Effects capable of breaking up soil and moving

Effects
of Silicon on Sorghum

Saad
Ur Rehman:C-037, Department of Agronomy, The Islamia University Bahawalpur

Abstract:

Silicon
Si is an agronomically important fertilizer element that enhances plant
tolerance to abiotic stresses. Sorghum is
an important crop in the world, which is often grown in areas of relatively low
rainfall, high temperatures and saline soils. This
paper provides a historical review of the literature on the effects Silicon on
different functions and physiology of Sorghum. The review covers the effects of Si
application on oxidative damage, root cell walls, transpiration rate, drought
stress and photosynthesis reactions of Sorghum.

Keywords:

silicon,
sorghum, root cell walls, transpiration rate, drought stress, salinity stress,
photosynthesis, si deposition

 

Introduction:

Sorghum
(Sorghum bicolor L. Moench) originates in the tropics and is cultivated
in various regions of the world. Sorghum is the fifth most significant cereal
crop worldwide in both area and metric tons harvested (FAO,2009).Sorghum is
mainly used as a lower-cost alternative to corn in feed rations. It is also an
important biomass producer in no-till farming and crop rotations (Landau and
Guimarães, 2010). Sorghum has a dense root system capable of breaking up soil
and moving nutrients through different soil layers. It is a salt and
aluminum-tolerant crop, making areas suitable for crop-growing, which would
otherwise be considered marginal for agriculture (Prasad et al., 2007;
Vasilakologlou et al., 2011). Besides, sorghum is also an important crop for
ethanol production due to the high sugar levels in its stems (Zhao et al., 2009;
Ratnavathi et al., 2011; Han et al., 2012; Zegada-Lizarazu and Monti, 2012).

Silicon
is the second most abundant mineral element in the soil after oxygen and
comprises 31 % of the earth’s crust, and is also a major constituent of many
plants (Epstein1999; Gong et al. 2006). Although silicon is not generally considered
to be an essential element for the majority of plants, its uptake has been
widely found to be beneficial in improving the biotic and abiotic stress
tolerance (Lianget al. 2007; Ma and Yamaji 2008; Epstein 2009); for instance,
alleviating heavy metal stress (Neumann and Nieden 2001), increasing tolerance
to salt and drought(Hattori et al. 2005; Kafi and Rahimi 2011), and improving the
resistance to pests and pathogens (Fauteux Het al. 2006).The Si content of the plant varies greatly
in different plant species, ranging from 0.1 to 10.0 % of dry weight (Ma etal.,
2006).In the soil solution, Si occurs mainly as mono silicic acid (O4SiH4) at concentrations
ranging from 0.1 to 0.6 mM and is taken up by plant in this form (Ma and
Takahashi, 2002). After the uptake, Si accumulates on the epidermis of various tissues
mainly as a polymer of hydrated amorphous silHica(Ma, 2004).The beneficial
effects of silicon on plant growth are particularly distinct under stress
conditions; however, we will discuss effect of Si under different conditions on
sorghum physiology.

