantly
increased by boron foliar application when compared with control non-sprayed
plants, except of number of spikes/plant and weights of straw/plant which
did not significantly affected. Similar results were recorded by Gunes
et al. (2003) and Mete et al. (2005) on wheat. Also the
interaction between boron and silicon take the same trend especially under
non-saline and at 6000 ppm soil salinity conditions (Table
5 and 6).
Table 5: |
Spikes number, straw yield (g)/plant and grains number
of wheat plants sprayed by different levels of silicon and boron grown
under different levels of soil salinity in pot experiment during two
successive seasons (2001-2002 and 2002-2003) |
 |
S: Soil salinity levels, A: Silicon treatment (Si 1
= 250 ppm SiO2 and Si 2 = 1000 ppm SiO2), B:
Boron treatment (0B = Tap water and 1B = 25 ppm boron), NS: Non Significant |
On the other hand, boron at both levels increased spikes number/plant
and grains yield when compared with control non-sprayed plants (Table
4). Meanwhile, straw yield and weight of 1000 grains were significantly
increased at the lowest level of boron. However, no significance differences
were obtained from grains number. In this respect, Subedi et al. (1997)
noted that, the soil addition of boron (1 kg B ha-1) significantly
increased the number of grains/spike thereby decreasing the total sterility
on wheat. Rahman et al. (2005) noted that, boron availability was
enhanced by liming which could have significant effects in preventing
spike-sterility and thereby increased yield of wheat and rice plants.
Table 6: |
Grains yield (g)/plant and weight of 1000 grain (g)of
wheat plants sprayed by different levels of silicon and boron grown
under different levels of soil salinity in pot experiment during two
successive seasons (2001-2002 and 2002-2003) |
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S: Soil salinity levels, A: Silicon treatment (Si 1
= 250 ppm SiO2 and Si 2 = 1000 ppm SiO2), B:
Boron treatment (0B = Tap water and 1B = 25 ppm boron), NS: Non Significant |
Chemical Composition
Inorganic Components
Concerning the effect of salinity on nutrients concentration, the
low values of both N and K concentrations were recorded by shoots growing
under salinity when compared with control plants grown under non-saline
soil (Table 7). The reduction was more pronounced at
the higher salinity level. In grains also a gradual reduction in N concentrations
were recorded with increasing salinity level. These results were agree
with those reported by Hu and Schmidhalter (1997) and Hanafy Ahmed et
al. (2002b) on wheat. It is important here to mention that, salinity
induced by using chloride or sodium salts might be implicated indirectly
in decreasing N and K concentrations of wheat plants. In this respect,
Hanafy Ahmed et al. (1996) mentioned that chloride seems to cause
a decrease in nitrate accumulation, since its uptake is antagonistic to
nitrate. Leidi et al. (1991) observed the competition between K+
and Na+ in wheat, especially by roots at different rates of
salinity. They pointed out that a reduction of transpiration recorded
at the highest salinity level could be correlated with a severe reduction
in K level.
Table 7: |
Nitrogen, phosphorus and potassium concentrations (mg
g-1 D.W) in the shoots of wheat plants sprayed by different
levels of silicon and boron grown under different levels of soil salinity
in pot experiment during two successive seasons (2001-2002 and 2002-2003) |
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S: Soil salinity levels, A: Silicon treatment (Si 1
= 250 ppm SiO2 and Si 2 = 1000 ppm SiO2), B:
Boron treatment (0B = Tap water and 1B = 25 ppm boron), NS Non Significant |
In addition, higher values of Na, Ca and Mg concentrations were detected
in saline stressed plants, which were more pronounced at higher than at
lower salinity level (Table 8). This higher internal
Na, Ca and Mg concentrations might be risen as a function of external
addition of NaCl, CaCl2 and MgSO4 in the soil. However,
no constant trend could be detected for P concentration at both seasons.
In this connection, Hanafy Ahmed et al. (2002b) suggested that
the several detrimental effects attributed to salinity stress on growth
characters and yield of wheat might be partially due to decreases in N
and K concentrations and/or increases in P, Na, Ca, Mg values. Leidi et
al. (1991) working on wheat reported that growth, leaf expansion and
yield under saline conditions were limited by lowering intake of N and/or
K by the plant in the presence of NaCl.
