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

Functions of silicon and phytolith in higher plants

Silicon (Si) is abundant in the lithosphere, and previous studies have confirmed that silicon plays an important role in plant growth. Higher plants absorb soluble silicon from soil through roots which is deposited in plant tissues mainly in the form of phytoliths. Based on previous studies, the research progress in silicon and phytoliths in the structural protection, enhancement on photosynthesis and transpiration of plants and plant growth and stress resistance was reviewed. Meanwhile, gaps in phytolith research, including phytolith morphology and function, impact of diverse environmental factors coupling with phytoliths, phytolith characteristics at different stages of plant development and phytoliths in regional vegetation are identified. The paper intends to promote the wider application of phytolith research findings and provides reference for further research on phytoliths.

Silicon-based anti-herbivore defense in tropical tree seedlings

Silicon-based defenses deter insect herbivores in many cultivated and wild grass species. Furthermore, in some of these species, silicon (Si) uptake and defense can be induced by herbivory. Tropical trees also take up Si and leaf Si concentrations vary greatly across and within species. As herbivory is a major driver of seedling mortality and niche differentiation of tropical tree species, understanding anti-herbivore defenses is pivotal. Yet, whether silicon is a constitutive and inducible herbivory defense in tropical forest tree species remains unknown. We grew seedlings of eight tropical tree species in a full factorial experiment, including two levels of plant-available soil Si concentrations (-Si/+Si) and a simulated herbivory treatment (-H/+H). The simulated herbivory treatment was a combination of clipping and application of methyl jasmonate. We then carried out multiple-choice feeding trials, separately for each tree species, in which leaves of each treatment combination were offered to a generalist caterpillar (Spodoptera frugiperda). Leaf damage was assessed. Three species showed a significant decrease in leaf damage under high compared to low Si conditions (by up to 72%), consistent with our expectation of Si-based defenses acting in tropical tree species. In one species, leaf damage was increased by increasing soil Si and in four species, no effect of soil Si on leaf damage was observed. Opposite to our expectation of Si uptake and defense being inducible by herbivory damage, simulated herbivory increased leaf damage in two species. Furthermore, simulated herbivory reduced Si concentrations in one species. Our results showed that tropical tree seedlings can be better defended when growing in Si-rich compared to Si-poor soils, and that the effects of Si on plant defense vary strongly across species. Furthermore, Si-based defenses may not be inducible in tropical tree species. Overall, constitutive Si-based defense should be considered part of the vast array of anti-herbivore defenses of tropical tree species. Our finding that Si-based defenses are highly species-specific combined with the fact that herbivory is a major driver of mortality in tropical tree seedling, suggests that variation in soil Si concentrations may have pervasive consequences for regeneration and performance across tropical tree species.

Why do plants silicify?

Despite seminal papers that stress the significance of silicon (Si) in plant biology and ecology, most studies focus on manipulations of Si supply and mitigation of stresses. The ecological significance of Si varies with different levels of biological organization, and remains hard to capture. We show that the costs of Si accumulation are greater than is currently acknowledged, and discuss potential links between Si and fitness components (growth, survival, reproduction), environment, and ecosystem functioning. We suggest that Si is more important in trait-based ecology than is currently recognized. Si potentially plays a significant role in many aspects of plant ecology, but knowledge gaps prevent us from understanding its possible contribution to the success of some clades and the expansion of specific biomes.

Climatic Drivers of Silicon Accumulation in a Model Grass Operate in Low- but Not High-Silicon Soils

Grasses are hyper-accumulators of silicon (Si), which is known to alleviate diverse environmental stresses, prompting speculation that Si accumulation evolved in response to unfavourable climatic conditions, including seasonally arid environments. We conducted a common garden experiment using 57 accessions of the model grass Brachypodium distachyon, sourced from different Mediterranean locations, to test relationships between Si accumulation and 19 bioclimatic variables. Plants were grown in soil with either low or high (Si supplemented) levels of bioavailable Si. Si accumulation was negatively correlated with temperature variables (annual mean diurnal temperature range, temperature seasonality, annual temperature range) and precipitation seasonality. Si accumulation was positively correlated with precipitation variables (annual precipitation, precipitation of the driest month and quarter, and precipitation of the warmest quarter). These relationships, however, were only observed in low-Si soils and not in Si-supplemented soils. Our hypothesis that accessions of B. distachyon from seasonally arid conditions have higher Si accumulation was not supported. On the contrary, higher temperatures and lower precipitation regimes were associated with lower Si accumulation. These relationships were decoupled in high-Si soils. These exploratory results suggest that geographical origin and prevailing climatic conditions may play a role in predicting patterns of Si accumulation in grasses.

