Leaf Silica Research Articles

Effectiveness of Bamboo Leaf Silica Applied with Nanosprayer and Growing Media on Yield of Red Beetroot (Beta vurgaris L.)

Silica protects plants from abiotic stress. Planting media provides a place to grow and nutrition for plant growth. The research aims to study the effect of bamboo leaf silica dosage and growing media on the yield of red beets cultivated in the lowlands. The research was designed using a completely randomized factorial design (CRD) with two treatment factors. The first factor is the dose of bamboo leaf silica fertilizer (0 ppm, 0.15 ppm, 0.30 ppm, 0.45 ppm, 0.60 ppm). The second factor is the growing media (soil growing media; soil growing media + goat manure, and soil growing media + goat manure + husk charcoal). Research results: bamboo leaf silica dosage (0.3 ppm, 0.45 ppm and 0.60 ppm) affects the chlorophyll b and total chlorophyll. Growing media soil + goat manure + husk charcoal affects the length of red beet plants, number of leaves, diameter and length of red beetroots, wet weight and dry weight of red beetroots. Combination of growing media treatment (soil + goat manure + husk charcoal) with all doses of bamboo leaf silica increased the chlorophyll a, chlorophyll b and total chlorophyll. The sugar content of red beetroots was not influenced by the dose of bamboo leaf silica or the growing medium. Keywords: Chlorophyll, Husk Charcoal, Manure, Soil.

Modification of The Synthesis of Silica from Litter Ori Bambusa (Bambusa blumeana) Leaves Using Sodium and Potassium With The Hydrothermal Sol-Gel Method As Agricultural Fertilizer

Bamboo plants in Indonesia are familiar plants and have not been utilized optimally. Most people only use the stems for daily needs and a small part for industry, while the leaves are left as waste. Bamboo leaves have a fairly high silica content. Bamboo leaves that have fallen and are scattered on the ground (bamboo leaf litter) are a potential source of natural silica. Silica fertilizer in agriculture is still very necessary, especially in nanosized. Through silica extraction from bamboo leaf litter, it is hoped that it will be able to meet agricultural silica needs and reduce bamboo leaf waste. The aim of this research is to modify the synthesis of silica from ori bamboo leaf litter (Bambusa blumeana) by extracting NaOH and KOH using the hydrothermal sol-gel method to produce nano silica. Nano silica production uses the hydrothermal method because it does not require large costs, is more effective, and has a high level of purity. Synthesis of original bamboo leaf litter silica using the hydrothermal sol-gel method. Bamboo leaf litter ash was analyzed using the XRF test, and silica was analyzed using the SEM-EDX mapping test to determine its morphology and content. The research results showed that synthesis with sodium produced higher levels of bamboo leaf silica compared to potassium. The size of the silica particles obtained is still in the form of micrometers (47–52 µm). Bamboo plants in Indonesia are familiar plants and have not been utilized optimally. Most people only use the stems for daily needs and a small part for industry, while the leaves are left as waste. Bamboo leaves have a fairly high silica content. Bamboo leaves that have fallen and are scattered on the ground (bamboo leaf litter) are a potential source of natural silica. Silica fertilizer in agriculture is still very necessary, especially in nanosized. Through silica extraction from bamboo leaf litter, it is hoped that it will be able to meet agricultural silica needs and reduce bamboo leaf waste. The aim of this research is to modify the synthesis of silica from ori bamboo leaf litter (Bambusa blumeana) by extracting NaOH and KOH using the hydrothermal sol-gel method to produce nano silica. Nano silica production uses the hydrothermal method because it does not require large costs, is more effective, and has a high level of purity. Synthesis of original bamboo leaf litter silica using the hydrothermal sol-gel method. Bamboo leaf litter ash was analyzed using the XRF test, and silica was analyzed using the SEM-EDX mapping test to determine its morphology and content. The research results showed that synthesis with sodium produced higher levels of bamboo leaf silica compared to potassium. The size of the silica particles obtained is still in the form of micrometers (47–52 µm).

