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Agricultural Water Management 98 (2011) 1808–1812
Contents lists available at ScienceDirect
Agricultural Water Management
journal homepage: www.elsevier.com/locate/agwat
Does silicon and irrigation have impact on drought tolerance mechanism of
sorghum?
Mukhtar Ahmed∗
, Fayyaz-ul-Hassen, Yasir Khurshid
PMAS, Arid Agriculture University, Muree Road, Shamsabad, Rawalpindi 46300, Pakistan
a r t i c l e i n f o
Article history:
Received 29 December 2010
Received in revised form 6 July 2011
Accepted 8 July 2011
Available online 2 August 2011
Keywords:
Drought
Leaf water potential
Growth dynamics
Irrigation
Synergistic
a b s t r a c t
Silicon absorption by crops in the form of silicic acid confers efficient utilization of available irrigation
water by minimizing transpiration losses. In present study, silicon and irrigation effects on sorghum
growth dynamics and drought tolerance mechanism were evaluated during 2007–2008. Two sorghum
cultivars: PARCSS2 and Johar1 were treated with two levels of silicon (Si0 = control and Si200 = 200 ml l−1
of
potassium silicate per kg of soil) and irrigation (W0 = control, crop lower limit and W40 = 40 mm of water,
crop upper limit). The results depicted that silicon absorption led to increased leaf water potential, growth,
transpiration, net photosynthetic rate and decreased shoot to root ratio in sorghum cultivars compared to
control treatment. It can be concluded that synergistic effect of silicon fertilization with ample irrigation
may improve the crop stand under drought and biotic stresses.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
The main component of soil solution which provides silicon
(second most abundant element in earth) is orthosilicic acid. Its
solubility is 1.7 mM at 25 ◦C with the pH < 9 (Knight and Kinrade,
2001). Silicon is not available in free form and it is absorbed by
plants in the form of uncharged silicic acid (Ranganathan et al.,
2006). Silicon is the main constituent of plants ranging from 0.1%
to 10% in top dry weight (Ma et al., 2001). The accumulation of sili-
con in plants differed significantly due to their roots ability to take
silicon (Ma et al., 2006). Silicon is agronomically important fertil-
izer as plant growth stimulant and tolerance builder to overcome
abiotic and biotic stresses (Liang et al., 2005). Silicon followed evap-
otranspiration path and deposited as hydrated silica (SiO2·nH2O) in
stems and leaves (Sangster et al., 2001). Water being limiting fac-
tor for crop growth and conventional irrigation technology does
not avoid elevated losses of crop available water due to evapo-
transpiration and leaching. Therefore, watering with silicon allows
a reduction in evapotranspiration and leaching (Gao et al., 2006).
Similarly, silicon fertilization led to increased volume and weight
of roots by 20–200% which ultimately enhanced drought resistance
in cultivated plants (Ahmed et al., 2011). Soil fertility and texture
have considerable correlation with Si compounds (Bocharnikova
and Matichenkov, 2008).
∗ Corresponding author. Tel.: +92 519290678/519290757; fax: +92 519290160.
E-mail address: ahmadmukhtar@uaar.edu.pk (M. Ahmed).
Silicon application as fertilizer to plants like maize depicted
decreased leaf transpiration under water stress and thereby
improved leaf water status (Gao et al., 2006). However, in sorghum
silicon application increased stomatal conductance (transpiration
rate) and alleviated the photosynthetic reduction by water stress
(Hattori et al., 2005). Similarly, antioxidants processes in crops were
activated by silicon under water stress (Gong et al., 2008). Thus,
silicon application may affect physiological traits to enhance crop
tolerance under deficit irrigation. Hattori et al. (2005) reported the
maintenance of physiological traits like transpiration, photosyn-
thesis and stomatal conductance in sorghum by silicon application.
In general, crop water uptake is regulated by root system. The
silicon fertilization improved water uptake in sorghum and conse-
quently improved the crop tolerance to water stress (Sonobe et al.,
2011).
