Barley (Hordeum vulgare L.) is the fourth most produced cereal crop globally and is cultivated in temperate climate regions. Nearly 140 million tonnes per year are produced worldwide, which are principally used as animal feed (70%) and for beer production (27%) [1]. Barley is susceptible to a wide array of harmful fungal diseases. Fusarium Head Blight (FHB) is an economically important disease of barley and other small grain cereals (i.e., wheat, oat, rye, and triticale) [2]. FHB reduces yield and impairs grain quality particularly due to the accumulation of Dangerous Mycotoxins (DON), which are harmful to human and animal health. Consequently, the contamination by DON makes barley-harvested kernels unacceptable for the malting and brewing industry [3]. Numerous Fusarium species differing in their predominance and mycotoxin spectra have been associated with FHB disease. By far, the most prevalent species are F. graminearum and F. culmorum, found in all barley-growing areas [4, 5]. During warm and wet conditions, FHB fungi can penetrate the rachis and spread via direct floret-floret contamination at anthesis. Disease symptoms are recognized by necrotic patches, bleaching of the florets, and discoloured kernels (tan, orange, brown, pink or red) scattered throughout the head [6].

The development and deployment of resistant barley cultivars are the simplest and the most effective approach in integrated disease management for decreasing the negative effects of FHB [7]. Two primary categories of polygenic resistance determined quantitatively by several Quantitative Trait Loci (QTLs) in barley to disease infection are generally recognized as Type I (resistant to initial penetration of the pathogen) and Type II (resistant to fungal spread within a spike), with Type I as the predominant type [8]. However, the lack of FHB resistant barley cultivars makes it difficult to achieve complete control of disease due to (1) potential presence of fungal inoculum on crop residues, such as ascospores, macroconidia, chlamydospores, and hyphal fragments, (2) possible persistence of favorable environmental conditions during the flowering stage for FHB infection, (3) complex inheritance of QTLs resistance and (4) significant cultivar-by-environment interaction effects [6-8]. Given this evidence, new strategies in the context of integrated disease management need to be developed to diminish losses due to the FHB pathogen complex.

Silicon (Si) has been well-documented to play a vital role in enhancing growth, development, and yield for a wide array of field crops, particularly under various abiotic (i.e., nutrient imbalance, salinity, metal toxicity, water deficit, waterlogging, radiation damage, UV, and temperature extremes) and biotic (i.e., pathogens and insect pests) stresses [9-14]. Silicon is also regarded as an environment-friendly compound in relation to soil, fertilizers, and plant nutrition [15]. Several mechanisms have been suggested to explore the enhanced fungal resistance in plants by silicon. Firstly, the polymerization of silicon beneath the cuticle and in the cell walls increases the physical barrier to fungal pathogens. Secondly, silicon plays a metabolic function in the plant–fungal interaction by enhancing the activities of plant defensive enzymes, leading to increased accumulation of defensive compounds, such as phenolics and phytoalexins, and in turn enhancing plant resistance to fungal pathogens. Thirdly, silicon can induce systemic resistance in plants [11, 13, 14, 16]. Thus, the decrease in disease symptom expression is due to the effect of silicon on some components of plant resistance [11].

Barley is a silicon-accumulator monocot having more than 1% dry weight; its absorption capacity from the soil is recorded in the range of 50-150 kg Si/ha [15]. However, there is little information about the potential positive effects of silicon on the resistance of barley to fungal pathogens, except against powdery mildew (Blumeria graminis) [17]. The decrease in the powdery mildew disease was due to the fact that Kunoh and Ishizaki study focused on barley leaf tissue [17]. The effect of silicon deposition on leaves might have been stronger against foliar pathogens because silicon played both: physical and biochemical roles [11, 13, 14, 16]. Until recently, no report is available in the literature examining the effectiveness of silicon in reducing the development of FHB fungi on barley. However, in the wheat–FHB association, Yobo et al. found that granulated and foliar potassium silicate has a limited potential to decrease F. graminearum severity on day 12, after pathogen inoculation as compared to the pathogen-inoculated control treatment in bread (Triticum aestivum L.) plants [18].