Silicon and Epichloë‐endophyte defences in a model temperate grass diminish feeding efficiency and immunity of an insect folivore

Abstract Plants deploy diverse anti‐herbivore defences which reduce feeding and performance of herbivores. Temperate grasses use silicon (Si) accumulation and Epichloë‐endophytes for physical and chemical (i.e. endophytic‐alkaloids) defence against insect herbivores. Recent studies suggest that Epichloë‐endophytes increase Si accumulation in their host grass. It is unknown, however, how this affects Si‐deposition on the leaf surface, their impacts on insect herbivore feeding efficiency and their immunity to potential infection/parasitism. To address this knowledge gap, we grew tall fescue (Festuca arundinacea) hydroponically with and without Si, in the absence or presence of the novel AR584 Epichloë‐strain. We exposed plants to Helicoverpa armigera (Lepidoptera: Noctuidae) in both in situ (intact leaves) and ex situ (excised leaves) feeding trials and determined the effects of Si and endophyte defences on herbivore feeding efficiency, growth rates and immunity against potential infection/parasitism. Endophytic plants supplied with Si showed 110% and 143% increases in leaf silica density and leaf Si concentrations, respectively, when exposed to herbivory, compared to non‐endophytic plants that were herbivore‐free. Despite the endophyte‐mediated increases in Si concentrations, H. armigera was only affected by Si supply; growth rates decreased by 87% and most feeding efficiency indices decreased by at least 30%. Si supply also increased mandibular wear by 16%, which was negatively correlated with H. armigera growth rates. Cellular and humoral immunity of H. armigera were negatively affected by both Si and endophytes. Endophytic‐loline alkaloid concentrations were unaffected by Si supply or herbivory, whereas herbivory increased peramine concentrations by 290%. To our knowledge, this is the first report of Si defences and Epichloë‐endophyte derived alkaloids compromising insect immunity via reduced melanisation response. Using tall fescue and H. armigera, our study suggests that deploying both physical (i.e. Si accumulation) and chemical (i.e. endophytic‐alkaloids) defences acting against multiple insect herbivore traits, including feeding efficiency, growth and immunity, may be a successful defence strategy in temperate grasses. This multi‐faceted defence may be particularly difficult for insect herbivores to overcome. Read the free Plain Language Summary for this article on the Journal blog.

Utilization of bamboo leaf silica as a superhydrophobic coating using trimethylchlorosilane as a surface modification agent

In recent years, there has been an increasing research interest regarding the development of superhydrophobic coating for various purposes with tetraethyl orthosilicate (TEOS) as the source of silica. Unfortunately, TEOS is hazardous as an alkoxide and need long step as well as time consuming to synthesize it. Typically, previous researches using fluoroalkylsilane (FOK) as the surface modification agent is containing fluoro compound which is not safety to use. This paper is aimed to develop a cost-effective and environmental-friendly superhydrophobic coating by utilizing bamboo leaf silica. This work was particularly conducted to develop the method of purification of bamboo leaf ash as the source of silica to produce superhydrophobic coating material. This method could be an effective and efficient way of preparing a superhydrophobic coating using silica bamboo leaf with eco-friendly solvent exchange agents. This observation used n-hexane, cyclohexane, isooctane as a solvent exchange, and trimethylchlorosilane (TMCS) as a surface modification agent which is not containing fluoro component. The initial bamboo leaf silica and purified silica were examined using XRD, BET, XRF, and SEM EDX Mapping to obtain essential information such as structure, porosity, purity, and surface topography. Also, the coated zinc plat with different solvents was assessed hydrophobicity by contact angle measurement and surface morphology by SEM. The efficient and effective formulation of the superhydrophobic coating was attained by applying n-hexane solvent, 13 %-v/v TMCS, 1.75 %-w/v of bamboo leaf silica was the highest of contact angle.

Silica and secondary metabolites as chemophenetic markers for characterization of bamboo species in relation to genetic and morphometric analysis.

Bamboo is a non-timber forest product and one of the most important grass plants of industrial and domestic use. It is widely distributed in tropical countries including India, China and Southeast Asian countries with wide genetic diversity. The diversity in the available genotypes becomes an important resource for the selection and improvement of the plants for ecological and commercial use. This study investigates eight commercially and ecologically important bamboo species of six genera (Bambusa, Dendrocalamus, Thyrsostachys, Vietnamosasa, Cephalostachyum and Indocalamus) from India, Thailand and Laos. These were evaluated for genetic differences by molecular makers, chemo-morphological variation and ability of silicon accumulation. The genetic cluster analyses of eight RAPD primers revealed genetic similarities in the ranges of 24-55%. The total silica content varied from 18.34 to 40.08ppm in leaves of different bamboo species. Chemical analysis of the silicon content by ICP-OES and secondary metabolite profiling on TLC depicted the prominent distinction among the species. The PCA analysis of quantitative morphological data grouped the species in two major clusters and found to correlate with chemical pattern and genetic similarity to some extent. This is the first report that summarizes species-specific variability of leaf silica content, secondary metabolites, and quantitative morphological data towards delineation of genetic phylogeny of bamboo species.