Silicon is never available in free form; it combines with oxygen
to form oxides and silicates. Since absorption of silicon by crops is in
the form of silicic acid which changes to irreversible amorphous sil-
ica. Therefore, availability of silicon is very little as most sources of
silicon are insoluble and not available to crops (Epstein, 1994). Soils
deficient of silicon are amended with silicon fertilizers to increase
quality and quantity of crops (Korndorfer and Lepsch, 2001). Simi-
larly, silicon foliar application increases pathogen resistance of crop
species which do not take up silicon efficiently. Silicon is an element
that does not cause severe injury to crops when present in larger
amounts; therefore, it is non-toxic to crops and can provide mul-
tiple benefits (Menzies et al., 1992). Similarly, silicon has no side
effects to environment when applied in fertilizer form. However,
its use as a silicone and other compounds in the plastic industry has
0378-3774/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.agwat.2011.07.003

M. Ahmed et al. / Agricultural Water Management 98 (2011) 1808–1812 1809
showed remarkable negative effects. Meanwhile, some products of
metallurgic aggregates (slag) are being sold as silicon source which
can cause problems if they have heavy metal. However past studies
showed that slag as a silicon source and soil corrective have addi-
tional benefits of controlling environmental pollution by utilizing
industrial waste effectively (Prado and Fernades, 2000).
Sorghum (Sorghum bicolor L.) is the crop of dry regions where
it can remain alive under permanent drought by adjustment in
water potential and root water uptake (Lux et al., 2002). There-
fore, sorghum is more tolerant to water stress than other crops. Its
adjustment to secondary lands has been well recognized. Sorghum
can be an optional crop in areas where moisture limitation and
heat stress is a trouble for maize (Farré and Faci, 2006). The min-
eral element like silicon could be used to enhance drought tolerance
in sorghum under deficit irrigation since it promotes water uptake
under water stress (Sonobe et al., 2011). As water scarcity demands
the maximum use of every drop of water, there is a need to use
silicon for sustainable productivity of crops. Further analysis on sil-
icon application and water uptake is required to understand silicon
enhanced crop tolerance to water stress. However, still information
regarding drought tolerance and water uptake ability in conjunc-
tion with silicon is lacking. Therefore, this study was aimed (i) to
document the effect of silicon application under deficit irrigation
on the water stress tolerance of sorghum and (ii) to enhance water
stress tolerance in sorghum.
2. Materials and methods
The study was conducted at PMAS, Arid Agriculture University
Rawalpindi, Pakistan during 2007 and repeated during 2008. The
plastic pots having an area of 0.05 m2 were filled with 8 kg of the
soil which was well drained alluvial loam (fine-silty, mixed, hyper-
thermic, vertic, ochraqualfs, USDA). Sorghum genotypes (PARCSS2
and Johar1) were grown at two silicon concentrations (0 and
200 ml l−1; Si0 and Si200) and two level of irrigation (0 and 40 mm:
W0 and W40). Thus, in total twelve growth treatments: Si0W0,
Si0W40, Si200W0, Si200W40, PARCSS2Si0, PARCSS2Si200, Johar1Si0,
Johar1Si200, PARCSS2W0, PARCSS2W200, Johar1W0 and Johar1W200
were in completely randomized design replicated thrice. Potas-
sium silicate as silicon source was applied in the silicon-applied
treatment (Si200) while potassium chloride in the silicon-deficient
treatment (Si0) before sowing. Similarly, pH (7.6) of the soil was
adjusted by adding calcium hydroxide. Ten seeds evenly spaced in
pots by hand on 10th July, 2007 and 2008 respectively. The irriga-
tion treatments (W40) were started after 5 DAS (days after sowing)
and soil kept at 100% field capacity by adding water every day. After
complete emergence of seedlings, evaporation from the pots sur-
face was prevented by placing aluminium foil which remained on
the soil surface till 50 DAS to prevent an increase in soil tempera-
ture due to solar radiation. Twenty days after sowing the seedlings
were thinned to one and till 50 DAS only one plant were managed.