Table 8: |
Sodium, calcium and magnesium concentrations (mg g-1
D.W) in the shoots of wheat plants sprayed by different levels of
silicon and boron grown under different levels of soil salinity in
pot experiment during two successive seasons (2001-2002 and 2002-2003) |
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S: Soil salinity levels, A: Silicon treatment (Si 1
= 250 ppm SiO2 and Si 2 = 1000 ppm SiO2), B:
Boron treatment (0B = Tap water and 1B = 25 ppm boron), NS: Non Significant |
Moreover, high Na concentration may negatively affect plant growth due
to the decrease in the absorption and translocation of water in these
plants (Bernstein and Pearson, 1956), disturbing the fine structure of
the plant cell causing swelling of the chloroplasts and plastids which
may decreasing the net photosynthesis (Yeo et al., 1985) and affect
the availability of most macronutrients in the plant (Hu and Schmidhalter,
1997). In addition, Hu and Schmidhalter (1997) reported that relative
growth rate and yield were positively correlated with potassium content
and negatively correlated with sodium content in root and shoot of wheat
plants.
In pot experiments, the two successive seasons, high values of N, P,
K and Ca concentrations were recorded by the shoots and grains with silicon
application either alone or combined with boron comparing with control
silicon non-sprayed plants, especially in shoots sprayed with boron, with
some exceptions (Table 7-10). On the other hand, a reverse
trend was recorded on Na concentration in grains. However, no constant
trend could be detected for Na concentration in shoot and for Mg concentration
in both shoots and grains by spraying silicon either alone or combined
with boron when compared with control non-sprayed plants.
In this respect, Chen et al. (2002) reported that, absorption
of N, P and K by rice plants was enhanced by silicon application. Moreover,
Liang et al. (1996) and Hanafy Ahmed et al. (2002b) reported
that, under saline soil conditions, low values of both Ca and Mg were
recorded by the plants treated with 500 ppm SiO2, while a reverse
trend was detected by spraying of 1000 ppm SiO2. Moreover,
Hanan (1996) working on rice, suggested that the beneficial effects of
Si might be ascribed to the isomorphus replacement of the phosphate ions
with the silicate ions. However, Marschner (1995) mentioned that Si had
no direct effect on P uptake or translocation to the roots. Liang et
al. (1996) pointed out that Si could enhance the uptake of K and inhibit
the uptake of Na in salt-stressed barley. In the same concern, Trivedi
et al. (2004) reported that addition of silicon to the nutrient
medium reversed the salt inhibited growth mainly by reducing the shoot
Na uptake.
In field experiment, silicon was significantly increased N and K concentrations
in shoots as well as N, P and Ca concentrations in grains, especially
in the plants treated with the higher rate of silicon (Table
11). In this respect, Hanan (1996), Liang et al. (1996) and
Hanafy Ahmed et al. (2002b) pointed out that N and K uptake was
significantly increased in shoots when Si was added to soil. The stimulating
effect of Si on N content might be due to the pH rise by sodium meta silicate
which stimulates ammonification (Kai, 1978). Moreover, Nayar et al.
(1982) reported that the application of silicate materials might help
further increasing in grain yields, especially through efficient use of
applied N. So, the data indicated that, the concentration of P increased
in shoots whereas concentration of Na and K in grains were increased but
not reached to significance, with some exceptions. However, Na and Mg
concentrations decreased significantly in shoots as well as concentration
of Mg in grains, with some exceptions, when compared with control silicon
non-sprayed plants.
In this respect, it can be suggested that the decrease in Na might be
induced by dilution effect as a result of beneficial effects of Si application
on dry matter accumulation. Similar suggestion was recorded by Hanafy
Ahmed et al. (2002b) on wheat plants. Moreover, Ma and Takahashi
(1993) working on rice, suggested that Ca uptake and translocation are
closely related to the transpiration. They also mentioned that transpiration
rate was significantly decreased by the Si addition and caused the decreased
on Ca uptake.
Thus, concerning the effect of silicon on alleviating the harmful effects
of salinity, it can be suggested that, Si addition may improve nutritional
balance under saline soil conditions, thereby a better growth performance
and consequence yield production obtained. In this respect, Gharib and
Hanafy Ahmed (2005) noted that, silicon foliar applications enhanced N,
K and Na concentrations in pea shoots. It might be suggested that, B could
improve the nutritional balance under saline soil conditions leading to
better performance of silicon.