Lineage and latitudinal variation in Phragmites australis tolerance to herbivory: implications for invasion success

Herbivores play a critical role in plant invasions either by facilitating or inhibiting species establishment and spread. However, relatively few studies with invasive plant species have focused on the role of plant tolerance and how it varies geographically to influence invasion success. We conducted a common garden study using two lineages (native and invasive) of the grass Phragmites australis that are prevalent in North American wetlands. Using 31 populations collected across a broad geographic range, we tested five predictions: 1) the invasive lineage is more tolerant to simulated folivory than the native lineage, 2) tolerance to herbivory decreases with increasing latitude of origin of the populations, 3) estimates of tolerance are correlated with putative tolerance traits and plasticity in those traits, 4) a tradeoff exists between tolerance and resistance to herbivory and 5) tolerance has a fitness cost. Response to folivory varied substantially among populations of P. australis, ranging from intolerance to overcompensation. Our model selection procedure deemed lineage to be an important predictor of tolerance but, contrary to our prediction, the native lineage was 19% more tolerant to folivory than the invasive lineage. Tolerance for both lineages exhibited a u‐shaped relationship with latitude. A tolerance–resistance tradeoff was evident within the invasive but not the native lineage. Also, tolerance was positively correlated with belowground biomass allocation, leaf silica concentrations, specific leaf area and plasticity in stem density, and negatively correlated with the relative growth rate of the population and plasticity in putative resistance traits. Lastly, although we did not detect costs of tolerance, our results highlight that fast growth rates can maintain high fitness in the presence of herbivory. Herbivory and plant defense strategies for P. australis lineages in North America exhibit complex biogeographic patterns that cause substantial heterogeneity in enemy release and biotic resistance and, consequently, invasion success.

Effects of leaf length and development stage on the triple oxygen isotope signature of grass leaf water and phytoliths: insights for a proxy of continental atmospheric humidity

Abstract. Continental relative humidity (RH) is a key climate parameter, but there is a lack of quantitative RH proxies suitable for climate model–data comparisons. Recently, a combination of climate chamber and natural transect calibrations have laid the groundwork for examining the robustness of the triple oxygen isotope composition (δ′18O and 17O-excess) of phytoliths, that can preserve in sediments, as a new proxy for past changes in RH. However, it was recommended that besides RH, additional factors that may impact δ′18O and 17O-excess of plant water and phytoliths be examined. Here, the effects of grass leaf length, leaf development stage and day–night alternations are addressed from growth chamber experiments. The triple oxygen isotope compositions of leaf water and phytoliths of the grass species F. arundinacea are analysed. Evolution of the leaf water δ′18O and 17O-excess along the leaf length can be modelled using a string-of-lakes approach to which an unevaporated–evaporated mixing equation must be added. We show that for phytoliths to record this evolution, a kinetic fractionation between leaf water and silica, increasing from the base to the apex, must be assumed. Despite the isotope heterogeneity of leaf water along the leaf length, the bulk leaf phytolith δ′18O and 17O-excess values can be estimated from the Craig and Gordon model and a mean leaf water–phytolith fractionation exponent (λPhyto-LW) of 0.521. In addition to not being leaf length dependent, δ′18O and 17O-excess of grass phytoliths are expected to be impacted only very slightly by the stem vs. leaf biomass ratio. Our experiment additionally shows that because a lot of silica polymerises in grasses when the leaf reaches senescence (58 % of leaf phytoliths in mass), RH prevailing during the start of senescence should be considered in addition to RH prevailing during leaf growth when interpreting the 17O-excess of grass bulk phytoliths. Although under the study conditions 17O-excessPhyto do not vary significantly from constant day to day–night conditions, additional monitoring at low RH conditions should be done before drawing any generalisable conclusions. Overall, this study strengthens the reliability of the 17O-excess of phytoliths to be used as a proxy of RH. If future studies show that the mean value of 0.521 used for the grass leaf water–phytolith fractionation exponent λPhyto-LW is not climate dependent, then grassland leaf water 17O-excess obtained from grassland phytolith 17O-excess would inform on isotope signals of several soil–plant-atmosphere processes.

