LEAF APPLICATION OF SILICIC ACID TO WHITE OAT AND WHEAT ( 1 )

Silicon (Si) is beneficial to plants in several aspects, but there are doubts about the effectiveness of leaf application. The purpose of this work was to evaluate the effects of Si, applied in a newly developed stabilized silicic acid form to the leaf, on nutrition and yield of irrigated white oat and wheat. Two experiments were performed (one per crop) in winter 2008, in Botucatu-SP, Brazil. A completely randomized block design with 14 replications was used. Treatments consisted of a control (without Si application) and Si leaf spraying, at a rate of 2.0 L ha-1 of the commercial product containing 0.8 % soluble Si. Silicon rate was divided in three parts, i.e. applications at tillering, floral differentiation and booting stages. Silicon leaf application increased N, P, K, and Si concentrations in white oat flag leaf, resulting in higher shoot dry matter, number of panicles per m2, number of grains per panicle and grain yield increase of 34 %. In wheat, Si leaf application increased K and Si concentrations, shoot dry matter and number of spikes per m2, resulting in a grain yield increase of 26.9 %.


INTRODUCTION
White oat (Avena sativa L.) and wheat (Triticum aestivum L.) are important winter cereal crops in South Central Brazil.In 2010/2011, the total area of these two species was 3.0 million ha, of which 2.1 and 0.9 million ha was cultivated with wheat and oat, respectively, producing 5.9 million tons of grains (CONAB, 2011).
Several studies have shown that silicon (Si) application is beneficial to crops such as rice, sugar cane, barley, maize, sorghum, and wheat (Ma et al., 2001;Gong et al., 2005;Hattori et al., 2005), which are considered Si-accumulating species.Nevertheless, Si is still relatively unknown and applied in agriculture.In many cases, increasing Si availability has increased crop development and yield, once this nutrient can indirectly influence some photosynthetic and biochemical aspects, especially in plants under biotic or abiotic stress conditions (Ma & Yamaji, 2006;Abdalla, 2011).Stress caused by extreme temperatures, drought, heavy or toxic metals, for instance, can be attenuated by Si application (Barbosa Filho et al., 2000;Crusciol et al., 2009;Abdalla, 2011;Prabagar et al., 2011).Si fertilization can also increase plant resistance to several fungal diseases and pests (Goussain et al., 2002;Berni & Prabhu, 2003).
In Brazil, Si is mostly supplied to soil through slug materials with calcium (Ca) and magnesium (Mg) silicates, which are low-water soluble sources (Sousa et al., 2010) and, depending on the origin, can contain heavy metals.Silicon uptake can occur actively, in an energy-spending process, even when roots are exposed to high concentrations of this nutrient (Malavolta, 2006), once plants absorb Si exclusively as monosilicic acid, also known as orthosilicic acid [Si(OH) 4 ] (Elawad & Green Junior, 1979).This monomeric form of silicic acid is found in freshwater and seawater at low concentrations (< 10 -4 mol L -1 ) and can gelatinize into silica gel at high concentrations or when the pH is low (Calomme et al., 2000).In this way, sources containing Si as stabilized silicic acid, which form mainly orthosilicic acid after dilution, can hypothetically be an alternative to increase Si uptake by plants.
In this regard, studies have shown that supplying Si via leaf, in small amounts, can be a viable option, fully providing Si and/or stimulating its absorption, with beneficial effects (Wang & Galleta, 1998;Buck et al., 2008;Reis et al., 2008;Carré-Missio et al., 2010;Figueiredo et al., 2010;Sousa et al., 2010).Thus, leaf fertilization with soluble Si sources seems promising to supply this element to plants, for its effectiveness and feasibility, low doses and because applications can be performed with the sprayers usually used by farmers (Figueiredo et al., 2010).However, there are practically no scientific studies available in the literature on supplying Si via leaf application, as stabilized silicic acid.
The purpose of this work was to evaluate the effects of Si leaf application, in the form of stabilized silicic acid, on nutrition and yield of sprinkle-irrigated white oat and wheat.