In addition, in field experiment, boron significantly increased P and
K concentrations in shoots, while N was significantly decreased at the
highest level (50 ppm) (Table 11). Meanwhile, Mg and
Ca concentrations in shoots decreased significantly by the lowest level
of boron (25 ppm) but increased significantly at the highest level of
boron. While, no significantly differences could be detected on Na concentrations
under both levels of boron. Moreover, the highest level of boron was significantly
increased N whereare significantly decreased P and Ca in grains. While,
no significant trend could be reported at the lowest level of boron on
all of the studied minerals concentrations.
Table 9: |
Nitrogen, phosphorus and potassium concentrations (mg
g-1 D.W) in grains of wheat plants sprayed by different
levels of silicon and boron grown under different levels of soil salinity
in pot experiment during two successive seasons (2001-2002 and 2002-2003) |
 |
S: Soil salinity levels, A: Silicon treatment (Si 1
= 250 ppm SiO2 and Si 2 = 1000 ppm SiO2), B:
Boron treatment (0B = Tap water and 1B = 25 ppm boron), NS: Non Significant |
In this respect, Schon et al. (1990) noted that B deficiency induced
inhibition of H+-ATPase activity causes a decrease
in cation uptake, such as K+. Moreover, El-Magid et al.
(2000) reported that, spraying wheat with B as boric acid at 0.06%
reduced N content. Meanwhile, Shaaban et al. (2004) reported on
wheat that, the highest uptake of K and Ca was obtained with foliar application
of B (25 or 50 ppm), which suggested that boron stimulates metabolic functions
facilitate absorption and translocation of these nutrients, especially
in plants grown under lime stress conditions.
Table 10: |
Sodium, calcium and magnesium concentrations (mg g-1
D.W) in grains of wheat plants sprayed by different levels of silicon
and boron grown under different levels of soil salinity in pot experiment
during two successive seasons (2001-2002 and 2002-2003) |
 |
S: Soil salinity levels, A: Silicon treatment (Si 1
= 250 ppm SiO2 and Si 2 = 1000 ppm SiO2), B:
Boron treatment (0B = Tap water and 1B = 25 ppm boron), NS: Non Significant |
Also, Lopez-Lefebre et al. (2002) noted that, increasing B application
(5, 10 and 20 mmol L-1 H3BO3) induced
a general improvement in the nutritional state, particularly of the essential
macronutrients N and P in tobacco plants (Nicotiana tabacum L.)
which could explain the positive effects of B on increase dry-material
production in the roots and leaves.
Organic Components
Concerning the effects of salinity on studied organic components of
wheat plants, there is a gradual reduction on total chlorophyll of the
two successive seasons by increasing salinity level when compared with
control plants grown under non-saline soil (Table 12).
In this respect, El-Bagoury et al. (1999) suggested that, biosynthesis
of chlorophylls in generally might be inhibited by the depressive effect
of stress conditions on the absorption of some ions involved in the chloroplast
formation, such as Mg and Fe and/or an increase of growth inhibitors,
such as ethylene or abscisic acid production which enhance senescence.
Salt could induce weakening of protein-pigment-lipid complex thus decreasing
chlorophyll concentration (Strogonove et al., 1970) or increased
chlorophyllase activity (Stivesev et al., 1973).
Table 11: |
Nitrogen, phosphorus, potassium, sodium, calcium and
magnesium concentrations (mg g-1 D.W.) in shoots as well
as grains of wheat plants sprayed by different levels of silicon and
boron grown under saline field (4400 ppm) experiment |
 |
b: Silicon treatment, Si 1 = 250 ppm SiO2,
Si 2 = 500 ppm SiO2, Si 3 = 1000 ppm SiO2, (a)
Boron treatment, 0B = Tap water, 1B = 25 ppm boron, 2B = 50 ppm boron,
NS: Non Significant |
In addition, the higher values of total sugars, free amino acids, soluble
phenols and proline concentrations were detected in saline-stressed plants
and these increases were more pronounced at the highest salinity level
(Table 12 and 13).