Leaf water potential («w) was recorded with pressure chamber
(Model 1000, PMS Instrument Co., Corvallis, OR) during the day-
time (10.00–14.00 h) while leaf area was recorded with leaf area
meter (CI-202 area meter CID, Inc), thereafter leaf area index (LAI)
was calculated. Lux et al. (2002) methodology was used to measure
silicon concentration of the fully expanded leaves, shoots and roots
at 50 DAS. The physiological parameters like transpiration (E) and
net photosynthetic rate (An) was measured by Infra-Red Gas Ana-
lyzer (IRGA) (Long and Bernacchi, 2003) while chlorophyll contents
were taken from top, middle and base of leaves and then average
value were used to represent SPAD chlorophyll contents. The plant
samples (leaves, shoots and roots) were ashed in a muffle oven at
500 ◦C for 5 h. The diluted HCL (1:1; 10 ml) was used to dissolve ash
at 100 ◦C and the process was repeated thrice. The samples were
then added into diluted HCL (1:1; 15 ml) and heated at 100 ◦C. The
samples were filtered, placed into a ceramic crucible and ashed
again in the oven at 540 ◦C for 5 h. The weight of Si was determined
gravimetrically after cooling.
The data obtained was subjected to STATISTICA 9 (Statsoft, Inc.
2010) software to check the effect of each treatment on crop growth
and silicon accumulation. Since effect of silicon on sorghum growth
was more prevalent under stress in past studies (Hattori et al.,
2008; Sonobe et al., 2009) therefore, we analysed data separately
for irrigated and non-irrigated treatments but pooled over years.
Two-way analysis of variance was performed on the data of leaf
water potential («w), leaf area index (LAI), specific leaf weight
(SLW), SPAD chlorophyll contents, leaf dry weight (LDW), shoot
dry weight (SDW), total dry weight (TDW), shoot to root ratio (S/R),
transpiration (E), net photosynthetic rate (An), silicon in leaf (SIL)
and silicon in root (SIR). The Scheffe’s test was used for comparison
of means at 5% level of probability. A coefficient of determination
(R2) was determined with 95% confidence interval to confirm the
relationship of treatments on crop growth and silicon accumula-
tion.
3. Results and discussion
Leaf water potential («w (−MPa)) was highest in the irrigated
treatment than non-irrigated, while silicon application signifi-
cantly increased leaf water potential (Table 1). Water use of
sorghum was greater in the silicon treatment with irrigation than
in non-silicon and non-irrigated treatments. For both irrigation
treatments, silicon application increased leaf water potential which
resulted to good water use by increased photosynthesis (Table 1).
However, under no irrigation sorghum maintained leaf water
potential to osmotically regulate leaf turgor pressure but this could
be improved prominently by applying silicon with deficit irrigation
(Table 1). Leaf water potential among genotypes depicted max-
imum values under silicon application while in the absence of
silicon it decreased significantly. However, under irrigation treat-
ments leaf water potential remained non-significant with lowest
R2 (Table 1). The results of present study confirmed the earlier
findings in which silicon application resulted to increased plant
water uptake and highest leaf water potential (Hattori et al., 2005).
Similarly, the adjustment of leaf water potential under deficit irri-
gation is one of the physiological mechanisms which might led to
highest leaf water potential (Hsiao and Xu, 2000). The results sup-
port our argument that the silicon application activated an active
water potential adjustment in sorghum leaves which consequently
enhanced the crop tolerance to water stress. Similar results were
reported by Sonobe et al. (2011) who concluded that silicon applica-
tion actively promoted water uptake which led to the development
of highest water potential.
Leaf area index (LAI), the determinant factor of crop growth
and productivity depicted significant variability due to irrigation
and silicon treatments (Table 1). The highest LAI was recorded for
Si200 while lowest noted for silicon deficient treatments. However,
effect of irrigation on LAI remained non-significant under silicon
applied treatments while it was significant under silicon deficient
treatments. LAI (at 50 DAS) in Si200 and W40 was 58% and 23%
higher than that in Si0 and W0 treatments. The increased LAI due
to silicon treatments could be due to rapid leaf area expansion
as silicon promoted cell wall extensibility (Hossain et al., 2002).