MATERIAL AND METHODS
This study consisted of two experiments, one with white oat and the other with wheat, conducted in the winter of 2008, in Botucatu-SP, Brazil (22º 51' S, 48º 26' W, 740 m asl).According to Köppen's classification, R. Bras. Ci. Solo, 36:1538-1544 the climate is Cwa, characterized as tropical altitude, with dry winters and hot wet summers.
The experiment had a completely randomized block design with 14 replications.Treatments consisted of a control (with no Si application) and Si leaf application, at 2.0 L ha -1 of the commercial product (0.8 % soluble Si as stabilized concentrated silicic acid that forms orthosilicic acid [Si(OH) 4 ] and disilicic acid after dilution, 0.15 % Mo and 48 % polyethyleneglycol -PEG400).For both crops, each plot consisted of thirteen 5-m rows of plants, of which the nine central rows were evaluated, disregarding 0.5 m at the row ends.
Both crops were sown on April 24, 2008, in rows spaced 0.17 m apart and 50 seeds m -1 .The cultivars white oat IAC 7 and wheat Coodetec 107 were used.At sowing, both crops were fertilized with 17, 60 and 34.5 kg ha -1 of N, P 2 O 5 and K 2 O, respectively, applied as 04-14-08 NPK formula, according to soil chemical analysis and recommendations of Cantarella et al. (1997).Sowing and fertilization were carried out mechanically, in a no tillage system.Seeds were treated with fungicide carboxin+thiram (50 + 50 g of the active ingredient per 100 kg of seeds).Seedlings emerged eight days after sowing.Side dressing was applied at tillering, with 65 kg ha -1 N as ammonium sulphate.
Silicon was applied at the stages recommended by the fertilizer manufacturer, i.e. three applications of 0.67 L ha -1 , at tillering, floral differentiation and booting, with a CO 2 -pressurized sprayer (volume 200 L ha -1 ).
Plant protection products were applied to control the rose-grain aphid (Metopolophium dirhodum Walker 1849) and leaf rust (Puccinia triticina Erikss.).Crops were irrigated by a sprinkler system, whenever water tension in soil reached -0.06 MPa, in the 0.10m layer, as recommended by Guerra et al. (1994).By these measures, it was possible to isolate the physiological effects of Si nutrition from other influences, such as pests, diseases and water stress.
At flowering, plants were harvested from 2 m of two rows to evaluate dry matter production.Also, the flag leaf of 50 plants per plot was sampled (Cantarella et al., 1997) for macronutrient (N, P, K, Ca, Mg and S) and Si analyses, according to Malavolta et al. (1997) and Korndörfer et al. (2004).
The crops were harvested on September 4, 2008.At this stage, yield components (white oat: number of panicles per m 2 , number of filled spikelets per panicle and 1,000 grain weight; wheat: number of spikes per m 2 , number of grains per spike and 1,000 grain mass) and total yield were determined.
Data were subjected to analysis of variance and means were compared by the t test (LSD) at a probability level of 5 %.