Field application of silicon alleviates drought stress and improves water use efficiency in wheat

Detrimental impacts of drought on crop yield have tripled in the last 50 years with climate models predicting that the frequency of such droughts will intensify in the future. Silicon (Si) accumulation, especially in Poaceae crops such as wheat (Triticum aestivum L.), may alleviate the adverse impacts of drought. We have very limited information, however, about whether Si supplementation could alleviate the impacts of drought under field conditions and no studies have specifically manipulated rainfall. Using field-based rain exclusion shelters, we determined whether Si supplementation (equivalent to 39, 78 and 117 kg ha^-1) affected T. aestivum growth, elemental chemistry [Si, carbon (C) and nitrogen (N)], physiology (rates of photosynthesis, transpiration, stomatal conductance, and water use efficiency) and yield (grain production) under ambient and drought (50% of ambient) rainfall scenarios. Averaged across Si treatments, drought reduced shoot mass by 21% and grain production by 18%. Si supplementation increased shoot mass by up to 43% and 73% in ambient and drought water treatments, respectively, and restored grain production in droughted plants to levels comparable with plants supplied with ambient rainfall. Si supplementation increased leaf-level water use efficiency by 32-74%, depending on Si supplementation rates. Water supply and Si supplementation did not alter concentrations of C and N, but Si supplementation increased shoot C content by 39% and 83% under ambient and drought conditions, respectively. This equates to an increase from 6.4 to 8.9 tonnes C ha^-1 and from 4.03 to 7.35 tonnes C ha^-1 under ambient and drought conditions, respectively. We conclude that Si supplementation ameliorated the negative impacts of drought on T. aestivum growth and grain yield, potentially through its beneficial impacts on water use efficiency. Moreover, the beneficial impacts of Si on plant growth and C storage may render Si supplementation a useful tool for both drought mitigation and C sequestration.

Plant-herbivore interactions: Combined effect of groundwater level, root vole grazing, and sedge silicification

Accumulation of silica (Si) by plants can be driven by (1) herbivory pressure (and therefore plant-herbivore interactions), (2) geohydrological cycles, or (3) a combination of (1) and (2), with (1-3) possibly affecting Si concentration with a 1-year delay.To identify the relative significance of (1-3), we analyzed the concentration of Si in fibrous tussock sedge (Carex appropinquata), the population density of the root vole (Microtus oeconomus), and the groundwater level, over 11 years.The largest influence of autumn Si concentration in leaves (Sileaf) was on the level of the current-year groundwater table, which was positive and accounted for 13.3% of its variance. The previous year's vole population density was weakly positively correlated with Sileaf, and it alone explained 9.5% of its variance.The only variable found to have a positive, significant effect on autumn Si concentration in rhizomes (Sirhiz) was the current-year spring water level, explaining as much as 60.9% of its variance.We conclude that the changes in Si concentration in fibrous tussock sedge are predominantly driven by hydrology, with vole population dynamics being secondary.Our results provide only partial support for the existence of plant-herbivore interactions, as we did not detect the significant effects of Si tussock concentration on the vole density dynamics. This was mainly due to the low level of silicification of sedges, which was insufficient to impinge herbivores.Future studies on plant-herbivore interactions should therefore aim at disentangling whether anti-herbivore protection is dependent on threshold values of herbivore population dynamics. Furthermore, studies on Si accumulation should focus on the effect of water-mediated Si availability.

