Silicon: A Benefic Element to Improve Tolerance in Plants Exposed to Water Deficiency

In plants this nutrient is assimilated mainly by roots, and capacity to accumulate in tissues is variable [11], being several monocots such as Oryza sativa and Triticum aestivum considered silicon accumulator, with absorption active by root system, and it present leaf levels normal‐ ly higher that 10.0 g kg-1 of Si [12]. In other hand, many dicots like as Phaseolus vulgaris and Glycine max are characterized as not accumulator of silicon, and its presents passive absorp‐ tion, with leaf tenors minors that 5.0 g kg-1 of Si [13].

Drought is one of the key sources of abiotic stress, since it induces smaller growth and development rates, flower aborting, and decreases crop yields during the vegetative, reproductive, and maturation stages [ ]. Usually, drought consequences on plants are studied in controlled/artificial conditions through water restriction.
Vascular plants present several strategies to minimize the negative effects induced by water deficiency, being morphological modifications like increase in root size [ ] and reduction in leaf area [ ]. Other responses are frequently reported, such as reductions in CO assimilation by leaf through stomatal closing, membrane damage and disturbed activity of various enzymes, especially those of CO fixation and adenosine triphosphate ATP synthesis. Enhanced metabolite flux through the photorespiratory pathway increases the oxidative load on the tissues as both processes generate reactive oxygen species. Injury caused by reactive oxygen species to biological macromolecules under drought stress is among the major deterrents to growth.
The stress occasioned by lower water supply to plant is defined as water deficit, being responsible to active responses in plant such as over-expression of genes linked to osmotic ad-justment [ ], modifications in physiological parameters as water potential, stomatal closing [ ], and decrease in photosynthesis rate [ ], besides reduction in cell metabolism with negative consequences on growth and production.
Gas exchanges like as leaf water potential, stomatal conductance, and transpiration rate has been used mainly to explain mechanisms affected during drought [ ]. When the water availability in soil decrease, normally the water potentials of soil and leaf also are reduced, and it will provoke as consequence turgescence loss in plant cells, mainly in leaf, causing stomatal closing and limiting the gas exchanges [ ].
The water potential describes the water amount, in which the water is moved in direction to potentials more negatives, and it differences of potential reveal water flux in soil-plant-atmosphere system [ ]. In general, the leaf water potential is depending to soil water potential, water flux in system, and transpiration rate [ ].
Attenuation of negative effects induced by the silicon application has been frequently described in plants exposed to water deficiency, such as Hattori et al. [ ]

. Objective
Aims of this chapter is to define i silicon and water deficit, to explain ii on silicon sources, uptake system and transporters into plant, and detection form in tissue, and to present iii the silicon action on gas exchanges and photosynthetic pigments in plants exposed to water restriction.

. Silicon sources
The silicon uptake using metal salts of silicic acid normally requires their hydrolysis prior to their uptake. In either case they would affect the ionic balance of the system. The proposed mechanism for the solubilization of silica by PNO or MNO is novel and probably involves polarization of surface silica layer through interaction with the oxygen of the pyridine Noxides. In the solubilization, PNO and MNO are regenerated, as evidenced by the fact that clear water containing freshly prepared PNO/MNO-silica complexes slowly deposits granular silica [ -].
Based in overview related, a research was conduced by Ranganathan et al. [ ] with Oryza sativa plants exposed to pyridine N-oxide PNO , -morpholino pyridine N-oxide MNO , and sodium metasilicate SMS aiming to test silicon sources on their ability to enhance the plant silicon content.
The suggestion that enhanced silica deposition is linked to the ability of the rice plant to combat abiotic and biotic stresses is further supported by SEM and EDX analysis of silicon distribution in the rice leaves. In the absence of added silicon, the leaves exhibited a scattered profile of silicon distribution Figure . The leaves treated with MNO, PNO or SMS showed enhanced silicon content and localization of silicon bodies in leaf bundle sheath cells, particularly in the primary and secondary cell wall. Electron microscopy and in situ X-ray analysis of rice leaves reflect the differences in silicon distribution and cell wall structure between silicon treated and untreated plants [ -]. The SEM pictures show that PNO and MNO enhanced the silica deposition on the leaves of rice plants concomitant with the localization of silicon bodies in leaf bundle sheath cells and in the primary and secondary cell walls [ ].