There is a need to understand how silicon affects the two components of barley resistance, Type I and Type II, in growth chambers where all biotic and abiotic conditions are strictly controlled. In order to develop a deeper understanding of the pivotal role of silicon in decreasing the negative disease effects in the barley–FHB system, here we aimed to (1) investigate the effect of soluble silicon applied to moderately resistant and susceptible barley plants via incorporation into the soil and via foliar spraying on the reduction of bleaching of spikes (disease incidence) and spikelets (disease severity) caused by four FHB species, i.e., F. culmorum, F. solani, F. verticillioides and F. equiseti in a growth chamber, and (2) propose the potential resistance mechanisms related to the reduction of the bleaching of spikes and spikelets.

Despite the urgent need to find alternative methods for genetic resistance for decreasing FHB infections on barley, no single strategy has yielded successful control [7]. There is a paucity of reports concerning the application of silicon as a protective measure for fungal pathogens in barley [17], and it has been proven only in the barley-Blumeria graminis pathosysem. In the case of head blight on barley, no study has investigated the potential impact of silicon. From a pathogenic point of view, we have, therefore, attempted to fill this gap by providing novel information on how silicon affects the two common quantitative components on moderately resistant “AS, MR” and susceptible “AB, S” barley, bleaching of spikes was used as an indicator of Type I resistance and bleaching of spikelets was used as a representative of Type II resistance, to four FHB pathogens. AS and AB were selected because they are currently the most important barley cultivars in Syria. AS is adapted to drier areas and is popular in northeast Syria. AB is adapted and primarily planted in the wetter areas in western and northwestern Syria. Both genetically different cultivars are two-rowed with thin stems and high tillering ability [19-22]. For reasons still debated, it appears that silicon is particularly effective against biotrophic fungi [11, 13, 14, 16]. However, the use of silicon in this work has shown novel and important observations that could be exploited to fend off four hemibiotrophic Fusarium fungi, i.e., F. culmorum, F. solani, F. verticillioides, and F. equiseti. This brings further concrete evidence that silicon inputs could be a valuable tool in integrated management against FHB agents by reducing the disease development on barley.

In this investigation, three proposed mechanisms could elucidate the efficient decrease in the incidence (DI), and severity (DS) of FHB disease conferred by foliar and root silicon applications. Firstly, the hypothesis of fungicidal impact suggests that silicon directly affects FHB pathogens due to the fact that foliar sprays were continued during the period ranging from 7 to 28 dpi in which the disease evaluations were conducted, thus silicon was present on barley heads at the periods of primary infection and pathogen invasion. However, this mechanism was doubted by novel findings showing that silicon did not hamper mycelial growth of the four FHB species used at any of the tested concentrations (1.67, 3.33, 5.00, or 6.67 mM soluble silicon). Being non-fungicidal, it is in agreement with other results [unpublished data]. The second and third postulated mechanisms are through improved overall mechanical strength and an outer protective layer at infection sites in barley heads and the priming of the plants for enhanced production of antifungal compounds [18]. These hypotheses are established on (1) FHB pathogens penetrate glumes, palea, and rachilla directly by constructing invasive mycelia which extend internally throughout vascular bundles in the spikelet [25, 26], and (2) it is unlikely that FHB species can invade the external surfaces of the floret due to the fact that they have strongly thickened lignified walls [25]. The second mechanism, silicon-enhanced cell wall fortification in bracts of inflorescence (silicon is present in glumes, palea, and rachilla [27] acts as a physical barrier against fungal infestation. However, this hypothesis can explain to a limited extent the lowering of FHB damage because 90% of absorbed silicon is approximately located in the epidermis cells of the barley leaves and their cell walls [28]. The third mechanism, germ tubes which successfully invaded the epidermis in barley heads, were controlled by the production of defense-related enzymes as well as the higher accumulation of antifungal components, reinforced by the existence of soluble silicon in the intracellular spaces inside the cells and in the cell wall, as well as in conducting vessels [11, 13]. Barley is known as a silicon absorber and accumulator plant [15]. This last hypothesis seems to be the best admitted and functional mechanism to reduce FHB bleaching of spikes and spikelets [18]. However, biochemical analyses are needed to explore which kind of antifungal compounds is related to the reduction of FHB symptoms on barley plants feed with silicon.

