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The influence of foliar application of silicon (Si) on chlorophyll contents, chlorophyll fluorescence, and growth of four wheat cultivars differing in drought tolerance (Sirvan and Chamran, as relatively drought tolerant, and Shiraz and Marvdasht, as drought sensitive) was examined under water deficit (100% and 40% F.C.) created in a greenhouse. The results showed that water deficit decreased shoot and root lengths, shoot dry weight, root dry weight, water utilization efficiency, chlorophyll a and b, and chlorophyll stability index. In contrast, foliar application of Si improved plant growth parameters and chlorophyll pigment concentration under water deficit; however, it did not significantly affect wheat growth under control conditions. Limited water supply reduced the values of minimal fluorescence from dark-adapted leaf (F 0 ), maximal fluorescence from dark-adapted leaf (F m ), maximum quantum yield of PSII (F v /F m ), effective quantum yield of PSII (ΦPSII), photochemical quenching (q P ), and apparent photosynthetic electron transport rate (ETR). However, under water deficit, foliar application of Si application increased the earlier mentioned parameters. In contrast, nonphotochemical quenching (q N ) and F 0 /F m increased under water deficit, and application of Si further improved these parameters. Chlorophyll fluorescence analysis suggested that Si alleviated water deficit-induced adverse effects by reducing nonphotochemical quenching, while increasing F v /F m and q P , so that it improved the light use efficiency in the four wheat cultivars under stress. Overall, we concluded that drought-sensitive cultivars (Shiraz and Marvdasht) could resemble resistant cultivars upon foliar application of silicon.

The effects of silicon (Si) and salicylic acid (SA) applications on proline content and expression of Δ 1pyrrolin-5-carboxylate synthetase (P5CS) were examined under different drought levels and different drought exposure times. Two wheat cultivars, a drought tolerant and a drought sensitive were used. The experiment was a factorial based on completely randomized design with three replicates. Expression analysis by the quantitative real time PCR showed that the tolerant cultivar had significantly higher P5CS expressions compared to the sensitive one under drought stress. In sampling time points, the maximum level of mRNA was observed at 48 h after stress was applied. At 48 h after stress induction, the expression of P5CS was almost 3.1 fold higher in the tolerant cultivar compared to the sensitive one. In both cultivars, gene expression decreased from 48 to 72 h. The stressed plants treated with Si + SA showed a higher expression. Proline content started to increase by Si and SA treatments and the maximum proline content was obtained at simultaneous application of Si + SA. Drought stress significantly reduced chlorophyll content, relative water content and leaf water potential of both cultivars, while increased electrolyte leakage (EL) of the leaves. In contrast, foliar-applied Si and SA significantly increased these parameters and reduced EL, and the effect of simultaneous application of Si and SA was greater. The results suggest that the P5CS is a stress inducible gene. This gene has the potential to be used for improvement of drought stress tolerance in wheat. Network analysis highlighted positive interaction of osmotic stress, drought and cold stress on P5CS1 and the regulatory role of MYB2, ERF-1, and EIN3 transcription factors. In conclusion, alleviation of drought stress by application of Si and SA was associated partially with enhanced expression of P5CS gene and following proline accumulation.

The role of exogenous individual or combined application of silicon (Si) and salicylic acid (SA) (control, 6mm Si, 1mm SA, and 6mm Si+1mm SA) on grain yield and some key physiological characteristics of wheat (Triticum aestivum L.) cvv. Shiraz (drought-sensitive) and Sirvan (drought-tolerant) was investigated under field water-stress conditions (100% and 40% field capacity). Drought stress caused a considerable reduction in biological yield, yield and yield components, relative water content and leaf water potential of both cultivars. Application of Si and SA effectively improved these parameters in water-deficit treatments. Moreover, water-limited conditions markedly promoted the activities of key antioxidant enzymes including peroxidase, ascorbate peroxidase, catalase and superoxide dismutase as well as the levels of malondialdehyde (MDA) and hydrogen peroxide (H2O2), while enhancing the accumulation of soluble sugars, potassium, magnesium and calcium in leaf tissues. Application of Si and SA further enhanced the activities of the key antioxidant enzymes and accumulation of osmolytes, and decreased the levels of H2O2 and MDA in drought-stressed plants; the positive effects of Si were greatest when it was applied with SA. Synergistic effects of Si+SA application on yield and physiological parameters were apparent compared with Si or SA applied separately. Water-stress alleviation and yield improvement in the wheat cultivars by Si and SA application was attributable to partly improved osmotic adjustment and antioxidant activity as well as to more favourable water status under stress conditions. Overall, Si and SA application proved to have great potential in promoting grain yield of wheat in drought-prone areas.

A greenhouse investigation was performed to examine the influence of salicylic acid (SA; 0, 0.5, and 1 mM), 24-epi-brassinolide (BR; 0, 0.5, and 0.75 μM), and silicon (Si; 0, 6, and 12 mM) in alleviating arsenic (As) toxicity in wheat under different As levels (0, 50, and 100 μM). Arsenic stress enhanced electrolyte leakage (EL), activities of some critical antioxidant enzymes, and the concentrations of malondialdehyde (MDA) and hydrogen peroxide (H 2 O 2) as well as raised the accumulation of key organic osmolytes in wheat leaves. Indeed, application of Si and also of SA and BR considerably elevated the activities of antioxidant enzymes and the concentrations of osmolytes, and decreased the levels of H 2 O 2 and MDA as well as EL in As-stressed plants. Arsenic toxicity resulted in reduced relative water content (RWC), photosynthetic rate, stomatal conductance, transpiration, and chlorophyll content of wheat. In contrast, Si, SA, and BR improved RWC and gas exchange characteristics under varying As regimes; however, the positive effects of SA and BR were high when applied with Si. The adverse effect of As on wheat was: root > leaf > grain. Under As-stress, supplementation with Si, SA, and BR applied separately or in combination caused a significant reduction in arsenic concentration in wheat, but the effect of Si + SA and especially of Si + BR was greater than that of Si, BR, or SA applied separately. Overall, SA, BR, and Si application proved to have a great potential in improving growth and quality of wheat and reducing As content in different parts of the wheat plant under high levels of As.

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Role of exogenously-applied Si on antioxidant enzyme activities was investigated in wheat under drought stress using a completely randomized factorial design with four replications. Drought stress significantly enhanced activities of ascorbate peroxidase, peroxidase, superoxide dismutase and catalase, and elevated accumulation of osmotically active molecules, soluble sugars and proline. Si application further enhanced activities of enzymes involved in oxidative defense system and accumulation of osmotically active molecules in drought-stressed plants. Under drought stress conditions, water shortage decreased protein content in all cultivars; however, application of Si increased it. Pollen area ratio was lower than 1 for cvs. Shiraz and Marvdasht under drought, but greater than 1 for cvs. Chamran and Sirvan. Water-limited regimes resulted in decreased leaf Ψw in all cultivars, but Si supply was effective in improving Ψw under water-limited regimes. Water shortage increased leaf 2 K, Mg, and Ca concentrations. Under drought stress, Si-treated plants had higher K concentration than the none-treated plants.

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