In this respect, Hanafy Ahmed et al. (2002b) working on wheat
mentioned that salinized plants accumulated soluble carbohydrate and showed
a remarkable higher levels of amino acids, soluble phenols and proline
for osmoregulation. This might be explained on the assumption that such
salt-stressed plants could have less efficiency to condensate simple organic
compounds into more complex ones. Margna (1977) reported that the major
determinant of phenol production is the supply of the prerequisites for
its synthesis, namely the amino acid precursor`s phenylalanine and tyrosine.Under
pot experiment, in the second season, the total free-polyamines concentrations
gradually increased with increasing salinity level then decreased under
the highest salinity level (6000 ppm), but, still higher than control
non-saline plants (Table 14). These results are in agreement
with those obtained by Santa-Cruz et al. (1997) on tomato and Hanafy
Ahmed et al. (2002a) on Myrtus communis plants. Katiyar
and Dubey (1990) working on rice, indicated that NaCl stress induced accumulation
of Polyamines (PAs) which possibly might helping the cells in maintaining
more favorable ionic environment and in regulating cellular pH.
Table 12: |
Effect of silicon and boron spraying at different levels
on concentrations of total chlorophyll in leaves (mg g-1
F.W.) as well as total sugars (mg glucose/g F.W.) and total free amino
acids (mg g-1 F.W.) in shoots of wheat plants grown under
different soil salinity levels in pot experiment during two successive
seasons (2001-2002 and 2002-2003) |
 |
S: Soil salinity levels, A: Silicon treatment (Si 1
= 250 ppm SiO2 and Si 2 = 1000 ppm SiO2), B:
Boron treatment (0B = Tap water and 1B = 25 ppm boron), NS: Non Significant
|
Recently, Legocka and Kluk (2005) reported that salinity stress
(NaCl 260mM) induced biosynthesis of Putrescine (Put) and other PAs in
the roots, as well as induced Put accumulation in the leaves, which might
indicate for translocation of Put from the roots to the shoot in lupin
(Lupinus luteus L. var. Ventus). In this connection, Willadino
et al. (1996) pointed out that Put is synthesized in plants from
arginine and/or ornithine by the action of the biosynthetic enzymes Arginine
Decarboxylase (ADC) and Ornithine Decarboxylase (ODC). ADC activity showed
significant increments in relation to salt stress, which may be related
to Put and some amino acid variations. Several studies have demonstrated
that the accumulated Put in stressed tissues is derived from L-arginine
and is due largely to increased activities of the ADC pathway. Krishnamurthy
and Bhagwat (1989) pointed out that the Put accumulation observed in rice
in response to salt stress might have resulted from an enhancement of
Put synthesis, alternatively, from an inhibition of spermidin and spermin
synthesis through the inhibition of the activity of the enzyme S-adenosylmethionine.
Starvation of K might accumulate Put in leaves to replace K+
as a result of unbalanced uptake of ions under saline conditions to reflect
a homeostatic mechanism for controlling cellular pH in highest plants
(Watson et al., 1998).
Table 13: |
Effect of silicon and boron spraying at different levels
on concentrations of total soluble phenol (mg g-1 F.W.)
and proline (mg g-1 D.W.) in shoots of wheat plants grown
under different soil salinity levels in pot experiment during two
successive seasons (2001-2002 and 2002-2003) |
 |
S: Soil salinity levels, A: Silicon treatment (Si 1
= 250 ppm SiO2 and Si 2 = 1000 ppm SiO2), B:
Boron treatment (0B = Tap water and 1B = 25 ppm boron), NS: Non Significant
|
Concerning the effect of silicon on organic components, the pot experiment
of both seasons, total chlorophyll concentrations increased by silicon
foliar application, especially at the lowest level of silicon under both
non-saline and saline soil conditions (Table 12, 15).
Meanwhile, under field experiment chlorophyll concentration increased
at the higher levels of silicon, e.g., 500 and 1000 ppm. Similar results
were reported by Trivedi et al. (2004) on wheat.
Table 14: |
Total free polyamine concentration (mmol Put/g F.W.)