The two components quantified in the present work were negatively affected by silicon. The finding that there was a decrease in bleaching of spikes and spikelets is of great pathogenic value for two reasons. Firstly, the possible mycotoxins released by the four analyzed FHB pathogens, which may have had their ability to efficiently diffuse within the head, decreased. The damage to the cells was avoided due to the silicon deposition in the epidermis cells of the barley leaves, and their cell walls may stimulate the host to produce defense-related enzymes and antifungal compounds. It is known that the damage caused by mycotoxins released by the four tested Fusarium species causing head blight is due to the loss of chloroplast pigment and browning of the barley head [25]. Secondly, bleaching of heads by FHB species may be a consequence of synthesized DON or the clogging of phloem and xylem by the pathogens [25, 29]. In addition, mycotoxins are diffusible, i.e., they move into barley tissues that are not colonized by FHB species [30]. The clogging of phloem and xylem vessels perturbs the movement of nutrients and water to un-colonized tissues, resulting in the premature death of spikelets [25]. The decrease in the bleaching of spikes and spikelets in silicon-applied plants indirectly suggests that although the fungi still gain full access to plant tissue, host colonization, diffusion of DON, and clogging of phloem and xylem vessels can be affected by the action of certain mechanisms of resistance.

During the initial infection stage, occurring up to 7 dpi, FHB fungi infection is established by forming several infection features, such as the foot structure, infection cushion, infection hyphae, lobate appressorium, papillae silica cell, and runner hyphae or necrotic lesions surrounding them [31]. This initial infection stage is associated with other virulent factors, apart from trichothecene synthesis, such as secretion of hydrolytic enzymes [31]. Taking into account that no effects of silicon were observed on FHB incidence (DI) and severity (DS) during the initial infection stage, it seems that potential silicon mediated responses need more time to stimulate as observed during the infection of the heads after day 14 until 28 or virulent factors associated with this earlier stage do not stimulate plant defense mediated with silicon. Our results suppose that silicon triggers defense processes in barley plants, acting as an elicitor, in the latest infection stages to diminish DI and DS with a diversity depending on FHB species byaffecting trichothecene synthesis and other virulent factors in fungi [31]. Therefore, the potential enhancement in the activity of these antifungal elements indicates the synthesis of plant defense compounds against external agents. It is widely accepted that plant’s defense system is activated by the previous infestation of fungi, silicon fertilization, or both [11, 13, 14, 16]. In contrast to our findings, Yobo et al. reported that mycotoxins production impaired protein synthesis in heads, and this would have suppressed the ability of wheat plants to resist progressive infection via delaying plant disease responses in the latest infection stages rated at 22 and 28 dpi [18]. It is important to measure toxin characteristics directly, and their variations to the four tested Fusarium species in the presence and absence of silicon.

There is a direct link between the ability of a plant to absorb silicon and the benefits derived from it [15]. During our research, the content of silicon in leaf tissue seemed to be quite appropriate based on the innate physiological capability of this plant species to absorb this element from the soil solution and the potential uptake by leaves to negatively influence FHB fungi growth. As barley control plants were not fed with silicon, it can be suggested that variations in silicon content delivered to barley plants via root and foliar applications accounted for differences in the level of disease response observed in this study. At 28 dpi, solid and foliar treatments reduced the bleaching of spikes (DI) by 26.6% and 22.9%, respectively, on “MR” barley and by 19.4% and 19.5%, respectively, on “S” barley and decreased the bleaching of spikelets (DS) by 20.4% and 19.5%, respectively, on “MR” plants and by 18.8% and 18.4%, respectively, on “S” plants. Our results agree with those found by Yobo et al. [18]; they showed that a reduction of FHB DS in the presence of potassium silicate was found. The differences in silicon rates provided by Yobo et al. to wheat and applied herein to barley may be attributable to the contrasting isolates and host cultivars used in this study and previous work [18]. Our data revealed that “MR” and “S” barley plants treated with the highest (3.00 g and 3.4 ppm) and the lowest (0.5 g and 0.8 ppm) tested rates delivered through root and foliar applications performed similarly to the control plants. The lack of significantly lower silicon treatments may relate to insufficient accumulation of silicon. Our data have been substantiated by Dogramaci et al. through their study on Chilli thrips Scirtothrips dorsalis populations on pepper plants [32]. Regarding higher silicon doses, the level of soluble silicon inside the plant probably exceeded the critical concentrations at which silicic acid polymerizes; here, the polymerized form of silicon is no longer physiologically active [33]. The content of silicon in leaf tissues in barley plants treated with solid formulation and foliar sprays will be investigated in further studies that will aim to explore the role of silicon in reducing FHB symptoms. However, previous studies have found that the increased use of silicon enhanced host resistance to fungal diseases [34].