Among genotypes, Johar1 yielded good response for leaf area with
silicon application. The results were contradictory to earlier conclu-
sion of Tsuji et al. (2001) while in our study PARCSS2 decreased its
LAI confirming that genotypes could adapt, by rolling leaf accord-
ing to changing environmental conditions like moisture stress. In
addition silicon deposited in the leaves helped to improve light

1810 M. Ahmed et al. / Agricultural Water Management 98 (2011) 1808–1812
Table1
Leafwaterpotential,growth,physiologyandsiliconaccumulationparametersinsorghumplantsgrownattwosiliconconcentration(0and200mll−1
)andundertwolevelsofirrigation(0and40mm)for50DAS.
IrrigationtreatmentsSitreatments«w(−MPa)LAISLWSPADLDWSDWRDWTDWS/REAnSILSIR
W0Si00.51c2.95b189.1756.83b7.69b23.66c14.54c38.20c1.64a3.25c15.75c0.0033c2.44b
Si2000.86b3.71a188.6776.21a9.50b29.16b23.65b52.81b1.23b4.47b17.00b1.23b5.38a
W40Si01.22a2.33c177.8356.58b5.78c19.55d15.90c35.46c1.29b3.82c14.00c0.0075c2.28b
Si2000.38d3.84a19986.05a15.27a35.10a27.03a62.14a1.29b6.27a22.70a1.88a5.94a
R2
0.550.940.880.90.90.9930.820.940.080.760.790.890.87
Sourceof
variation
Irrigation(W)***NSNS***********
Silicon(Si)****NS**************
W×Si***NS***********
GenotypesSitreatments«w(−MPa)LAISLWSPADLDWSDWRDWTDWS/REAnSILSIR
PARCSS2Si00.99a2.66c147.22d57.00b6.7422.56c15.99c38.55c1.464.47b7.83c0.0045c2.44b
Si2000.67c3.58b190.33b80.83a13.3337.05a28.42a65.47a1.36.00a21.00a1.26b5.44a
Johar1Si00.86b2.62c167.17c56.41b6.7420.65c14.44c35.10c1.464.40b11.00b0.0063c2.29b
Si2000.57bc3.97a202.33a81.43a11.4427.21b22.26b49.47b1.226.46a23.00a1.86a5.87a
R2
0.470.640.430.7990.880.90.940.920.610.670.750.60.76
Sourceof
variation
Genotypes(G)****NSNS***NS***NS
Silicon(Si)**************
G×Si*****NS***NS****
GenotypesIrrigationtreatments«w(−MPa)LAISLWSPADLDWSDWRDWTDWS/REAnSILSIR
PARCSS2W01.083.33185.177312.61a32.81a23.03a55.85a1.474.15c6.80c0.75b3.9
W401.013.4618069.8810.36b25.94b18.53bc44.48bc1.466.43a20.00b1.13a4.47
Johar1W00.462.9120364.837.47c26.79b21.38ab48.17b1.294.20c5.97c0.51b3.98
W400.423.13186.567.967.82c21.92c18.16c40.09c1.235.46b23.22a0.73b3.68
R2
0.270.070.060.140.370.920.930.970.360.650.70.080.0044
Sourceof
variation
Genotypes(G)NSNSNSNS****NS***NS
Irrigation(W)NSNSNSNS********NS
G×WNSNSNSNS****NS***NS
Parametervalueswithinacolumnfollowedbydifferentlettersdiffersignificantly(P<0.05,Scheffe’stest).NS,*,and**representnon-significanceandsignificanceatP<0.05and<0.01,respectively(two-wayANOVA).The
parametersare:«w:leafwaterpotential;LAI:leafareaindex;SLW:specificleafweight;SPAD:chlorophyllcontents;SDW:shootdryweight(g);RDW:rootdryweight(g);TDW:totaldryweight(g),S/R:shootrootratio,E:
transpirationrate(mmolm−2
s−1
);An:netphotosyntheticrate(␮molm−2
s−1
);SIL:siliconinleaf(mg);SIR:siliconinroot(mg).