Versatile role of silicon in cereals: Health benefits, uptake mechanism, and evolution

Silicon (Si) is an omnipresent and second most abundant element in the soil lithosphere after oxygen. Silicon being a beneficial element imparts several benefits to the plants and animals. In many plant species, including the cereals the uptake of Si from the soil even exceeds the uptake of essential nutrients. Cereals are the monocots which are known to accumulate a high amount of Si, and reaping maximum benefits associated with it. Cereals contribute a high amount of Si to the human diet compared to other food crops. In the present review, we have summarized distribution of the dietary Si in cereals and its role in the animal and human health. The Si derived benefits in cereals, specifically with respect to biotic and abiotic stress tolerance has been described. We have also discussed the molecular mechanism involved in the Si uptake in cereals, evolution of the Si transport mechanism and genetic variation in the Si concentration among different cultivars of the same species. Various genetic mutants deficient in the Si uptake have been developed and many QTLs governing the Si accumulation have been identified in cereals. The existing knowledge about the Si biology and available resources needs to be explored to understand and improve the Si accumulation in crop plants to achieve sustainability in agriculture.

Variation of foliar silicon concentrations in temperate forbs: effects of soil silicon, phylogeny and habitat

Silicon (Si) accumulation is known to alleviate various biotic and abiotic stressors in plants with potential ecological consequences. However, for dicotyledonous plants our understanding of Si variation remains limited. We conducted a comparative experimental study to investigate (1) interspecific variation of foliar Si concentrations across 37 dicotyledonous forbs of temperate grasslands, (2) intraspecific variation in foliar Si concentration in response to soil Si availability, the influence of (3) phylogenetic relatedness, and (4) habitat association to moisture. Foliar Si differed markedly (approx. 70-fold) across the investigated forbs, with some species exhibiting Si accumulation similar to grasses. Foliar Si increased with soil Si availability, but the response varied across species: species with higher Si accumulation capacity showed a stronger response, indicating that they did not actively upregulate Si uptake under low soil Si availability. Foliar Si showed a pronounced phylogenetic signal, i.e., closely related species exhibited more similar foliar Si concentrations than distantly related species. Significant differences in foliar Si concentration within closely related species pairs nevertheless support that active Si uptake and associated high Si concentrations has evolved multiple times in forbs. Foliar Si was not higher in species associated with drier habitats, implying that in dicotyledonous forbs of temperate grasslands high foliar Si is not an adaptive trait to withstand drought. Our results demonstrated considerable inter- and intraspecific variation in foliar Si concentration in temperate forbs. This variation should have pervasive, but so far understudied, ecological consequences for community composition and functioning of temperate grasslands under land-use and climate change.

Silicon in the Soil-Plant Continuum: Intricate Feedback Mechanisms within Ecosystems

Plants' ability to take up silicon from the soil, accumulate it within their tissues and then reincorporate it into the soil through litter creates an intricate network of feedback mechanisms in ecosystems. Here, we provide a concise review of silicon's roles in soil chemistry and physics and in plant physiology and ecology, focusing on the processes that form these feedback mechanisms. Through this review and analysis, we demonstrate how this feedback network drives ecosystem processes and affects ecosystem functioning. Consequently, we show that Si uptake and accumulation by plants is involved in several ecosystem services like soil appropriation, biomass supply, and carbon sequestration. Considering the demand for food of an increasing global population and the challenges of climate change, a detailed understanding of the underlying processes of these ecosystem services is of prime importance. Silicon and its role in ecosystem functioning and services thus should be the main focus of future research.

Plant carbohydrate content limits performance and lipid accumulation of an outbreaking herbivore

Locusts are major intermittent threats to food security and the ecological factors determining where and when these occur remain poorly understood. For many herbivores, obtaining adequate protein from plants is a key challenge. We tested how the dietary protein : non-structural carbohydrate ratio (p : c) affects the developmental and physiological performance of 4th-5th instar nymphs of the South American locust, Schistocerca cancellata, which has recently resurged in Argentina, Bolivia and Paraguay. Field marching locusts preferred to feed on high carbohydrate foods. Field-collected juveniles transferred to the laboratory selected artificial diets or local plants with low p : c. On single artificial diets, survival rate increased as foods became more carbohydrate-biased. On single local plants, growth only occurred on the plant with the lowest p : c. Most local plants had p : c ratios substantially higher than optimal, demonstrating that field marching locusts must search for adequate carbohydrate or their survival and growth will be carbohydrate-limited. Total body lipids increased as dietary p : c decreased on both artificial and plant diets, and the low lipid contents of field-collected nymphs suggest that obtaining adequate carbohydrate may pose a strong limitation on migration for S. cancellata. Anthropogenic influences such as conversions of forests to pastures, may increase carbohydrate availability and promote outbreaks and migration of some locusts.