in shoots of wheat plants sprayed by different levels of silicon and
boron grown under different soil salinity levels in pot experiment
during season 2002-2003 |
 |
S: Soil salinity levels, A: Silicon treatment (Si 1
= 250 ppm SiO2 and Si 2 = 1000 ppm SiO2), B:
Boron treatment (0B = Tap water and 1B = 25 ppm boron) NS: Non Significant
|
Table 15: |
Effect of silicon and boron spraying at different levels
on concentrations of total chlorophyll in leaves (mg g-1
F.W.) as well as proline (mg g-1 D.W.), total free polyamine
(m mol Put/g F.W.), total sugars (mg glucose/g F.W.), total free amino
acids (mg g-1 F.W.) and total soluble phenol (mg g-1
F.W.) in shoots of wheat plants grown under saline filed (4400 ppm)
experiment |
 |
b: Silicon treatment, Si 1 = 250 ppm SiO2,
Si 2 = 500 ppm SiO2, Si 3 = 1000 ppm SiO2, 2B
= 50 ppm boron, NS: Non Significant |
In these respect, Ma et al. (2004) reported that, under drought
stress silicon decreased the decomposition of chlorophyll in cucumber
plants, limited plasma membrane permeability, alleviated malondialdehyde
content in leaves, as well as the activities of peroxidase, superoxide
dismutase and catalase. These physiological biochemical reactions showed
positive correlations with the amount of silicon supply.
Total chlorophyll concentrations decreased by boron foliar application
either alone or combined with silicon under both non-saline and saline
soil conditions when compared with control boron non-sprayed plants. Also
under field experiment, revealed that, chlorophyll decreased by increasing
boron concentration. Similar results were reported by Hanafy Ahmed et
al. (1996) on faba bean.
Concerning total sugars concentration, both pot experiments, silicon
either alone or combined with boron under saline soil conditions significantly
decreased total sugars concentration. However, under non-saline soil conditions
a reverse trend was obtained with some exceptions. Meanwhile, total soluble
phenols concentration was decreased in shoots of both seasons when compared
with control non-sprayed plants under both saline and non-saline soil
conditions. In this respect, Hanafy Ahmed et al. (2002b) suggested
that silicon could inhibit the synthesis of total soluble phenols under
saline soil conditions. Moreover, Parry and Kelso (1975) reported that,
Si seems to influence the content and metabolism of polyphenols in xylem
cell walls.
In addition, in both pot experiments, silicon either alone or combined
with boron significantly increased total free amino acids and proline
concentrations in shoots under both non-saline and saline soil conditions
when compared with control non-sprayed plants, with some exceptions. In
field experiment, silicon significantly decreased total sugars, total
free amino acids, total soluble phenols and proline concentrations in
shoots when compared with control non-sprayed plants. In this connection,
Hanafy Ahmed et al. (2002b) noted that, the favorable effects of
silicon on alleviating the harmful effects of salinity on wheat plants
could be indirectly through increase the ability of plants under salinity
stress to condensate simple organic compounds, e.g., sugars, free amino
acids, soluble phenols into more complex ones, e.g., carbohydrates, protein,
polyphenols. They added that, silicon might be an adaptive response to
salinity, through raising proline concentration. In addition, Schmidt
et al. (1999) noted that silicon might positively affect the activity
of some enzymes involved in the photosynthesis of turfgrass Regarding
the effect of boron, total sugars and total free amino acids concentrations
significantly increased in shoots by boron foliar application in both
pot experiments especially under high salinity levels. Also, total sugars,
total free amino acids and proline concentrations increased under field
experiment when compared with control boron non-sprayed plants. However,
proline concentration in shoots of both pot experiments did not affected
by boron spraying. On the other hand, total soluble phenols concentration
in shoots was significantly decreased in the first season of pot experiment
as well as under field conditions at the lower level of boron (25 ppm)
(Table 12, 13 and 15).
In this connection, Mengel and Kirkby (1979) pointed out that B might
modify the activity of the enzyme 6-phosphogluconate dehydrogenase by
complexing with its substrate 6-phosphogluconate. The enzyme regulates
the first step in the pentose phosphate pathway. It is therefore argued
that when B is present the activity of the pentose phosphate pathway is
decreased in favor of glycolysis. On the other hand, when boron is deficient
the pentose phosphate pathway is favored and consequently induces the
accumulation of phenolic compounds. They added that B and Cu may have
possible effects on DNA and RNA synthesis, which could increase the rate
of protein synthesis.
In this respect, Eweida et al. (1984) reported that spraying sugar
beet plants with boron at 100 ppm gave the highest sugar concentration.
Moreover, Acerbo et al. (1973) and Dugger (1983) mentioned that
the conversion of soluble sugars or sugar phosphate to starch was inhibited
by boron deficiency in plants.
Concerning the effect of silicon and/or boron on concentration of total
free-polyamines, under pot experiment conditions, all studied treatments
reduced total free-polyamines when compared with control non-sprayed plants
especially under saline soil conditions with some exceptions (Table
14). In this connection, Watson et al. (1998) reported that,
boron deficiency induced free putrescine accumulation in Arabidopsis
due to the differential and unbalanced uptake of ions and cell pH.