      
I.           
Deposition of
Si in sorghum root endodermis

There
is a group of plants that deposits Si into endodermal cell walls. Silicon
impregnation of the root endodermis is the significant characteristic of  sorghum (Sangster & Parry, 1976a,b,c;
Hodson , 1989b, 1993; Lux et al., 2002), The function of
endodermal Si impregnation is the mechanical strengthening of walls, which
makes them effective as a barrier against pathogens and parasites (El Hiveris,
1978).A role in plant water relations has also been shown; drought resistant
sorghum cultivars have considerably higher amounts of Si deposited in the
endodermis than nonresistant cultivars(Lux et al., 2002). The addition
of Si to the soil improves the growth of sorghum under drought conditions
(Hattoriet al.,2001).Sorghum, together with other species of the tribe
Andropogoneae(family Poaceae), is unique because solid silicon, in the form of regularly-distributed,
dome-shaped Si aggregates is deposited on the inner tangential endodermal
walls. These structures were first described in the Andropogoneae by Borissow
(1924, 1925, 1928), and called ‘RasdorskysKörpchen’ (Rasdorsky bodies). Silicon
deposition into root endodermal cell walls has a clear basipetal direction in
both sorghum (Sangster & Parry, 1976a) and rice (Luxet al., 1999)
with low content in apical parts and increasing content towards the base of
the root. Silica aggregates in the sorghum root endodermis start to form after
the onset of secondary wall thickening, and aggregates are incorporated into
the wall matrix (Sangster & Parry, 1976c).The uptake of Si by silicon
accumulators is fast and considered to exceed the transpiration stream, rather
than being dependent on it. The specific transporters of Si, found in diatoms
(Hildebrand et at., 1998), have not been found in higher plants.
However, the isolation of rice mutants defective in active Si uptake indicates
the existence of Si transporters inthe roots of higher plants (Tamai et al.,
2002).

    
II.           
Effects of
Silicon application on oxidative damage of sorghum

Sorghum
is moderately tolerant to salinity and can grow well in saline soils (Maas et
al., 1986).However, at higher levels of salinity, considerable reduction in
its growth takes place, therefore, improvement of its salinity tolerance by any
means is a great challenge for plant scientists. Although a variety of strategies
are currently in vogue to counteract the salinity problem, application of Si
considered as one of the convenient and cost-effective approaches of overcoming
the salinity menace. MOHAMMAD KAFI et al., 2011 shows that Si may alleviate
salt stress ins orghum by increasing antioxidants activity. Supplied1.44
g.kg-1soil Si, caused increased activity of APX, CAT,SOD, PRO, GR, total
antioxidant and total phenol concentration, while, 1.92 g.kg-1soil Si
application caused increase in MSI, soluble sugars and total phenol concentration,
CAT, SOD and total antioxidant activity. It seems that Si increased antioxidant
activity and inhibited the lipid peroxidation of cell plasma membranes to maintain
integrity in high levels of Na+ concentration inthe cytoplasm but osmotic
stress occurred in plant cells .Despite increased antioxidant activity at 1.44
g.kg-1soil Si, growth and dry matter accumulation of salt-stressed sorghum
plants was not improved by this amount of Si application.

 
III.           
Effect on
root cell walls of Sorghum

It
has been reported that silicon is deposited on the inner tangentiall walls
(ITW) of root endodermal tissue in rice, sorghum and other gramineous species
(Parry and Kelso 1975, Sangster and Parry 1976a). Silicon is deposited on the
endodermal ITW in roots in an amorphous form and is called “silicon deposition”
or “silicon aggregation”. This deposition is distinguished from gel-form
silicon accumulated in the shoot. Silicon deposition in sorghum roots has been
well investigated anatomically by Parry and Kelso (1975), Sangster and Parry
(1976a), Sangster and Parry (1976b), Sangster and Parry (1976c), Hodson  and Sangster (1989a), Hodson and Sangster
(1989b), and Hodsonand Sangster (1993). In sorghum, the mechanical
strengthening of root endodermal cell walls by silicon deposition is documented
as being related to resistance to invasion by root parasites(Maiti et al.
1984). Lux et al. (1999) found, by anatomical analysis, that the intensity of
silicification in the roots of rice was higher in an upland rice cultivar than
in a lowland rice cultivar. Lux et al. (2002) further demonstrated that the
drought tolerant sorghum cultivar accumulated more silicon in roots than a
drought-susceptible sorghum cultivar. They suggested that silicon deposition
might be related to drought tolerance through the increase of resistance to radial
water leakage of roots or to protection of stele tissues from mechanical damage
caused by drying soil.