It is mainly accepted that silicon-mediated resistances were stimulated when plants were grown in silicon-enriched soil [11, 13]. More importantly, our data confirmed that barley plants treated with a solid source of silicon (1.50 g) suffered similarly lower levels of FHB disease as compared with those treated with foliar spray (1.7 ppm). Our findings are somewhat in accordance with Yobo’s et al.'s study, which reported that granulated potassium silicate gave slightly better control of F. graminearum than the liquid one [18]. Thus, this observation is of great importance because of the potentially considerable quantities of silicon accumulated inside leaf tissues from foliar applications. However, no report has ever affirmed the efficient absorption of silicon in plant tissues following foliar sprays [35]. For the beneficial effects to manifest, it is commonly accepted that silicon should be absorbed by plant roots in the form of silicic acid, along with water through which it follows the transpiration stream to finally polymerize into insoluble silica, known as species-specific solid bodies (phytoliths) silica [15]. There is a significant body of literature showing that decrease of disease damage through foliar silicon treatment is the result of a direct effect on the pathogen rather than one alleviated by the plant. This was reviewed in Wang et al. [13] and Sakr [16].

The silicon effect seemed to be species-specific at 21 and 28 dpi. The four FHB species utilized in the current work are known as capable of mycotoxin production [6]. Thus, the ability of silicon to reduce bleaching of spikes and spikelets in varying amounts on barley proposes that Fusarium species differ in their abilities to resist potential silicon mediated responses. In addition, it seems that silicon accumulates much more in barley plants infected with F. culmorum followed by F. equiseti, and the least with F. solani and F. verticillioides. Such pathogen specificity has also been shown for certain fungal and viral infections [36, 37].

The present work showed that silicon significantly decreased DI and DS on both “MR” and “S” barley plants. Our results propose that silicon participates actively in enhancing the basal resistance of AS and AB to Fusarium species infection. However, the effects of the higher resistance in AS were pronounced under controlled conditions, significantly reducing DI and DS compared to AB, regardless of FHB species and silicon applications at 14, 21, and 28 dpi. We would point out that, generally, DI and DS were lower in AS plants treated with silicon than AB plants treated with silicon. Our findings are in accordance with those reported by Pazdiora et al. [38], who studied the same effect for wheat cultivars infected with Pyrenophora tritici-repentis, the causative agent of the tan spot. The greatest control of tan spot was obtained with the moderately resistant cultivar grown in soil amended with calcium silicate [38]. Intrinsic differences were observed between the two tested cultivars regarding Type I resistance; moderately resistant plants responded to silicon applications better than susceptible ones, irrespective of FHB species and silicon treatments at 14, 21, and 28 dpi. It seems that head infection is more applicable to distinguish specific responses between barley plants with different degrees of quantitative resistance than spikelet infection determined for Type II resistance. Field experiments are in progress in our station to confirm the efficacy of soluble silicon in reducing FHB disease on AS and AB.

To summarize, methods used to protect barley plants against FHB infection focus principally on using genetic quantitative resistance. In the current work, the use of silicon inputs has highlighted novel and important phenomena that might be exploited in efforts to fend off FHB species. Our data provide the first evidence that root and foliar application of silicon can decrease both FHB DI and DS with a diversity depending of FHB species on “MR” and “S” barley cultivars. It is noteworthy that there exists an interesting observation within barley plants in terms of silicon absorption conferred by foliar spray. Whether this absorption is attributed to the presence of silicon transporters remains to be elucidated. The application of silicon would be an ideal environment-friendly policy and a valuable choice that may be used in an integrated disease management strategy, especially when completely FHB resistant barley cultivars are not available. Enhanced resistance occurred in barley plants with different levels of resistance to FHB infection.