M. Ahmed et al. / Agricultural Water Management 98 (2011) 1808–1812 1811
interception characteristics by keeping the LAI and specific leaf
weight high (Epstein, 1999). Similarly, improved LAI due to silicon
application could be due to improved crop water status (Romero-
Aranda et al., 2006) and modification in the ultrastructure of leaf
organelles (Shu and Liu, 2001).
Specific leaf weight (SLW) an important criterion to observe
drought tolerance in different crops like sorghum remained non-
significant under irrigation and silicon treatments. However, SLW
in the silicon treatments were higher in Johar1, decreased sig-
nificantly in the PARCSS2 under silicon deficient treatment. The
analysis of variance for Si and G × Si interaction remained signif-
icant for SLW. The effect of irrigation (W) and G × W interactions
depicted non-significant effect on SLW. Significantly higher value
of SLW under silicon applied irrigated treatment could be due
to the accumulation of silicon in leaves, therefore, increased leaf
dry weight (Table 1). Similarly, maintenance of SLW might led to
increased resistance to biotic stresses (Wiese et al., 2005), improved
mechanical stability of stems and leaf blades (Rafi et al., 1997). SPAD
chlorophyll content measured by SPAD meter (Minolta, Tokyo,
Japan) at 50 DAS recorded highest Si200W40 while lowest value
noted in silicon deficient irrigated treatment (Si0W40). Similarly,
the highest value of chlorophyll contents recorded for Johar1 due
to silicon which confirmed that silicon application boosted photo-
synthetic processes by maintaining chlorophyll structure (Table 1).
However, the effect of irrigation treatments on sorghum genotypes
remained non-significant for chlorophyll contents.
Application of silicon to the sorghum under irrigated treat-
ment increased the leaf dry weight (LDW) while decreased trend
observed for silicon deficient non-irrigated treatment. Similar
results were reported by Tsuji et al. (2003) who concluded that
silicon application led to maximum plant dry matter. Similarly,
treatments effect (Si and W) as sole on LDW of sorghum remained
significant (R2 = 0.90). However, genotypes and G × Si interaction
were statistically non-significant while effect of irrigation treat-
ments remained significant for LDW (Table 1). Similarly, highest
shoot (R2 = 0.99), root (R2 = 0.82) and total (R2 = 0.94) dry weight
observed for silicon applied irrigated treatment (Si200W40). Mean-
while, the effects of genotypes, Si, irrigation (W), G × Si and
W × Si remained statistically significant for shoot, root and total
dry weight. The increased production of drymatter (LDW, SDW,
RDW and TDW) due to silicon and deficient irrigation treatments
revealed that silicon application to sorghum resulted to enhanced
growth (Sonobe et al., 2009). This led to conclusion that silicon
application improved water uptake and consequently enhanced the
plant growth under limited water (Hattori et al., 2008; Sonobe et al.,
2011). However, increased root growth could be due to modifica-
tion in the soil characteristics like soil porosity and bulk density.
As, in this study increased soil porosity and decreased bulk density
facilitated root penetration (Table 2).
Silicon application to sorghum with irrigated treatments
decreased shoot root ratio (S/R) which could be due to increased
root surface area. Similarly, silicon application increased root
dry weight, thus demonstrated that under silicon irrigated treat-
ment water uptake increased significantly. Similar results were
reported by Hattori et al. (2008) who concluded that silicon appli-
cation improved water uptake by sorghum root and consequently
enhanced crop tolerance to deficit irrigation. These results sug-
gested that silicon application was mainly beneficial to the growth
of root and its effect become more prominent in the presence
of irrigation which stimulated the development of root system,
allocating more matter to root system of plants (Taiichiro et al.,
2003). The highest root to shoot ratio obtained under silicon applied
irrigated treatments was close with the findings of Hattori et al.
(2005). The crop physiological parameters like transpiration (E)
and net photosynthetic rate (An) remained maximum under sil-
icon applied irrigated treatment while minimum value recorded
Table 2
Soil physiochemical characteristic before and after the experiments.