High silicon concentrations in grasses are linked to environmental conditions and not associated with C4 photosynthesis

The uptake and deposition of silicon (Si) as silica phytoliths is common among land plants and is associated with a variety of functions. Among these, herbivore defense has received significant attention, particularly with regard to grasses and grasslands. Grasses are well known for their high silica content, a trait which has important implications ranging from defense to global Si cycling. Here, we test the classic hypothesis that C₄ grasses evolved stronger mechanical defenses than C₃ grasses through increased phytolith deposition, in response to extensive ungulate herbivory ("C₄ -grazer hypothesis"). Despite mixed support, this hypothesis has received broad attention, even outside the realm of plant biology. Because C₃ and C₄ grasses typically dominate in different climates, with the latter more abundant in hot, dry regions, we also investigated the effects of water availability and temperature on Si deposition. We compiled a large dataset of grasses grown under controlled environmental conditions. Using phylogenetically informed generalized linear mixed models and character evolution models, we evaluated whether photosynthetic pathway or growth condition influenced Si concentration. We found that C₄ grasses did not show consistently elevated Si concentrations compared with C₃ grasses. High temperature treatments were associated with increased concentration, especially in taxa adapted to warm regions. Although the effect was less pronounced, reduced water treatment also promoted silica deposition, with slightly stronger response in dry habitat species. The evidence presented here rejects the "C₄ -grazer hypothesis." Instead, we propose that the tendency for C₄ grasses to outcompete C₃ species under hot, dry conditions explains previous observations supporting this hypothesis. These findings also suggest a mechanism via which anthropogenic climate change may influence silica deposition in grasses and, by extension, alter the important ecological and geochemical processes it affects.

Soil nutrients and precipitation are major drivers of global patterns of grass leaf silicification

Grasses accumulate high concentrations of silicon (Si) in their tissues, with potential benefits including herbivore defense, improved water balance, and reduced leaf construction costs. Although Si is one of the most widely varying leaf constituents among individuals, species, and ecosystems, the environmental forces driving this variation remain elusive and understudied. To understand relationships between environmental factors and grass Si accumulation better, we analyzed foliar chemistry of grasses from 17 globally distributed sites where nutrient inputs and grazing were manipulated. These sites span natural gradients in temperature, precipitation, and underlying soil properties, which allowed us to assess the relative importance of soil moisture and nutrients across variation in climate. Foliar Si concentration did not respond to large mammalian grazer exclusion, but significant variation in herbivore abundance among sites may have precluded the observation of defoliation effects at these sites. However, nutrient addition consistently reduced leaf Si, especially at sites with low soil nitrogen prior to nutrient addition. Additionally, a leaf‐level trade‐off between Si and carbon (C) existed that was stronger at arid sites than mesic sites. Our results suggest soil nutrient limitation favors investment in Si over C‐based leaf construction, and that fixing C is especially costly relative to assimilating Si when water is limiting. Our results demonstrate the importance of soil nutrients and precipitation as key drivers of global grass silicification patterns.

Silicon and Plant-Animal Interactions: Towards an Evolutionary Framework

Herbivory is fundamental in ecology, being a major driver of ecosystem structure and functioning. Plant Si and phytoliths play a significant antiherbivory role, the understanding of which and of its evolutionary context will increase our understanding of this phenomenon, its origins, and its significance for past, extant, and future ecosystems. To achieve this goal, we need a superdisciplinary evolutionary framework connecting the role of Si in plant–herbivore interactions, in global processes, and in plant and herbivore evolution. To do this properly, we should acknowledge and incorporate into our work some basic facts that are too often overlooked. First, there is great taxonomic variance both in plant Si contents, forms, and roles, but also in herbivore responses, dietary preferences, and in fossil evidence. Second, species and their traits, as well as whole ecosystems, should be seen in the context of their entire evolutionary history and may therefore reflect not only adaptations to extant selective factors but also anachronistic traits. Third, evolutionary history and evolutionary transitions are complex, resulting in true and apparent asynchronisms. Fourth, evolution and ecology are multiscalar, in which various phenomena and processes act at various scales. Taking these issues into consideration will improve our ability to develop this needed theoretical framework and will bring us closer to gaining a more complete understanding of one of the most exciting and elusive phenomena in plant biology and ecology.