Statistical Approach for Water Glass Precursor Preparation from Bamboo Leaf Silica

The bamboo leaf consists of high silica content. In this study, the bamboo silica was utilized as water glass. Water glass can also be used as eco-friendly precursors for the preparation of silica aerogel. The extraction process of silica from bamboo leaf have been carried out through acid leaching and combustion process at 750°C based on thermal gravimetry analysis. This paper is aimed to assess the effect of two types of alkali hydroxide and concentrations producing water glass of bamboo leaf silica. The result was analyzed by using a 22-factorial design with six replications. The alkali hydroxide types were sodium hydroxide and potassium hydroxide with a concentration of 4 M and 6 M, respectively. The study was executed in several steps, i.e. pre-treatment of bamboo leaf, leaching process, combustion, purification of bamboo leaf silica and synthesis of water glass. The optimum condition of water glass products was obtained at silica concentration of 41.08 ppm and density of 1.23 g/L for potassium hydroxide, while at 40.15 ppm and 1.30 g/L for sodium hydroxide. ANOVA analysis resulted in a significant effect for type of hydroxide and concentration releasing model for silica concentration response. This study also characterized the silica structure and composition by instrumentation analysis for silica solid obtained from optimum water glass condition with potassium hydroxide. It can be denoted the physical behavior of silica solid from water glass by potassium hydroxide did not differ from previous research of water glass by sodium hydroxide.

Silicon Conversion From Bamboo Leaf Silica By Magnesiothermic Reduction for Development of Li-ion Baterry Anode

Silicon (Si) is a promising alternative material for the anode Lithium ion Battery (LIB). Si has a large theoretical capacity about 3579 mA hg-1, ten times greater than the commercial graphite anode (372 mA hg-1). Bamboo is a source of organic silica (bio-silica). Most part biogenetic content of SiO2 is obtained in bamboo leaves. This paper aims to investigate the synthesis nano Si from bamboo leaves through magnesiothermic reduction after silica extraction using sol–gel method and to observe nano Si of bamboo leaf as mixed material for lithium ion baterry. Silica and silicon content was determined using XRF. Silica product has 96,3 wt. % yield of extraction from bamboo leaf, while silicon yield was obtained 61.2 wt. %. The XRD pattern revealed that silica and silicon product were amourphous. The extracted silica and silicon from bambo leaf has spherical shape and agglomerated form. As anoda material for LIB, silicon product achieved 0,002 mAh capacity for 22 cycle.

Mechanism of silica deposition in sorghum silica cells.

Grasses take up silicic acid from soil and deposit it in their leaves as solid silica. This mineral, comprising 1-10% of the grass dry weight, improves plants' tolerance to various stresses. The mechanisms promoting stress tolerance are mostly unknown, and even the mineralization process is poorly understood. To study leaf mineralization in sorghum (Sorghum bicolor), we followed silica deposition in epidermal silica cells by insitu charring and air-scanning electron microscopy. Our findings were correlated to the viability of silica cells tested by fluorescein diacetate staining. We compared our results to a sorghum mutant defective in root uptake of silicic acid. We showed that the leaf silicification in these plants is intact by detecting normal mineralization in leaves exposed to silicic acid. Silica cells were viable while condensing silicic acid into silica. The controlled mineral deposition was independent of water evapotranspiration. Fluorescence recovery after photobleaching suggested that the forming mineral conformed to the cellulosic cell wall, leaving the cytoplasm well connected to neighboring cells. As the silicified wall thickened, the functional cytoplasm shrunk into a very small space. These results imply that leaf silica deposition is an active, physiologically regulated process as opposed to a simple precipitation.

Early silicification of leaves and roots of seedlings of a panicoid grass grown under different conditions: anatomical relations and structural role.

