Supplementation on Plants Stress Responses

Silicon has been proved to be beneficial for the healthy growth and development of many plant species, particularly graminaceous plants such as

14.1 Drought Tolerance

Optimization of silicon nutrition results in increased mass and volume of roots, giving increased total and adsorbing surfaces (Kudinova 1975). These plants could more efficiently extract water from drying substrate than plants without Si supplementation. Experiment with citrus

(Citrus spp.) has demonstrated that with increasing monosilicic acid concentration in irrigation water, the weight of roots increased more than that of shoots (Matichenkov et al. 1999b). The same effect was observed for Bahia grass (Paspalum notatum Fl├╝gge) (Matichenkov et al. 2000). Greater root-shoot mass ratio provides greater water absorption surface and lower transpiration area leading to a considerable increase in plants tolerance to drought. In the cell walls of xylem elements Si deposition increases resistance of vessels to collapses caused by negative sap pressures particularly under conditions of high transpiration and drought or heat stress. In addition, the silicon-cellulose membrane in epidermal tissue also protects plants against excessive loss of water by cuticular transpiration. This action occurs owing to a reduction in the diameter of stomatal pores and, consequently, a reduction in leaf transpiration (Snyder et al. 2007). In rice plants, Si can alleviate water stress by decreasing transpiration. Rice plants have a thin cuticle and the formation of a cuticle-Si double layer significantly decreases cuticular transpiration. Since water stress causes stomata closure and reduction of photosynthetic rate, Si stimulates the growth and photosynthesis of rice more clearly under water-stresses than nonstressed conditions (Ma et al. 2001). Furthermore, deposition of Si in rice increases the thickness of the culm wall and the size of the vascular bundle preventing lodging. Sterility is related to many factors including excess water loss from the hull. Transpiration from the panicles occurs only from the cuticle of the hull because the hull has no sto-mata. Silicon deposition on the hull decreases the transpiration from panicles by about 30% at either milky or maturity stage, preventing excess water loss. This is the reason why Si application significantly increases the percentage of ripened grain (Ma et al. 2001).

14.2 Tolerance to Flooding, Lodging and Mutual Shading

Si deposition in the epidermal layer of the leaves is thought to be responsible for improvement of exposure to light, increase of resistance to lodging, and reduction of mutual leaf shading particularly in dense stands of cereals (Marschner 1995). Leaf erectness is an important factor affecting light interception in dense plant stands. The effect of Si on leaf erectness is mainly a function of the Si deposition in the epidermal layers of the leaves and, thus, over a wide range closely related to the Si concentration supplied (Balasta et al. 1989). Also in dicotyledonous species, such as cucumber, Si increases the rigidity of mature leaves, which are held more horizontally, increases their Chl content, and delays their senescence (Adatia and Bestford 1986).

14.3 Tolerance to Frost Damage and Cold

Proper silicon nutrition can increase frost resistance by plants (Matichenkov et al. 1999a). However, this mechanism remains poorly understood. Cool summers (low temperature an insufficient sunlight) usually cause serious damage to rice production. Low temperatures decrease Si uptake by rice and in sufficient sunlight lowers the Si:N ratio, which induces blast. Application of Si under such conditions markedly reduces the incidence of blast in rice (Ohyama 1985). The effect of Si is most evident under low light intensity. The Si effect on rice growth under shaded conditions is larger than that without shading but the mechanism responsible for these phenomena is unknown. It has been hypothesized that Si deposited on the leaf epidermal system might act as a window to facilitate the transmission of light into photosynthetic mesophyll tissue (Agarie et al. 1996). However, evidence supporting this hypothesis could not be obtained at this time.

14.4 Resistance Against Pathogens and Pests

Two possible mechanisms of Si-enhanced plant resistance to pathogens have been proposed. In the first mechanism, polymerized Si can reinforce the cell walls by physically inhibiting fungal germ tube penetration of the epidermis, thereby impeding infections (Hayasaka et al. 2008). Polymerization of monosilicic acid into polysili-cic acid and its transform to amorphous silica forms a thickened silicon-cellulose membrane (Aleshin 1988) which can be associated with pectin and Ca ions (Waterkeyn et al. 1982). Such a double-cuticular layer protects plants against attack by fungal pathogens (Yoshida 1975). Silicon forms also some complexes with cell wall components and decreases its sensitivity to enzymes released by the rice blast fungus (Magnaporthe grisea M.E. Barr). Indeed, silicon can be associated with lignin-carbohydrate complexes in the cell wall of rice epidermal cells (Inanaga et al. 1995) . In the second mechanism, soluble form of Si within plants can induce defense response and Si may act locally as a signal in triggering natural defense response in both dicots and monocots, by stimulating the activity of such enzymes as chitinases, PODs, polyphenol oxidase, and/or by increasing the production of phenolic compounds, phytoalexins, antimicrobial compounds and systemic stress signals (salicylic acid, jasmonic acid, and ethylene) (Ghanmi et al. 2004). Silicone has a similar saturable effect and can significantly change the activity of signaling systems in cells after elicitation, including the mitogen activated protein (MAP) kinases (Fauteux et al. 2005). Only the soluble form of Si within plants can induce defense responses, while the polymerized fraction is almost inert. Therefore, Si-induced plant resistance to pathogens vanishes when Si supply to plants is stopped, even though Si had irreversibly accumulated (Fauteux et al. 2005).