. Uptake system and transporters linked to silicon
For decades, rice has been known as the most effective silicon-accumulating species, although the mechanisms involved in high silicon uptake are poorly understood. One of the reasons is, unlike other minerals, the genotypic difference in silicon concentration of rice is too small to be utilized for comparative study on silicon uptake by rice A kinetic study showed that the silicon concentration in the root-cell symplast increased with increasing silicon concentration in external solution but saturated at a higher silicon concentration in both lines Figure A . Again, there was no significant difference in the silicon concentration of symplastic solution between the wild type and the mutant. These results suggest that silicon transport from the external solution to the root cortical cells is mediated by a type of transporter and that the transporter of the mutant is identical to that of the wild type [ ].
Other kinetic study on xylem loading of silicon was then conducted in the wild-type and mutant rice. In contrast to the silicon concentration in the root cortical cell symplast, the sili-con concentration in the xylem sap was much higher in the wild type than in the mutant Figure B . In the mutant, the silicon concentration in sap increased gradually with increasing silicon concentration in the external solution without saturation. In the wild-type rice, the silicon concentration in the xylem sap also increased with increasing silicon concentration in the external solution Figure A , but it was saturated at a higher concentration [ ].
The silicon concentration in the xylem sap of the wild type was higher than mM at . mM silicon supply Figure B . This concentration was much higher than that in root-cell symplast Figure A , suggesting that silicon is transported from the root cells to the xylem also against a concentration gradient. The curve of Figure B also suggests that the release of silicon into the xylem is mediated by a type of transporter in the wild type [ ]. Above results suggest that at least two transporters are involved in the silicon uptake by rice roots Figure . One is located on the plasma membrane of root cortical cells SIT , silicon transporter , which transport silicon from external solution to the root cortical cells. The other is located on the plasma membrane of xylem parenchyma cells SIT , silicon transporter , which is responsible for releasing silicon into the xylem. These transporters may have different affinities for silicic acid Figure . Our results also clearly showed that the mutant is defective in xylem loading of silicon rather than transport of silicon from the external solution to the root cell [ ].