Grasses accumulate high amounts of silica deposits in tissues of all their organs, especially at mature stage. However, when and under which conditions do grass seedlings begin to produce these silica deposits and their relation with anatomy and development is little known. Here we investigated the silicification process in the first leaves and roots of seedlings of Bothriochloa laguroides grown in different substrate and Si treatments. The distribution and content of silica deposits in the organs of the seedlings grown under different conditions were analyzed through staining techniques and SEM-EDAX analyses. Leaf silica deposits were accumulated 3-4 days after the first leaf emergence, also under low silica solution (0.17-0.2mM). Their location was mainly restricted to short costal cells from basal sectors, and scarcely in trichomes and xylem at tips. Silica content in leaves increased with the age of the seedlings. Roots presented dome-shaped silica aggregates, between 4-12μm of diameter, located in the inner tangential wall of endodermal cells and similar to those produced at maturity. Silicification begins early in the first photosynthetic leaf, and silica distribution is opposite to that found in mature plants, mainly restricted to basal sectors, probably acting as a reinforcing element. The fast incorporation of solid amorphous silica in leaves and roots, may be useful for farm applications in species that are Si-fertilized.

Leaf silica concentration in Serengeti grasses increases with watering but not clipping: insights from a common garden study and literature review.

Grasses (Poaceae) lack the complex biochemical pathways and structural defenses employed by other plant families; instead they deposit microscopic silica (SiO2) granules in their leaf blades (i.e., phytoliths) as a putative defense strategy. Silica accumulation in grasses has generally been considered an inducible defense; other research suggests silica accumulation occurs by passive diffusion and should therefore be closely coupled with whole plant transpiration. We tested the hypothesis that grasses increase leaf silica concentration in response to artificial defoliation in a common garden study in the Serengeti ecosystem of East Africa. Additionally, a watering treatment tested the alternative hypothesis that leaf silica was largely driven by plant water status. Leaf silica content of two dominant C4 Serengeti grass species, Themeda triandra and Digitaria macroblephara, was quantified after a 10-month clipping × water experiment in which defoliation occurred approximately every 2 months and supplementary water was added every 2 weeks. Themeda had greater silica content than Digitaria, and Themeda also varied in foliar silica content according to collection site. Clipping had no significant effect on leaf silica in either species and watering significantly increased silica content of the dominant tall grass species, Themeda, but not the lawn species, Digitaria. Our data, and those collected as part of a supplementary literature review, suggest that silicon induction responses are contingent upon a combination of plant identity (i.e., species, genotype, life history limitations) and environmental factors (i.e., precipitation, soil nutrients, grazing intensity). Specifically, we propose that an interaction between plant functional type and water balance plays an especially important role in determining silica uptake and accumulation.

From bamboo leaf to aerogel: Preparation of water glass as a precursor

In this study, water glass was synthesized from bamboo leaf as a precursor to produce silica aerogel. Bamboo leaf was combusted to produce bamboo leaf silica (BLS) and reacted with sodium hydroxide to form a water glass solution. The effects of the processing parameters such as the temperature, the time, and the agitation speed on the silica yield in water glass were studied. These processing parameters were optimized based on the regressed correlation to synthesize water glass. It was found that bamboo leaf contains approximately 20wt.% of silica, which is higher than the silica content in rice husk. Characterizations of BLS confirmed that it has identical purity, amorphicity and chemical nature as rice husk silica. Optimization study also showed that BLS can be completely dissolved in NaOH in at least 2h at 30°C and an agitation speed of 20rpm, depending on the combination of the parameters.

More than herbivory: levels of silica‐based defences in grasses vary with plant species, genotype and location

Silica defences in grasses have recently been suggested to be a potential driver of vole population dynamics. However, the ability of grasses to induce silica in response to herbivory has not been tested in northern ecosystems where small rodents are important herbivores.We conducted a large‐scale field experiment in subarctic tundra using three river catchments differing in herbivore densities, and examined the effects of small rodent and/or reindeer exclusion on leaf silica levels in five grass species (Avenella flexuosa, Anthoxanthum nipponicum, Calamagrostis phragmitoides, Deschampsia cespitosa and Phleum alpinum). We also conducted a greenhouse experiment using three of these species (A. flexuosa, A. nipponicum and D. cespitosa) and Festuca ovina to determine whether intraspecific genotypic variation affects baseline silica concentrations and the capacity to induce silica in response to simulated grazing. Baseline leaf silica concentrations and silica induction varied with plant species in both experiments, with catchment in the field experiment and with genotype in the greenhouse experiment. These findings show that the allocation to silica defences in grasses is highly variable, and suggest that the combined effects of grazing pressure, plant species and intraspecific genotypic differences are likely to determine the circumstances under which silica induction may be an optimal defence strategy. A better understanding of the interplay between grazing and other factors influencing silica induction is necessary to interpret the role of silica in plant–herbivore interactions.

