Effects of various tissues mainly as a

Effectsof Silicon on SorghumSaadUr Rehman:C-037, Department of Agronomy, The Islamia University BahawalpurAbstract:SiliconSi is an agronomically important fertilizer element that enhances planttolerance to abiotic stresses.

Sorghum isan important crop in the world, which is often grown in areas of relatively lowrainfall, high temperatures and saline soils. Thispaper provides a historical review of the literature on the effects Silicon ondifferent functions and physiology of Sorghum. The review covers the effects of Siapplication on oxidative damage, root cell walls, transpiration rate, droughtstress 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 cultivatedin various regions of the world.

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Sorghum is the fifth most significant cerealcrop worldwide in both area and metric tons harvested (FAO,2009).Sorghum ismainly used as a lower-cost alternative to corn in feed rations. It is also animportant biomass producer in no-till farming and crop rotations (Landau andGuimarães, 2010). Sorghum has a dense root system capable of breaking up soiland moving nutrients through different soil layers. It is a salt andaluminum-tolerant crop, making areas suitable for crop-growing, which wouldotherwise be considered marginal for agriculture (Prasad et al., 2007;Vasilakologlou et al., 2011). Besides, sorghum is also an important crop forethanol 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).

Siliconis the second most abundant mineral element in the soil after oxygen andcomprises 31 % of the earth’s crust, and is also a major constituent of manyplants (Epstein1999; Gong et al. 2006). Although silicon is not generally consideredto be an essential element for the majority of plants, its uptake has beenwidely found to be beneficial in improving the biotic and abiotic stresstolerance (Lianget al. 2007; Ma and Yamaji 2008; Epstein 2009); for instance,alleviating heavy metal stress (Neumann and Nieden 2001), increasing toleranceto salt and drought(Hattori et al. 2005; Kafi and Rahimi 2011), and improving theresistance to pests and pathogens (Fauteux Het al. 2006).

The Si content of the plant varies greatlyin 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 concentrationsranging from 0.1 to 0.6 mM and is taken up by plant in this form (Ma andTakahashi, 2002). After the uptake, Si accumulates on the epidermis of various tissuesmainly as a polymer of hydrated amorphous silHica(Ma, 2004).

The beneficialeffects of silicon on plant growth are particularly distinct under stressconditions; however, we will discuss effect of Si under different conditions onsorghum physiology.      I.           Deposition ofSi in sorghum root endodermisThereis a group of plants that deposits Si into endodermal cell walls. Siliconimpregnation of the root endodermis is the significant characteristic of  sorghum (Sangster & Parry, 1976a,b,c;Hodson , 1989b, 1993; Lux et al., 2002), The function ofendodermal Si impregnation is the mechanical strengthening of walls, whichmakes them effective as a barrier against pathogens and parasites (El Hiveris,1978).

A role in plant water relations has also been shown; drought resistantsorghum cultivars have considerably higher amounts of Si deposited in theendodermis than nonresistant cultivars(Lux et al., 2002). The additionof Si to the soil improves the growth of sorghum under drought conditions(Hattoriet al.,2001).Sorghum, together with other species of the tribeAndropogoneae(family Poaceae), is unique because solid silicon, in the form of regularly-distributed,dome-shaped Si aggregates is deposited on the inner tangential endodermalwalls. These structures were first described in the Andropogoneae by Borissow(1924, 1925, 1928), and called ‘RasdorskysKörpchen’ (Rasdorsky bodies). Silicondeposition into root endodermal cell walls has a clear basipetal direction inboth sorghum (Sangster & Parry, 1976a) and rice (Luxet al.

, 1999)with low content in apical parts and increasing content towards the base ofthe root. Silica aggregates in the sorghum root endodermis start to form afterthe onset of secondary wall thickening, and aggregates are incorporated intothe wall matrix (Sangster & Parry, 1976c).The uptake of Si by siliconaccumulators is fast and considered to exceed the transpiration stream, ratherthan 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 indicatesthe existence of Si transporters inthe roots of higher plants (Tamai et al.,2002).

    II.           Effects ofSilicon application on oxidative damage of sorghumSorghumis moderately tolerant to salinity and can grow well in saline soils (Maas etal., 1986).However, at higher levels of salinity, considerable reduction inits growth takes place, therefore, improvement of its salinity tolerance by anymeans is a great challenge for plant scientists. Although a variety of strategiesare currently in vogue to counteract the salinity problem, application of Siconsidered as one of the convenient and cost-effective approaches of overcomingthe salinity menace. MOHAMMAD KAFI et al., 2011 shows that Si may alleviatesalt stress ins orghum by increasing antioxidants activity. Supplied1.

44g.kg-1soil Si, caused increased activity of APX, CAT,SOD, PRO, GR, totalantioxidant and total phenol concentration, while, 1.92 g.kg-1soil Siapplication caused increase in MSI, soluble sugars and total phenol concentration,CAT, SOD and total antioxidant activity. It seems that Si increased antioxidantactivity and inhibited the lipid peroxidation of cell plasma membranes to maintainintegrity in high levels of Na+ concentration inthe cytoplasm but osmoticstress occurred in plant cells .Despite increased antioxidant activity at 1.