Differential stoichiometric responses of shrubs and grasses to increased precipitation in a degraded karst ecosystem in Southwestern China

The elemental concentrations of both plants and soils are sensitive to variations in precipitation due to the limiting roles of water on soil processes and plant growth in karst ecosystems of Southwestern China; however, precipitation is predicted to increase in this region. Nevertheless, it is unclear how the elemental composition of soils and plants might respond to such increases in moisture. Particularly, how potassium (K) may behave as a key mediator in the regulation of the water potential of plants. For this study, the responses of the elemental composition of both soils and plants to the variable addition of water were investigated. Two grasses (Cymbopogon distans and Arundinella setosa) and two shrubs (Carissa spinarum and Bauhinia brachycarpa) were investigated under four levels of watering treatments 0%, +20%, +40%, and +60%, relative to the annual rainfall, respectively. Compared to the control (CK), the soil water content (SWC) increased to 3.75, 3.86, and 4.34 mg g^-1 in T1, T2, and T3 groups, respectively (p < 0.05). Non-metal elements (C, H, N, S, and P, except for Si) in the soil were relatively stable with water addition; however, metal elements (Al, Na, Mg, Fe, and K, along with Si) increased significantly, whereas Zn and Ca decreased (p < 0.05). With water addition, leaf N and P remained unchanged in all four species, while K, Mg, and S decreased in both shrubs (higher C:K, N:K, and P:K). Increases in Fe, Si, and K were observed in both grasses (lower C:K, N:K, and P:K), which suggested that K played distinct roles for water regulation in shrubs and grasses. These findings implied that the elemental compositions of both soils and plants might be altered with increasing precipitation in the future, where different plant types may adopt distinct K-regulation strategies to cope with variable soil moisture.

Are ants attracted to herbivorized leaves of Caryocar brasiliense Camb. (Caryocaraceae)?

Abstract Different profiles of secondary compounds are released by plants after herbivore attack. Many of these compounds are used by predators and parasitoids to locate herbivores that are damaging leaves. Such an induced indirect defense was tested with the Caryocar brasiliense-ant system in the dry season, when C. brasiliense has old leaves, and in the rainy season, when C. brasiliense has new leaves. A total of 20 plants were analyzed per season. Two opposite leaves of the same branch were selected for each plant. Approximately 40% of the area of one leaf was removed (treatment leaf) while the other leaf remained intact (control). The number of ants that visited each leaf was counted simultaneously for a period of 15 minutes and the mean difference in ant number was tested by paired t-test. The mean number of ants differed significantly between treatment and control only in the rainy season (t = 3.004, df = 19, p = 0.007). This finding suggests the presence of induced defense in this system only when the leaves are young. The study supports the Optimal Defense Theory since young leaves of C. brasiliense with artificial damage attracted significantly more ants than leaves without damage and represents the first evidence of an induced defense mechanism in the C. brasiliense-ant system.

Silicon and Plant Natural Defenses against Insect Pests: Impact on Plant Volatile Organic Compounds and Cascade Effects on Multitrophic Interactions

Environmental factors controlling silicon (Si) accumulation in terrestrial plant are key drivers to alleviate plant biotic stresses, including insect herbivory. While there is a general agreement on the ability of Si-enriched plant to better resist insect feeding, recent studies suggest that Si also primes biochemical defense pathways in various plant families. In this review, we first summarize how soil parameters and climate variables influence Si assimilation in plants. Then, we describe recent evidences on the ability of Si to modulate plant volatile emissions, with potential cascade effects on phytophagous insects and higher trophic levels. Even though the mechanisms still need to be elucidated, Si accumulation in plants leads to contrasting effects on the levels of the three major phytohormones, namely jasmonic acid, salicylic acid and ethylene, resulting in modified emissions of plant volatile organic compounds. Herbivore-induced plant volatiles would be particularly impacted by Si concentration in plant tissues, resulting in a cascade effect on the attraction of natural enemies of pests, known to locate their prey or hosts based on plant volatile cues. Since seven of the top 10 most important crops in the world are Si-accumulating Poaceae species, it is important to discuss the potential of Si mobility in soil-plant systems as a novel component of an integrated pest management.