Characteristic Unit Before silicon After silicon
EC dS m−1
0.35 0.75
pH 1:1 7.5 7.45
Saturation % % 33.5 34.75
CEC C mol kg−1
9.15 9.85
Organic matter % 0.41 0.42
Silicon in soil (SiO3
2−
) mg kg−1
20.850 33.06
Soluble cations
Ca2+
Mg2+
meq l−1
2.70 2.60
Soluble anions
CO3
2−
meq l−1
0.50 0.41
HCO3
1−
meq l−1
2.45 2.10
Cl1−
meq l−1
3.00 2.45
SO4
2−
meq l−1
0.37 0.40
Textural class Loam Loam
Sand % 57.30 57.30
Silt % 23.80 23.80
Clay % 19.00 19.00
Total nitrogen % 0.031 0.024
K+
mg kg−1
78 69
Available P mg kg−1
6.75 6.25
DTPA extractable Zn mg kg−1
0.23 0.22
Bulk density mg m−3
1.57 1.40
Total porosity % 50.4 51.2
for silicon deficient non-irrigated treatment (Table 1). However,
among genotypes maximum E and An recorded for Johar1 under
silicon application (Si200) while irrigation treatments also showed
significant effect on physiological parameters (Table 1). The results
were in line with previous findings who concluded that silicon
application to sorghum crop increased transpiration rate therefore,
lessened the reduction of photosynthesis by water stress (Hattori
et al., 2005). Therefore, results of growth and physiological param-
eters confirmed that silicon and irrigation have synergistic effect
which could be used to improve crop stand under water stress.
The results of silicon concentration in leaves were significant
for silicon application under irrigated treatments while non-
significant for silicon deficient non-irrigated treatments (Table 1).
Similarly, among genotypes the highest silicon concentration
recorded for Johar1 under Si200 which could be due to the active
uptake of silicon by crop and its deposition in the leaves through
transpiration pathway. The results suggested that silicon fertiliza-
tion and irrigation treatments could help in maintaining pathogen
resistance mechanism in plant leaves by providing strong silicifi-
cation. Similarly, crops could maintain good turgor potential under
drought with the silicon fertilization while depositions of Si pro-
tect plants from multiple abiotic and biotic stresses. These data,
together with the high speed of silicon uptake and deposition by
sorghum root (Lux et al., 2003), and the effects of losing root cell
wall in sorghum (Hattori et al., 2005), confirmed the possibility that
silicon could play an important role in water transport and growth
of sorghum under drought conditions. Similarly, increased concen-
tration of silicon in root depicted maximum root dry weight which
led to decreased S/R ratio (Table 1). Therefore, this beneficial effect
of silicon on the sorghum growth might be due to good growth of
root. Moreover, these changes were more distinct in the combina-
tion of silicon fertilization and irrigation. In terms of improvement
in growth under dry conditions, silicon and other elements seem
to do enhancement of dry matter production itself, rather than
through the enhancement of properties responsible for drought
tolerance (Egilla et al., 2001). The positive effect of high silicon
availability in leaves, increased source strength was well correlated
with the drought tolerance and resistance (Table 1). The results
were in line with similar experiments in which source strength
was manipulated by the application of silicon. Silicon containing
products are thought that they could play an active role in plant
protection against diseases (Jian and Yamaji, 2008). We propose

1812 M. Ahmed et al. / Agricultural Water Management 98 (2011) 1808–1812
that the overall effect of higher silicon availability in leaves and
roots, increases water uptake, source strength and provide strength
against diseases.
4. Conclusions
Nutrients like silicon have significant role to build drought tol-
erance in crops. The modification of fertilizer composition with
addition of silicon could improve crop productivity under water
stress. The present research on Si with irrigation boosted crop
stand because it showed synergistic relationship even with lim-
ited water. In conclusion, the present study demonstrated that the
crop growth (leaf, shoot and root), physiology (E and An) and sili-
con accumulation of sorghum were promoted by silicon application
with deficit irrigation especially under water stress. Similarly, it
can be concluded that synergistic effect of silicon fertilization with
ample irrigation may improve the crop stand under drought and
biotic stresses. Further studies to clarify the way in which silicon
enhanced water uptake would help in accelerating the utilization
of Si in sorghum cultivation in arid region.
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