Tradeoffs between foliar silicon and carbon‐based defences: evidence from vegetation communities of contrasting soil types

Silica is ubiquitous in plants and can constitute up to 10% of plant dry mass, varying with phylogeny and soil silicon availability. Plant silicon is an important alleviator of abiotic (salinity, heavy metal, drought) and biotic (herbivore and fungal pathogen) stress. As well as playing an important role in reducing the impact of abiotic stresses, silicon may be an alternative to carbon‐based and other chemical defences. Knowledge of silicon function is predominantly derived from agricultural species and model systems. We investigated the abundance and role of plant silicon at a community level by comparing leaf silicon concentration with defence chemicals, carbon compound concentrations and invertebrate assemblages in vegetation communities from two different soil types with contrasting levels of plant available silicon. We found that the concentrations of silicon in the leaves did not reflect the silicon availability in the soil at a community level. The leaf silica concentration range in the vegetation communities was comparable to other diverse communities reported in the literature, suggesting that the species rather than the environment determine leaf silica concentration. Across sites, leaf silica concentration was significantly negatively correlated with concentrations of carbon, total phenols and weakly with tannins but not with other measured defence compounds. Leaf silica concentration was also negatively correlated with Coleoptera abundance, but not the abundance of any other invertebrate groups measured. Our results suggest that tradeoffs exist between phenolic‐ and tannin‐based defences and provide evidence that leaf silicification may be a more effective defence against some chewing herbivore groups than others.

Carbon bio‐sequestration within the phytoliths of economic bamboo species

AbstractThe rates of carbon bio‐sequestration within silica phytoliths of the leaf litter of 10 economically important bamboo species indicates that (a) there is considerable variation in the content of carbon occluded within the phytoliths (PhytOC) of the leaves between different bamboo species, (b) this variation does not appear to be directly related to the quantity of silica in the plant but rather the efficiency of carbon encapsulation by the silica. The PhytOC content of the species under the experimental conditions ranged from 1.6% to 4% of the leaf silica weight. The potential phytolith carbon bio‐sequestration rates in the leaf‐litter component for the bamboos ranged up to 0.7 tonnes of carbon dioxide (CO2) equivalents (t‐e‐CO2) ha−1 yr−1 for these species. Assuming a median phytolith carbon bio‐sequestration yield of 0.36 t‐e‐CO2 ha−1 yr−1, the global potential for bio‐sequestration via phytolith carbon (from bamboo and/or other similar grass crops) is estimated to be ∼1.5 billion t‐e‐CO2 yr−1, equivalent to 11% of the current increase in atmospheric CO2. The data indicate that the management of vegetation such as bamboo forests to maximize the production of PhytOC has the potential to result in considerable quantities of securely bio‐sequestered carbon.

The origins and diversification of C4 grasses and savanna‐adapted ungulates

AbstractC4 grasses constitute the main component of savannas and are pervasive in other dry tropical ecosystems where they serve as the main diet for grazing animals. Among potential factors driving C4 evolution of grasses, the interaction between grasses and grazers has not been investigated. To evaluate if increased grazing pressure may have selected for higher leaf silica production as the grasses diverged, we reconstructed the phylogeny of all 800 genera of the grass family with both molecular (combined multiplastid DNA regions) and morphological characters. Using molecular clocks, we also calculated the age and number of origins of C4 clades and found that shifts from C3 to C4 photosynthesis occurred at least 12 times starting 30.9 million years ago and found evidence that the most severe drop in atmospheric carbon dioxide in the late Oligocene (between 33 and 30 million years ago) matches the first origin of C4 photosynthesis in Chloridoideae. By combining fossil and phylogenetic data for ungulates and implementing a randomization procedure, our results showed that the appearance of C4 grass clades and ungulate adaptations to C4‐dominated habitats match significantly in time. An increase of leaf epidermal density of silica bodies was found to correspond to postulated shifts in diversification rates in the late Miocene [24 significant shifts in diversification (P<0.05) were detected between 23 and 3.7 million years ago]. For aristidoid and chloridoid grasses, increased grazing pressure may have selected for a higher leaf epidermal silica production in the late Miocene.