RESULTS AND DISCUSSION
Silicon application increased N, P, K and Si concentrations in the flag leaves of white oat (Table 1).This result can be a consequence of improved root growth, once Si application enhances root structures, as reported by Vlamis & Williams (1967) and Carvalho-Pupatto et al. (2003).Despite the effects of Si treatments, only P, K, Mg and S concentrations were within the ranges considered appropriate by Cantarella et al. (1997), which are 20-30, 2.0-5.0,15-30, 2.5-5.0,1.5-5.0 and 1.5-4.0g kg -1 , respectively.Nitrogen and Ca concentrations were above the optimum ranges.
Although ranges of Si deficiency, sufficiency and excess have not been established for white oat in Brazil so far, both treatments resulted in higher concentrations than those reported in the literature, i.e. 10.9 g kg -1 , considering the total shoots (Korndörfer et al., 2004).This can be explained by the analysis of the flag leaf only, in this experiment.From these results, it was possible to infer that white oat plants were nutritionally well-balanced, being classified as Si-accumulating, with contents above 10 g kg -1 (Ma et al., 2001).
Shoot dry matter was increased by Si leaf application, which may be associated to better plant nutrition (Table 1).Silicon positively influences plant growth and biomass production, especially monocotyledons, as a consequence of improved tissue rigidity, better angle of leaves and light interception, improving photosynthesis (Ma & Yamaji, 2006;Abdalla, 2011;Gong & Chen, 2012).Gong et al. (2003) also found that Si fertilization in soil increased wheat growth, even under appropriate water supply, probably due to growth stimuli.According to Elawad et al. (1982a,b) and Abdalla (2011), Si is involved in cell elongation and division processes as well as in hormone balance.
Evaluations of yield components revealed that Si application increased the number of panicles per area by 22.2 % compared to the control (Table 1).This corroborates findings of Ma et al. (1989), Takahashi (1995) and Artigiani (2008), who also attributed higher number of panicles per area to a better Si nutrition in rice.
The average number of filled spikelets per panicle was also increased by Si leaf application (Table 1), which may be related to higher spikelet fertility.Takatsuka & Makihara (2001) associated low spikelet fertility with low Si concentrations in rice plants.In rice and barley, Ma (2004) reported that the yield component number of filled spikelets per panicle is most influenced by Si.
The mass of 1,000 grains was the only component not affected by the treatments (Table 1).That can be explained by the fact that this yield component is intimately related to genetic characteristics, and is little influenced by other factors (Fornasieri Filho & Fornasieri, 1993).
Silicon application increased grain yield as a consequence of significant changes in yield components compared to the control (Table 1).Yield was increased by 1,783 kg ha -1 , i.e. 29.7 sacks (of 60 kg) per hectare or 34 %, compared to the treatment with no Si application.This result is mainly justified by the higher number of panicles m -2 and filled spikelets per panicle.
Silicon leaf application increased K and Si concentrations in the wheat flag leaves (Table 2).The concentrations of the other nutrients were not influenced, possibly due to higher dry matter production causing a dilution effect despite Si uptake increase, which led to similar contents in both treatments.Studies showed that Si supplied via leaf application, at lower rates, can be a viable alternative to provide this nutrient to plants, with particular beneficial effects, meeting the Si requirements and/ or stimulating Si uptake (Wang & Galleta, 1998;Buck et al., 2008;Reis et al., 2008;Carré-Missio et al., 2010;Figueiredo et al., 2010;Sousa et al., 2010).According to Rosolem (2002), leaf applications at low nutrient rates can increase nutrient concentrations above the initially applied amounts.This management is designated stimulating complementary leaf fertilization, indicated for vigorous, highly productive crops, with no nutritional deficiency.The treatments did not influence the concentrations of all other nutrients.
It is worth mentioning that, although Si application enhanced plant nutrition for the cited nutrients, concentrations were within the range considered appropriate for the wheat crop (Raij et al., 1997), both with and without Si application, as follows: 20-34, 2.1-3.3,15-30, 2.5-10.0,1.5-4.0 and 1.5-3.0g kg -1 , respectively for N, P, K, Ca, Mg and S.Although the ranges of Si sufficiency concentrations in wheat are not determined to date, the contents observed in both treatments can be considered appropriate, according to the range of 5.1 -16.9 g kg -1 established by Korndörfer et al. (2004) for leaves + stems.This shows that wheat is a Si-accumulating crop, similarly to white oat (Ma et al., 2001).
Silicon leaf application increased the wheat shoot dry matter by 47.3 %, probably as a result of better plant nutrition (Table 2).Gong et al. (2003) also reported that Si supplied in soil increased wheat development possibly due to stimulated growth, once Si may be involved in cell elongation and division processes, as well as in the hormone balance (Elawad et al., 1982a,b;Abdalla, 2011).The number of spikes per area was the only yield component affected by Si (Table 2).The other components were not influenced by the treatments.Silicon leaf application increased the number of spikes per area by 10.4 %.Ma et al. (1989), Takahashi (1995) and Artigiani (2008) had also confirmed the effects of Si on the number of rice panicles per area.
Grain yield was significantly increased by Si leaf application compared to the control (without Si application), as a result of the higher photosynthetic area, due to the higher dry matter production and higher number of spikes per m² (Table 2).Silicon leaf application increased yields by 1,377 kg ha -1 , Means followed by the same small letter in the row do not differ significantly by the t test at 5 %.* and ***: significant at 5 and 0.1 %, respectively; ns : not significant.R. Bras. Ci. Solo, 36:1538-1544 i.e. 23 sacks per hectare or 26.9 %, compared to the control.
Studies have demonstrated that Si is involved in a number of structural, physiological and biochemical aspects of the plant cycle, with diverse functions.As pests and diseases were controlled during the experiment and irrigation ensured the water supply, it is believed that Si effects may be correlated to the better crop response at the low temperatures (Liang et al., 2008) that are normal in the winter, or even related to the higher production and accumulation of total sugar and proline (Crusciol et al., 2009;Abdalla, 2011).Silicon is also beneficial to the plant metabolism, since the element can activate genes involved in phenol production and enzyme activity related to defense mechanisms (Ma & Yamaji, 2006;Buck et al., 2008;Abdalla, 2011), resulting in increased crop yields, as observed in both experiments.

CONCLUSIONS
1. Silicon leaf application as stabilized silicic acid efficiently supplied Si for white oat and wheat plants.
2. Leaf-supplied Si increased nutrient concentrations in flag leaves and yield of white oat and wheat cropped under sprinkler irrigation.Means followed by the same small letter in the row do not differ significantly by the t test at 5 %.* and ***: significant at 5 and 0.1 %, respectively; ns : not significant.