When resistance is futile, tolerate instead: silicon promotes plant compensatory growth when attacked by above- and belowground herbivores

Plants have evolved numerous herbivore defences that are resistance- or tolerance-based. Resistance involves physical and chemical traits that deter and/or harm herbivores whereas tolerance minimizes fitness costs of herbivory, often via compensatory growth. The Poaceae frequently accumulate large amounts of silicon (Si), which can be used for herbivore resistance, including biomechanical and (indirectly) biochemical defences. To date, it is unclear whether Si improves tolerance of herbivory. Here we report how Si enabled a cereal (Triticum aestivum) to tolerate damage inflicted by above- and belowground herbivores. Leaf herbivory increased Si concentrations in the leaves by greater than 50% relative to herbivore-free plants, indicating it was an inducible defensive response. In plants without Si supplementation, leaf herbivory reduced shoot biomass by 52% and root herbivory reduced root biomass by 68%. Si supplementation, however, facilitated compensatory growth such that shoot losses were more than compensated for (+14% greater than herbivore-free plants) and root losses were minimized to -16%. Si supplementation did not improve plant resistance since Si did not enhance biomechanical resistance (i.e. force of fracture) or reduce leaf consumption and herbivore relative growth rates. We propose that Si-based defence operates in wheat via tolerance either in addition or as an alternative to resistance-based defence.

Biochar applications decrease reproductive potential of the English grain aphid Sitobion avenae and upregulate defense-related gene expression

BACKGROUND

The application of biochar as a targeted strategy for managing herbivorous pests is a topic of growing interest. The present study first determined the influence of biochar amendments on the reproductive performance of the English grain aphid Sitobion avenae, and then examined defense-related gene expression in the wheat plant as a function of biochar amendments and aphid feeding.

RESULTS

Compared to the control, biocahr amendments decreased aphid lifetime fertility by 9.09% and 20.23% for amending levels at 3% and 5%, respectively; it reduced aphid population by 18.68%, 21.69%, and 28.70% for the levels at 1.5%, 3%, and 5%, respectively. Biochar applications increased silicon content more than 40% in wheat plants. Furthermore, biochar additions increased the expression of four defense-related genes (AOS, LOX, PAL and PR) in wheat plants with extension of feeding time by aphids.

CONCLUSION

Our results suggest that biochar amendments to soils have detrimental consequences on the reproductive potential of the aphid on the wheat, and the effect may result from aphid-induced plant defenses being raised by biochar applications. © 2018 Society of Chemical Industry.

Silicon uptake by a pasture grass experiencing simulated grazing is greatest under elevated precipitation

BACKGROUND

Grasses are hyper-accumulators of silicon (Si) and often up-regulate Si following herbivory. Positive correlations exist between Si and plant water content, yet the extent to which Si uptake responses can be mediated by changes in soil water availability has rarely been studied and never, to our knowledge, under field conditions. We used field-based rain-exclusion shelters to investigate how simulated grazing (shoot clipping) and altered rainfall patterns (drought and elevated precipitation, representing 50% and 150% of ambient precipitation levels, respectively) affected initial patterns of root- and shoot-Si uptake in a native Australian grass (Microlaena stipoides) in Si-supplemented and untreated soils.

RESULTS

Si supplementation increased soil water retention under ambient and elevated precipitation but not under drought, although this had little effect on Si uptake and growth (tiller numbers or root biomass) of M. stipoides. Changes in rainfall patterns and clipping had strong individual effects on plant growth and Si uptake and storage, whereby clipping increased Si uptake by M. stipoides under all rainfall treatments but to the greatest extent under elevated precipitation. Moreover, above-ground-below-ground Si distribution only changed following elevated precipitation by decreasing the ratio of root:shoot Si concentrations.