Impacts of silica-based defences in grasses on the feeding preferences of sheep

Grasses, which dominate many terrestrial ecosystems, sustain high densities of grazing mammals, so are of great economic and ecological importance. Traditionally, grasses are thought to be adapted to tolerate grazing rather than defend against it; however, silica deposited in the leaves of grasses has recently been shown to act as a feeding deterrent to invertebrate herbivores and small mammals. This study assesses whether silica is effective as a feeding deterrent to larger mammalian herbivores. We assess the impact of manipulated silica levels in five grass species on the feeding preferences of sheep both within and between grass species. Sheep feeding behaviour was driven by between-species differences in palatability. Hence, within a single species silica addition did not cause significant changes in feeding preference. However, there were significant differences in both the feeding preferences and bite rates between grass species, and these differences were much more marked when the grasses had been exposed to high levels of silica. The impacts that silica had on preference were least pronounced in palatable species (e.g. Poa annua) compared with less-palatable species (e.g. Brachypodium pinnatum and Festuca ovina). Sheep fed for longer, took more bites and had a higher bite rate on the grass species with the lowest leaf silica concentrations, namely P. annua. Sheep were less affected by silica defences than smaller, non-ruminant herbivores, but the changes in species preference rankings caused by silica suggest it may lead to changes in sward composition. Further, in species that are already relatively low in palatability, silica-induced reductions in bite rate could potentially reduce forage intake rates, with consequences for sheep performance.

Grasses and the resource availability hypothesis: the importance of silica‐based defences

The resource availability hypothesis (RAH) predicts that allocation of resources to anti-herbivore defences differs between species according to their growth rate. We tested this hypothesis by assessing the growth and defence investment strategies of 18 grass species and comparing them against vole feeding preferences. In addition, we assessed the effectiveness of silica, the primary defence in many grasses, in influencing vole feeding behaviour. Across species, we found that there was a strong negative relationship between the overall investment in defence and growth rate, thus supporting predictions of the RAH. However, no such relationship was found when assessing the various individual anti-herbivore defences, suggesting that different grass species show significant variation in their relative investment in strategies such as phenolic concentration, silica concentration and leaf toughness. Silica was the most influential defensive factor in determining vole feeding preference. Experimentally induced increases in leaf silica concentration deterred vole feeding in three of the five species tested, and altered feeding preference ranks between species. The strong positive relationship between silica concentration and leaf abrasiveness, when assessed both within and between species, suggests that increased abrasiveness is the mechanism by which silica deters feeding. Although grasses are often considered to be tolerant of herbivore damage rather then defended against it, they do follow predictions of defence allocation strategy based on their growth rates, and this affects the feeding behaviour of generalist grass-feeding herbivores.

Boosting silica levels in wheat leaves reduces grazing by rabbits

Systemic application of sodium silicate can significantly enhance the levels of leaf silica in winter wheat (Triticum aestivum L. cv. Mercia), suggesting that this material could reduce the palatability of plants to vertebrate herbivores (e.g. rabbits, Oryctolagus cuniculus L.). A bioassay was developed using hydroponically grown wheat plants. Plants treated with sodium silicate were significantly more resistant to grazing by wild rabbits than untreated plants, with severe, potentially lethal feeding damage being reduced by over 50%. Further studies were carried out to develop more practical techniques for boosting silica levels in plants using silicon-rich 'fertilisers' including calcium silicate and calcium silicate slag (CSS). Silica levels were elevated in the plant 1.9-2.8 times over the control through the application of various silicon materials, in line with those of the hydroponic treatment. Encouragingly, levels of silica were elevated even in young wheat plants, which are most vulnerable to rabbit damage, and in a range of wheat varieties. The use of CSS is particularly promising because of its lower cost in comparison with calcium silicate, and it has a proven track record in slag fertilisation of rice and sugar cane crops. At the optimum CSS application rate of 3 g silicon L(-1) soil, wheat silica levels were approximately doubled, with no detrimental impacts on long-term growth or yield.