© Benaki Phytopathological Institute
Si increases plant resistance to pathogenic fungi
3
ed as one of the most beneficial elements
that increases plant resistance against abi-
otic and biotic stresses. However, the mech-
anisms responsible for alleviating biotic and
abiotic stresses remain unclear because they
may act in the soil, at the root surface and
in
planta
(roots and shoots) (Liang
et al.,
2007;
Van Bockhaven
et al.,
2013).
2.1. Absorption of silicon by plants
Silicon is as important as phosphorus
and magnesium (0.03%) in the biota (Exley,
1998). It is the second most abundant el-
ement on the earth’s crust after oxygen. It
comprises up to 70% of the soil mass in the
form of minerals and water-soluble monosil-
licic acid (H
4
SiO
4
) (Lowenstam, 1981). In soil
solution, silicon occurs mainly as monosili-
cic acid in concentrations between 0.1 and
0.6 mM (Savant
et al
.
,
1997).
Silicon is taken up by plant roots as non-
charged monosilicic acid (Ma and Yamaji,
2006), when pH of the soil solution is below
9 (Ma and Takahashi, 2002). Monosilicic acid
uptake is passive and largely determined by
transpiration rate (Datnoff
et al.,
2007). Once
it reaches a concentration of around 2 mM,
monosilicic acid is polymerized into insol-
uble silica, known as species-specific solid
bodies (phytoliths) (Mitani
et al.,
2005). It is
deposited in cell walls, intercellular spaces
and as a subcuticular layer outside the cells
of leaves (Datnoff
et al.,
2007). Moreover, sil-
icon accumulates in higher amounts in ma-
ture leaves than in young ones (Ma and
Takahashi, 2002). Plants absorb a significant
fraction of dissolved silicon that originates
from litterfall decomposition i.e phytolith
dissolution (Datnoff
et al.,
2007). The con-
centration level of absorbed silicon in plants
ranges from 0.1 to 10% dry weight, depend-
ing on the plant genotype, the concentra-
tion of silicon in soil and the environmental
conditions (Ma and Yamaji, 2006).
2.2. Agronomic importance of silicon in
plant crops
Silicon is reported to increase and en-
hance yield, growth and production of
plants. It improves some morphological
and mechanical characteristics (height, stat-
ure, root penetration into the soil, exposure
of leaves to light, resistance to lodging) in
several plant species. Silicon reduces tran-
spiration and enhances plant resistance to
drought stress, salinity and metal toxicity,
and increases enzyme activity (Datnofft
et
al.,
2007). On the other hand, regarding bi-
otic stresses, the accumulation of silicon in
plant plays an important role in plant de-
fense against insect herbivores. Several her-
bivorus insects suffer adverse effects when
feeding on silica-rich plants (Reynolds
et al.,
2009). Moreover, silicon has been shown to
improve resistance in many plants to various
fungal, viral and bacterial pathogens (Rodri-
ges and Datnoff, 2005; Silva
et al.,
2010; Zell-
ner
et al.,
2011; Van Bockhaven
et al.,
2013).
Most interesting, silicon protects plants
against a multitude of stresses without the
occurrence of resistance trade-offs and/or
growth and yield penalties (Fauteux
et al
.,
2005; Ma and Yamaji, 2006; Epstein, 2009;
Van Bockhaven
et al.,
2013).
2.3. Accumulation of silicon in plant spe-
cies
In the absence of abiotic and/or biot-
ic stresses, silicon was believed to have a
negligible effect on metabolism of healthy
plants, which suggests its nonessential role
(Epstein, 2009). However, silicon nutrition
promoted agronomic yields of unstressed
crops such as rice, as demonstrated by Rodri-
ges and Datnoff (2005). According to Ma and
Yamaji’s (2006) agricultural point of view, sil-
icon uptake in graminaceous plants, such as
wheat, oat, rye, barley, sorghum, maize, and
sugarcane, was much higher than its uptake
in other plant species. One typical example
was rice, which absorbed 150-300 kg Si/ha.
High accumulation of silicon in rice has been
demonstrated to be necessary for healthy
plant growth, and high and stable produc-
tion (Snyder
et al.,
2006). Moreover, grami-
naceous plants absorb silicon at concentra-
tion levels equal to or greater than some of
the essential nutrients like N and K (Savant
et al
., 1997). In rice, for example, silicon ac-
cumulation was about 108% greater than