CONCLUSIONS

Results highlight the importance of soil water availability for Si uptake and suggest a role for both active and passive Si transport mechanisms. Such manipulative field studies may provide a more realistic insight into how grasses initially respond to herbivory in terms of Si-based defence under different environmental conditions.

Elevated carbon dioxide and warming impact silicon and phenolic-based defences differently in native and exotic grasses

Global climate change may increase invasions of exotic plant species by directly promoting the success of invasive/exotic species or by reducing the competitive abilities of native species. Changes in plant chemistry, leading to altered susceptibility to stress, could mediate these effects. Grasses are hyper-accumulators of silicon, which play a crucial function in the alleviation of diverse biotic and abiotic stresses. It is unknown how predicted increases in atmospheric carbon dioxide (CO₂ ) and air temperature affect silicon accumulation in grasses, especially in relation to primary and secondary metabolites. We tested how elevated CO₂ (eCO₂ ) (+240 ppm) and temperature (eT) (+4°C) affected chemical composition (silicon, phenolics, carbon and nitrogen) and plant growth in eight grass species, either native or exotic to Australia. eCO₂ increased phenolic concentrations by 11%, but caused silicon accumulation to decline by 12%. Moreover, declines in silicon occurred mainly in native species (-19%), but remained largely unchanged in exotic species. Conversely, eT increased silicon accumulation in native species (+19%) but decreased silicon accumulation in exotic species (-10%). Silicon and phenolic concentrations were negatively correlated with each other, potentially reflecting a defensive trade-off. Moreover, both defences were negatively correlated with plant mass, compatible with a growth-defence trade-off. Grasses responded in a species-specific manner, suggesting that the relative susceptibility of different species may differ under future climates compared to current species rankings of resource quality. For example, the native Microlaena stipoides was less well defended under eCO₂ in terms of both phenolics and silicon, and thus could suffer greater vulnerability to herbivores. To our knowledge, this is the first demonstration of the impacts of eCO₂ and eT on silicon accumulation in grasses. We speculate that the greater plasticity in silicon uptake shown by Australian native grasses may be partly a consequence of evolving in a low nutrient and seasonally arid environment.

Population-level manipulations of field vole densities induce subsequent changes in plant quality but no impacts on vole demography

Grazing-induced changes in plant quality have been suggested to drive the negative delayed density dependence exhibited by many herbivore species, but little field evidence exists to support this hypothesis. We tested a key premise of the hypothesis that reciprocal feedback between vole grazing pressure and the induction of anti-herbivore silicon defenses in grasses drives observed population cycles in a large-scale field experiment in northern England. We repeatedly reduced population densities of field voles (Microtus agrestis) on replicated 1-ha grassland plots at Kielder Forest, northern England, over a period of 1 year. Subsequently, we tested for the impact of past density on vole life history traits in spring, and whether these effects were driven by induced silicon defenses in the voles' major over-winter food, the grass Deschampsia caespitosa. After several months of density manipulation, leaf silicon concentrations diverged and averaged 22% lower on sites where vole density had been reduced, but this difference did not persist beyond the period of the density manipulations. There were no significant effects of our density manipulations on vole body mass, spring population growth rate, or mean date for the onset of spring reproduction the following year. These findings show that grazing by field voles does induce increased silicon defenses in grasses at a landscape scale. However, at the vole densities encountered, levels of plant damage appear to be below those needed to induce changes in silicon levels large and persistent enough to affect vole performance, confirming the threshold effects we have previously observed in laboratory-based studies. Our findings do not support the plant quality hypothesis for observed vole population cycles in northern England, at least over the range of vole densities that now prevail here.