44g.kg-1soil Si, growth and dry matter accumulation of salt-stressed sorghumplants was not improved by this amount of Si application. III.           Effect onroot cell walls of SorghumIthas 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 theendodermal ITW in roots in an amorphous form and is called “silicon deposition”or “silicon aggregation”.

This deposition is distinguished from gel-formsilicon accumulated in the shoot. Silicon deposition in sorghum roots has beenwell 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 mechanicalstrengthening of root endodermal cell walls by silicon deposition is documentedas being related to resistance to invasion by root parasites(Maiti et al.1984). Lux et al.

(1999) found, by anatomical analysis, that the intensity ofsilicification in the roots of rice was higher in an upland rice cultivar thanin a lowland rice cultivar. Lux et al. (2002) further demonstrated that thedrought tolerant sorghum cultivar accumulated more silicon in roots than adrought-susceptible sorghum cultivar.

They suggested that silicon depositionmight be related to drought tolerance through the increase of resistance to radialwater leakage of roots or to protection of stele tissues from mechanical damagecaused by drying soil.  IV.           Effect ontranspiration rate of SorghumSiliconapplication in rice led to a decrease in transpiration via the formation of acuticle –silica double layer, maintaining a high leaf water potential(Yoshida1965, Matoh et al. 1991). Although there was a possibility that water loss fromthe cuticle might also have decreased with silicon application in sorghumplants, its effect on the total transpiration rate would have been quite smallcompared with that in rice. According to Matohet al. (1991), the contributionof 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 increasedtranspiration from the stomata caused by silicon application.   V.           Effect of Siunder drought StressSiliconapplication may also be effective in enhancing the drought tolerance of plants.Under water stress conditions, such as soil drying and high water demand from theatmosphere, silicon-applied cereal crops have been reported to be able toretain 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 beresponsible (Yoshida 1965,Matoh et al. 1991). The active reaction of stomata toatmospheric humidity in rice (Agarie et al. 1998) and the decrease in thespecific leaf area in wheat (Gong et al. 2003)have also been suggested to beinvolved in the inhibition of leaf water deficit.

However, these data areinsufficient to clarify completely the mechanism of improvement indroughttolerance caused by silicon application. As past studies have focused attentionmainly on the prevention of excess water loss, as mentioned above, the effectsof silicon application on water uptake ability, which cannot be ignored in adiscussion of drought tolerance, remain unknown. Multivariate statisticalanalysis showed that treatments water stress-free, regardless of silicon doseapplied, presented higher nitrate levels and lower carbohydrate, proline andsucrose levels in both leaves and roots. Differences were observed in theamount of biochemical compounds in sorghum roots and leaves, and this quantityalso varied according to soil water-stress conditions. Silicon application insorghum plants mitigates the negative effects of drought stress, favoring thiscrop cultivation in areas of low water availability.

Therefore, the applicationof this compound is highly recommended, especially in regions undergoing dryconditions.(Luma Castro de Souza et al,.2015)The studies show water useefficiency was significantly affected by silicon application among sorghumcultivars. It can be concluded that silicon application can enhance growth anddevelopment of sorghum and it can be recommended as supplemental fertilizer toenhance drought tolerance.

PARC SS-2 has been noted as the drought tolerantsorghum genotype and it must be used to develop future new potential droughtcombating cultivars.(Mukhtar et al., 2011) VI.

           Effect onphotosynthesis of SorghumThebeneficial effects of Si application are more significant when plants weregrown under stressful environments. For example, dry matter in the highest-Si treatmentwas increased at 10.5 and 23.1 dS m-1compares with 5.2 dS m-1 which is inaccordance to Lianget al. (2006). Review by Liang et al. (2007) showed the positiveeffects of Si on mitigating salinity in rice, mesquite, wheat, barley, cucumberand tomato in recent investigations.

There was a positive significantcorrelation(p<0.001) between photosynthesis and transpiration obtained andtranspiration rate was increased but not significantly at high level of Siapplication (Table 11).Matoh et al.

(1986) reported that Si-induced reductionin transpiration rate and partial blockage of the transpiration bypass flow.Therefore, Na concentration in the shoots of plant was decreased by reductionin transpiration rate but there were contradictory reports (Savant et al.,1997).Netondo et al. (2004a) reported that chlorophyll concentration of theleaves of sorghum grown at high NaCl concentrations reduced. The reduction indry matter in saline condition might be through inhibition of current photoassimilation because salinity reduces the contents of photosynthetic pigments.However, the highest level of Si application increased Cha and Chbconcentrations.

Conclusion Studies show that in salt stress condition supply ofSi could improve photochemical efficiency ofPSII by increased chlorophyllcontent, limiting the transpiration rate and detoxifying ROS by accumulation ofsilicon in leaves (Mohsenzadeh, et al., 2011; Alaghabaryet al., 2004). 

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