14.5 Silicon-Enhanced Tolerance to Salinity

The mitigative effect of Si on salinity has been examined in rice, mesquite, wheat, barley, cucumber and tomato (Liang et al. 2007) . The Na concentration in the shoots of rice and barley was reported to decrease by addition of Si that was attributed to Si-induced reduction in transpiration rate and to the partial blockage of the transpira-tional bypass flow. Reduction in the uptake and root-shoot transport of Na and increase of that for K has been attributed to Si-induced stimulation of the root plasma membrane H+ATPase under salt stress (Liang et al. 2006). Added Si decreased the permeability of the plasma membrane of leaf cells, and significantly improved the ultrastructure of chloroplasts which were damaged by the added NaCl with the double membranes disappearing and the grannae being disintegrated in the absence of Si (Liang et al. 2007). Silicon also increases activity of antioxidant enzymes, decreases the malondialdehyde (MDA) concentration, and suppresses membrane leakage in barley under salt stress (Liang et al. 2003) (Fig. 16.6). Silicone supplementation stimulated root H+ATPase and ffPRase activity in the plasma membranes and tonoplasts and mediated membrane fluidity, suggesting that Si may affect the structure, integrity and functions of plasma membranes by influencing the stress-dependent peroxidation of membrane lipids (Zhu et al. 2004). There are other hypotheses for the ameliorative effect of Si on salt stress include improved photo-synthetic activity, enhanced K/Na selectivity ratio, and increased concentration of soluble substances in the xylem, resulting in limited Na adsorption by plants (Snyder et al. 2007).

14.6 Mineral Stress

Mineral stress can be classified into the deficiency of essential elements and the excess of essential and other elements. Many reports have shown the beneficial effects of Si under mineral stress. In this section, the beneficial effects under P deficiency and excess of N are described.

14.6.1 Phosphorus Deficiency

According to the long term field experiment on rice and barley, the effect of Si on plants yield is larger when P is not supplied. Previously, such beneficial effects of Si were explained as an improvement of P availability in soil. However, later experiment showed that Si does not have any effect in P availability in soil and it seems unlikely that interaction between Si and P occurs in the soil. In a solution culture experiment, no

Fig. 16.6 Effect of Si supplementation (1 mM) on the activity of SOD (Unit mg-1 protein), GR (NADPH absorption g-1 protein) and concentration of GSH (mg g-1 FW) and malondialdehyde (MDA) (nmol g-1 FW), and electrolytic leakage (%) of the roots of barley plants (Hordeum vulgare L. cv. Jian 4) under salt (120 mM) stress (Liang et al. 2003)

significant effect of Si was observed on dry weight of shoot, root, and grain of rice when P was supplied at an adequate level. However, when the P level is decreased, the effects of Si are obvious. Phosphorus uptake is not enhanced by Si in rice, and this implies that Si improves internal P utilization (Ma et al. 2001).

14.6.2 Nitrogen Excess

Application of high levels of N is common for maximum yield in some crop species. Under such cultural conditions, leaf erectness is an important factor affecting light interception. Leaf erectness decreases with increasing N application, but Si application increases leaf erectness, decreasing mutual shading caused by dense planting and high N application. Excess N also increases susceptibility to disease such as blast in rice, but Si decreases the occurrence of blast disease in rice with high N fertilizer applications (Ma et al. 2001).

14.6.3 Mechanisms for Silicon-Mediated Alleviation of Metal Toxicity

Silicon-mediated alleviation of (heavy) metal toxicity in higher plants is widely accepted.

14.6.3.1 Manganese Toxicity

Recently the role of silicon in mitigating Mn tox-icity has been investigated extensively in barley, rice, bean, pumpkin, cowpea and cucumber (for a review see Liang et al. 2007I . Total Mn in the leaves was unaffected by Si but Si caused Mn to be more evenly distributed instead of being

GR GSH MDA Mambrane Leakage

concentrated in discrete necrotic spots. Silicon lowered the apoplastic Mn concentration in cow-pea, suggesting that Si may modify the cation binding capacity of the cell wall (Horst et al. 1999). Iwasaki et al. (2002) found that Si supply alleviates Mn toxicity not only by decreasing the concentration of soluble apoplastic Mn through enhanced adsorption of Mn on the cell walls, but also a role of soluble Si in the apoplast in the detoxicification of apoplastic Mn was indicated. Further research suggested that Si may affect the oxidation process of excess Mn mediated by POD through interaction with phenolic substances in the solution phase of the apoplast, maintaining the apoplast in a reduced state, which is thought to be a requirement for improved Mn tolerance of the leaf tissue. Study of Mn toxicity in cucumber clearly showed that plants not treated with Si had higher Mn concentrations in the intercellular washing fluid compared with plants treated with Si despite approximately the same total Mn content in the leaves. The Mn concentration of the intercellular washing fluid was positively correlated with the severity of Mn-toxicity symptoms and negatively correlated with the Si supply (Rogalla and Romheld 2002). Furthermore, in Si-treated plants less Mn was located in the sym-plast (<10%) and more Mn was bound to the cell wall (>90%) compared with non-Si-treated plants with about 50% in each compartment. Manganese present in Si-treated plants is therefore less available and for this reason less toxic than in plants not treated with Si (Rogalla and Romheld 2002).

Fig. 16.6 Effect of Si supplementation (1 mM) on the activity of SOD (Unit mg-1 protein), GR (NADPH absorption g-1 protein) and concentration of GSH (mg g-1 FW) and malondialdehyde (MDA) (nmol g-1 FW), and electrolytic leakage (%) of the roots of barley plants (Hordeum vulgare L. cv. Jian 4) under salt (120 mM) stress (Liang et al. 2003)

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