So, it may be suggested that boron could rebalance the uptake of ions
resulting in rebalance in cell pH, leading to decrease total free-polyamines
level under different salinity levels as compared with control non-sprayed
plants. Moreover, Camacho-Cristóbal et al. (2005) working
on tobacco plants reported that, levels of free Put and Spd in leaves
were similar in both boron-sufficient and boron-deficient plants. However,
boron-deficient plants had higher conjugated polyamine concentration than
boron-sufficient plants.
Under field experiment, both silicon levels, i.e., 250 and 500 ppm lowered
total free-polyamines, while the highest level (1000 ppm SiO2)
increased it when compared with control non-sprayed plants. On contrary,
a reverse trend was recorded for the plants treated with boron. This means
that polyamines increased with increasing boron concentration either alone
or combined with silicon when compared with control non-sprayed plants.
In this connection, it is well known that polyamine metabolism is altered
in response to an inadequate nutrition in K, Mg or P (Slocum et al.,
1984). For example, free putrescine dramatically increases in K-deficient
plants, suggesting that polyamines could play a role in maintaining cation-anion
balance in plant tissues (Flores and Martin-Tanguy, 1991). In relation
to the short-term effects caused by boron deficiency, Camacho-Cristóbal
et al. (2002) have described an accumulation of caffeoylputrescine
in leaves from boron-deficient tobacco plants, which lead to suggest a
possible role of boron in polyamine metabolism.
Several detrimental effects attributed to salinity stress on most of
the studied growth characters might be partially due to an increase in
the total free polyamine concentrations. In this respect, Smith (1977)
stated that salt stress caused a significant change in intracellular concentrations
of polyamine. Moreover, Di Tomaso et al. (1989) pointed out that
excess Put may have some negative effects on corn plant. Hsiao (1973)
reported that Put rise appears to be more sensitive to turgor deficit,
causing inhibition of cell growth, cell wall synthesis and ABA accumulation.
Moreover, N'Doye et al. (1994) working on tomato, proposed that
a high PA content in light might have a deleterious effects on the photosynthetic
apparatus. Additionally, Strogonove (1973) noted that, Put feeding is
the direct cause of the necrosis observed in salt-stressed plants. Thus,
it appears that, the effect of silicon and boron foliar application on
decreasing PAs accumulation might be indirectly implicated on reducing
the hazard effects attributed to salinity stress on most of the studied
growth characters.
Concerning plant hormones concentrations in the shoots of plants growing
under pot experiment in the second season, gradual decreases were detected
in IAA, GA3 and CK with increasing salinity level (Table
16). Meanwhile, an opposite trend was obtained with abscisic acid
(ABA). These findings are in agreement with those obtained by El-Shafey
et al. (2003) and Nesiem and Ghallab (1999) on wheat plants.
In this respect, Zeinab and Sallam (1996) reported that with increasing
salinity level, the tryptophan synthase α-monomers were
gradually dissociated from the oligomers producing less active soenzyme.
This reduced the biosynthesis of L-tryptophan and consequently IAA. Also,
CK activity decreased with increasing salt stress in barley plants. They
added that, gibberellin inhibitors accumulated, so plant growth was retarded
or even stopped.
Table 16: |
IAA, GA, ABA and CK concentrations (μg g-1
F.W.) in shoots of wheat plants sprayed by different levels of silicon
and boron grown under different soil salinity levels in pot experiment
during season 2002-2003 |
 |
S: Soil salinity levels, A: Silicon treatment (Si 1
= 250 ppm SiO2 and Si 2 = 1000 ppm SiO2), B:
Boron treatment (0B = Tap water and 1B = 25 ppm boron), NS: Non Significant |
Silicon either alone or combined with boron under different salinity
levels increased GA3 and CK concentrations. The boron-silicon
combinations have the superiority effect. However, no constant trend could
be detected from spraying silicon on IAA. IAA decreased by the lowest
level of silicon either alone or combined with boron, while a reverse
trend was detected at the highest level of silicon either alone or combined
with boron. On the other hand, ABA concentration decreased by both levels
of silicon either alone or combined with boron when compared with control
non-sprayed plants, with some exceptions under high salinity levels. Silicon
sprayed alone has the superiority effect on decreasing ABA than silicon
combined with boron.