. Detection technique
Despite the abundance of studies of the effects of Si fertilizers and electron-microscopic observations of silica depositions within plants [ , -], few of the morphological analysis of silica deposition in rice tissues have used histochemical staining and conventional microscopic techniques available to field researchers.
Although there are a few reports using X-ray scanning analytical microscopy, which described silica deposition detected in a dicot plant such as Arabidopsis halleri [ -], X-ray scan-ning microscopy have been broadly applied for analysis of silica bodies and other elements in rice tissues [ -].
Therefore, study carried out by Isa et al. [ ] optimized a technique for silica body specific staining to visualize silica deposits in rice tissue by bright field microscopy and a technique for determining the in situ content of elements, including silica, by X-ray scanning microscopy. Silica bodies were observed in the motor cells of the leaf blades of cv. Hinohikari at the maximum tillering stage Figure A , and silica opal was also observed in the blades Figure  B . In enlarged images of silica cells located along the vascular bundles of cv. Hinohikari treated with silicic acid for days, accumulation of silica was observed as clear X shapes Figure   Successful staining reactions for observing silica bodies in plant tissues have been developed they involve the use of methyl red, silver amine chromate, and crystal violet lactone [ , ]. In accordance with the reactivity of the silanol groups on the surfaces of the silica bodies, the crystal violet lactone dye stains the silica bodies exclusively, clearly allowing their shapes to be observed [ ].
. Attenuation of negative effects produced by silicon in physiological parameters of plants exposed to water limitation Benefits of silicon actuation recently reported on physiological parameters such as transpiration [ ], stomatal conductance [ ], and photosynthesis [ ] were reported in several specie. In addition, pepper crops, more specifically Capsicum annuum exercises strong influency on Brazilian market, and there is necessacity to investigate silicon action on this crop. A study aiming to respond these questions was organized with five water and silicon combinations control, deficit + . , deficit + . deficit + . , and deficit + . μM Si applied to two cultivars Ikeda and Vermelho Gigante with a total of treatments.
The water deficit promoted a decrease in leaf relative water content in two cultivars, and . , . , and . μM Si did not consistently increase this variable for Ikeda, although it was maintained at levels closer to the deficit. Leaf relative water content in Vermelho Gigante for all Si concentrations was slightly higher than the deficit + μM Si Figure A .
Stomatal conductance was significantly reduced due to water deficit in both cultivars, compared with the control. Ikeda applied with . and . μM Si had higher stomatal conductance compared with deficit + μM Si, while Vermelho Gigante had higher values at . and . μM Si Figure B .
The water deficit caused significant reduction in transpiration in Ikeda and Vermelho Gigante cultivars Figure C . Exogenous application of . , . , and . μM Si promoted attenuation of symptoms induced by water deficit. The treatments with added silicon were not statistically different.
The leaf relative water content of treatments under silicon application was maintained at levels closer to the control treatment, and this is linked to silicon action that was probably absorbed by plant, and deposited mainly in epidermal cell   Figure 6. Leaf relative water content (A), stomatal conductance (B), and transpiration rate (C) in two pepper cultivars treated with silicon and exposed to water deficiency. Means followed by the same letter are not significantly different by the Scott-Knott test at 5% of probability. The bars represent the mean standard error [15].
The Si application attenuated the effects of water deficit, indicating intermediary levels in relation to stomatal conductance, and consequently, possible maintenance in gas exchange. A fall in this parameter will affect directly water relations, limiting the assimilation of car-bon dioxide CO and water flux H O through stomata[ ]. The stomatal mechanism will reduce the CO assimilation, causing a reduction in photo-assimilate production and losses in Therefore, there is limited information linked to silicon action on chlorophyll levels in plants submitted to water deficit. Based in previous results described, Silva et al. [ ] carried out an experiment with Lycopersicon esculentum plants submitted to water deficiency and silicon, being five water and silicon combinations control, deficit + . , deficit + . , deficit + . , and deficit + . μM Si applied to two cultivars Super Marmande and Santa Cruz totalizing treatments. This study had aim of explain silicon action on chlorophylls.
Water deficit promoted reduction not significant in relation to level of chlorophyll a in control treatment for Super Marmande, while Santa Cruz presented significant reduction Figure A . Applications of silicon in treatments deficit + . , deficit + . , and deficit + . μM Si promoted an increase in levels of chlorophyll a to both cultivars.
Cultivars evaluated under water deficit presented a reduction in chlorophyll b Figure B , when compared to control plants. Silicon applications in concentrations of . , . , and . μM Si provoked oscillations in this parameter. In treatments exposed to silicon, the levels of chlorophyll b presented better performances in concentrations of . and . μM Si for Super Marmande and Santa Cruz cultivars, respectively, being these treatments statistically equals to control to both cultivars.
Water deficiency occasioned significant decrease in total chlorophyll evaluated in both cultivars Figure C . Silicon in concentrations of . , . , and . μM Si induced a progressive increase in total chlorophyll. The better result was obtained in treatment exposed to concentration of . μM Si, and this treatment is statistically equal to control.
The reduction in chlorophyll a for both cultivars is a consequence of water restriction, in which will induce probably the production of peroxidative enzymes, and these enzymes are associated to degradation of chlorophyll a in thylakoid membrane [ ]. In cultivars exposed to water deficit, silicon application proportioned increase in levels of chlorophyll a, indicating synthesis of new pigments, and maintenance of chlorophyll a previously existing. Donega [ ] also affirmed that silicon uses promotes improvement in plant architecture and increase in photosynthesis. In other hand, the deposition of silicon in cell wall increased your tissue resistance, and it will keep plants with better performance linked to leaf position and interception of light [ -] Water deficiency occasioned decrease chlorophyll b due to water restriction in substrate. Chlorophyll b was positively affected by silicon application, and this fact is associated to Si accumulation in epidermal cells localized in shoot, in which it will promote indirect protector effect on photosynthetic apparatus, and consequently decrease in damages provoked by water restriction in this parameter. The reduction in chlorophyll b in plants under water deficiency probably is resulting of disorders in chloroplast and modifications in relation proteins/lipids responsible by formation of pigment-protein complex [ -]. Similar results on increase in chlorophyll b were observed by Locarno et al. [ ] studying Rosa spp. plants under silicon application.
Results obtained in total chlorophyll indicate that plants under water deficit and treated with silicon presented increase in this parameter, and a relationship observed data not shows between leaf water potential and total chlorophyll can explain this fact, because adequate water amount in leaf tissue works probably maintaining stability of chloroplasts and consequently the functions carried out by the chlorophylls like as absorption and transport of energy.
In other hand, plants exposed to water deficit presented decrease, being probably linked to decrease in nitrogen absorption, in which is an essential element in formation of chlorophyll molecules. Plants under nitrogen application presented increase in chlorophyll amounts [ ], as well as silicon promotes modifications on nitrogen metabolism [ ]. Water works as vehicle responsible by nitrogen conduction and other nutrients during absorption through root system [ ]. In other hands, during situations of water deficiency can there lower water assimilation, producing a negative interference linked to nitrogen assimilation. Therefore, this fact will generate minor amount of δ-aminolevulinate, which is chlorophyll precursor  , and total chlorophyll (C) in two tomato cultivars treated with silicon and exposed to water deficiency. Means followed by the same letter are not significantly different by the Scott-Knott test at 5% of probability. The bars represent the mean standard error [16].

. Final considerations
This chapter was structured with novel informations that can represent an important source to students, teachers, researchers, scientists and farmers on silicon action linked to attenuation of water deficit in higher plants. It revealed results and concepts on water deficiency and your consequences on plants, as well as silicon utilization with finality to improve tolerance during inadequate water supply. In addition, it presented a simple and efficient technique to carry detection of element beneficial in tissue. Based in recent results, was also demonstrated as silicon is assimilated, transported, and accumulated in several plant organs. Besides to prove positive interference on gas exchanges and photosynthetic pigments in plants exposed to water restriction. [ ] Takahashi E, Ma JF, Miyake Y. The possibility of silicon as an essential element for higher plants. [ ] Parida AK, Dagaonkar VS, Phalak MS, Auramgabadkar LP. Alterations in photosynthetic pigments, protein and osmotic components in cotton genotypes subjected to