 
IV.           
Effect on
transpiration rate of Sorghum

Silicon
application in rice led to a decrease in transpiration via the formation of a
cuticle –silica double layer, maintaining a high leaf water potential(Yoshida
1965, Matoh et al. 1991). Although there was a possibility that water loss from
the cuticle might also have decreased with silicon application in sorghum
plants, its effect on the total transpiration rate would have been quite small
compared with that in rice. According to Matohet al. (1991), the contribution
of cuticular transpiration was about 25–39% of the total transpiration in rice,
whereas it was only 3–9% in sorghum (Hattori, T. et al.2004, unpublished data).
The decrease in water loss from the cuticle would be masked by the increased
transpiration from the stomata caused by silicon application.

   
V.           
Effect of Si
under drought Stress

Silicon
application may also be effective in enhancing the drought tolerance of plants.
Under water stress conditions, such as soil drying and high water demand from the
atmosphere, silicon-applied cereal crops have been reported to be able to
retain a higher leaf water potential than crops grown without silicon application
(Yoshida1965, Matoh et al. 1991, Agarie et al. 1998). The formation of a silica
– cuticle double layer on leaf epidermal tissue has been considered to be
responsible (Yoshida 1965,Matoh et al. 1991). The active reaction of stomata to
atmospheric humidity in rice (Agarie et al. 1998) and the decrease in the
specific leaf area in wheat (Gong et al. 2003)have also been suggested to be
involved in the inhibition of leaf water deficit. However, these data are
insufficient to clarify completely the mechanism of improvement indrought
tolerance caused by silicon application. As past studies have focused attention
mainly on the prevention of excess water loss, as mentioned above, the effects
of silicon application on water uptake ability, which cannot be ignored in a
discussion of drought tolerance, remain unknown. Multivariate statistical
analysis showed that treatments water stress-free, regardless of silicon dose
applied, presented higher nitrate levels and lower carbohydrate, proline and
sucrose levels in both leaves and roots. Differences were observed in the
amount of biochemical compounds in sorghum roots and leaves, and this quantity
also varied according to soil water-stress conditions. Silicon application in
sorghum plants mitigates the negative effects of drought stress, favoring this
crop cultivation in areas of low water availability. Therefore, the application
of this compound is highly recommended, especially in regions undergoing dry
conditions.(Luma Castro de Souza et al,.2015)The studies show water use
efficiency was significantly affected by silicon application among sorghum
cultivars. It can be concluded that silicon application can enhance growth and
development of sorghum and it can be recommended as supplemental fertilizer to
enhance drought tolerance. PARC SS-2 has been noted as the drought tolerant
sorghum genotype and it must be used to develop future new potential drought
combating cultivars.(Mukhtar et al., 2011)

 
VI.           
Effect on
photosynthesis of Sorghum

The
beneficial effects of Si application are more significant when plants were
grown under stressful environments. For example, dry matter in the highest-Si treatment
was increased at 10.5 and 23.1 dS m-1compares with 5.2 dS m-1 which is in
accordance to Lianget al. (2006). Review by Liang et al. (2007) showed the positive
effects of Si on mitigating salinity in rice, mesquite, wheat, barley, cucumber
and tomato in recent investigations. There was a positive significant
correlation(p<0.001) between photosynthesis and transpiration obtained and transpiration rate was increased but not significantly at high level of Si application (Table 11).Matoh et al. (1986) reported that Si-induced reduction in transpiration rate and partial blockage of the transpiration bypass flow. Therefore, Na concentration in the shoots of plant was decreased by reduction in transpiration rate but there were contradictory reports (Savant et al., 1997).Netondo et al. (2004a) reported that chlorophyll concentration of the leaves of sorghum grown at high NaCl concentrations reduced. The reduction in dry matter in saline condition might be through inhibition of current photo assimilation because salinity reduces the contents of photosynthetic pigments. However, the highest level of Si application increased Cha and Chb concentrations. Conclusion Studies show that in salt stress condition supply of Si could improve photochemical efficiency ofPSII by increased chlorophyll content, limiting the transpiration rate and detoxifying ROS by accumulation of silicon in leaves (Mohsenzadeh, et al., 2011; Alaghabaryet al., 2004).