It could suggest that, reducing the transpirational loss of water as
a beneficial effect of silicon under saline soil conditions might increase
the cell turgor, which could be more helpfully in rebalancing phytohormones
levels through its effect on either the biosynthesis (Amzallage et
al., 1992) or its destruction. The enhancement of the phytohormone
levels in treated plants within a suitable concentration range might improve
plant growth and yield of wheat plants. Al-aghabary et al. (2004)
studied the influence of silicon (2.5 mM), sodium chloride (NaCl 100 mM)
and Si (2.5 mM) + NaCl (97.5 mM) supply on H2O2
level and activities of some enzymes related to antioxidation protection
against the potentially cytotoxic forms of activated oxygen under salinity.
Silicon partially offset the negative impacts of NaCl stress by raising
superoxide dismutase and catalase activities. Also, Si addition decreased
ascorbate peroxidase activity and significantly reduced H2O2
level in the leaves of salt-treated plants (Rout and Show, 2001). It could
be suggested that, silicon might alleviate the effects of salt
stress by reducing peroxidase activity, which led to increase IAA by decreasing
its degradation.
Concerning the effect of boron foliar application on plant hormones concentrations
in shoots, data in Table 16 revealed that, boron increased GA3
and CK concentrations reaching about 10-fold either alone or combined
with silicon as compared to control non-sprayed plants under both non-saline
and saline soil conditions. Moreover, boron either alone or combined with
silicon decreased ABA under both non-saline and saline soil conditions,
except of a slight increases were found at 4000 or 6000 ppm soil salinity.
However, no constant trend could be detected for boron on IAA.
In this respect, Hanafy Ahmed et al. (1996) noted that, boron
treatments (25, 50 and 75 ppm) increased CK and GAs, but free auxins were
drastically reduced by all boron treatments on faba bean, while ABA did
not affect except of an increase was recorded with 75 ppm B treatment.
Moreover, Hanafy Ahmed et al. (1999) noted that, boron foliar application
(0.0, 100, 250 and 500 ppm as boric acid) increased IAA and GAs in peas
plants shoots, while ABA tended to decline.
In this connection, Wagner and Michael (1971) and Maynard and David (1987)
reported that, B deficiency has been found to depress CK concentrations
but an increase in IAA production was obtained as a result in the reduction
of IAA oxidase activity. Moreover, Das (2000) noted that boron deficiency
induced dramatic increase in the activity of IAA oxidase in the roots
which falls rapidly when boron is supplied. Moreover, Srivastava and Gupta
(1996) noted that boron could regulate endogenous auxin in plants by protecting
the IAA oxidase system through complexation of o-diphenols; inhibitors
of IAA oxidase. Excessive auxin activity found under B deficiency could
cause excessive proliferation of cambial cells, rapid and disproportionate
enlargement of cells and collapse of nearby cells.
It is well known that, in response to various environmental stresses,
such as drought and salinity, stomata is close as a process
that involves the phytohormone ABA (Schroeder et al., 2001).
Tanaka et al. (2005) found that, CK and auxins inhibit the
effects of ABA on induced stomatal closure. Also,
CK decreased the decomposition of chlorophyll and photosynthesis. So,
It could suggest that, the reducing of the transpirational loss of water
as a beneficial role of silicon (Agrie et al., 1992), could
kept the transpiration rate at a relatively steady rate during stress,
leading to increase the cells turgor which might be more helpful in rebalancing
of phytohormones by increasing IAA, GA and CK and decreasing ABA which
was reflected on improving plant growth and consequently on yield of wheat
plants.
CONCLUSION
In conclusion, the results of this study highlight the role of silicon
in improving wheat yield under non-saline or saline soil conditions either
alone or combined with boron. We suggest that under non-saline soil conditions
silicon should be sprayed by concentration 250 ppm combined with 0.25
ppm boron at 40 and 70 days after sowing with under investigation study
significantly increased grains yield/plant. Furthermore under different
saline soil all levels of silicon (250 and 1000 ppm SiO2 under
pots experiment as well as 250, 500 and 1000 ppm SiO2 under
field experiment) either alone or combined with boron significantly increased
grains yield/plant with some exceptions. However, the lowest level of
silicon combined with boron had the superiority effect in all measured
yield components under both pots and field experiments.
|
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