Taxonomic Demarcation of Setaria pumila (Poir.) Roem. & Schult., S. verticillata (L.) P. Beauv., and S. viridis (L.) P. Beauv. (Cenchrinae, Paniceae, Panicoideae, Poaceae) From Phytolith Signatures

Background and Aims: The role and significance of phytoliths in taxonomic diagnosis of grass species has been well documented with a focus on the types found in foliar epidermis and the synflorescence. The present paper is an attempt to broaden the scope of phytoliths in species diagnosis of grasses by developing phytolith signatures of some species of the foxtail genus Setaria P. Beauv. through in situ location and physico-chemical analysis of various phytolith morphotypes in different parts of the plant body. Methods: Clearing solution and dry ashing extraction methods were employed for in situ location and isolation of phytolith morphotypes respectively. Ultrastructural details were worked out by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy. Morphometric and frequency data of phytolith morphotypes were also recorded. Biochemical architecture of various phytolith types was worked out through SEM-EDX, XRD, and FTIR analysis. Data were analyzed through Principal Component Analysis and Cluster Analysis. Key Results:In situ location of phytoliths revealed species specific epidermal patterns. The presence of cystoliths (calcium oxalate crystals) in the costal regions of adaxial leaf surface of S. verticillata (L.) P. Beauv. is the first report for the genus Setaria. Our results revealed marked variations in epidermal ornamentation and undulation patterns with a novel "Λ" (Lamda) type of undulated ornamentation reported in S. verticillata. Dry ashing method revealed species specific clusters of phytolith morphotypes. Conclusions: The study revealed that phytoliths can play a significant role in resolution of taxonomic identity of three species of Setaria. Each species was marked out by a unique assemblage of phytolith morphotypes from various parts of the plant body. Apart from in situ location and epidermal patterning, diagnostic shapes, frequency distribution, size dimensions, and biochemical architecture emerged as complementary traits that help in developing robust phytolith signatures for plant species.

Evidence for Active Uptake and Deposition of Si-based Defenses in Tall Fescue

Silicon (Si) is taken up from the soil as monosilicic acid by plant roots, transported to leaves and deposited as phytoliths, amorphous silica (SiO2) bodies, which are a key component of anti-herbivore defense in grasses. Silicon transporters have been identified in many plant species, but the mechanisms underpinning Si transport remain poorly understood. Specifically, the extent to which Si uptake is a passive process, driven primarily by transpiration, or has both passive and active components remains disputed. Increases in foliar Si concentration following herbivory suggest plants may exercise some control over Si uptake and distribution. In order to investigate passive and active controls on Si accumulation, we examined both genetic and environmental influences on Si accumulation in the forage grass Festuca arundinacea. We studied three F. arundinacea varieties that differ in the levels of Si they accumulate. Varieties not only differed in Si concentration, but also in increases in Si accumulation in response to leaf damage. The varietal differences in Si concentration generally reflected differences in stomatal density and stomatal conductance, suggesting passive, transpiration-mediated mechanisms underpin these differences. Bagging plants after damage was employed to minimize differences in stomatal conductance between varieties and in response to damage. This treatment eliminated constitutive differences in leaf Si levels, but did not impair the damage-induced increases in Si uptake: damaged, bagged plants still had more leaf Si than undamaged, bagged plants in all three varieties. Preliminary differential gene expression analysis revealed that the active Si transporter Lsi2 was highly expressed in damaged unbagged plants compared with undamaged unbagged plants, suggesting damage-induced Si defenses are regulated at gene level. Our findings suggest that although differences in transpiration may be partially responsible for varietal differences in Si uptake, they cannot explain damage-induced increases in Si uptake and deposition, suggesting that wounding causes changes in Si uptake, distribution and deposition that likely involve active processes and changes in gene expression.

Effect of phytoliths for mitigating water stress in durum wheat

The role of silicon (Si) in alleviating biotic and abiotic stresses in crops is well evidenced by empirical studies; however, the mechanisms by which it works are still poorly known. The aim of this study is to determine whether or not phytolith composition and distribution in wheat are affected by drought and, if so, why. Durum wheat was grown using hydroponics in the presence of polyethylene glycol (PEG)-6000 to perform a water-stress simulation. We developed an original method for in situ analysis of phytoliths in leaves via X-ray imaging. PEG was efficient in inhibiting water uptake by roots and creating stress, and prevented a small fraction of Si from being accumulated in the shoots. The application of Si with PEG maintained shoot and root fresh weights (FW) and relative water content at higher values than for plants without Si, especially at PEG 12%. Our data show that, under water stress in the presence of Si, accumulation of phytoliths over the veins provides better support to the leaf, thus allowing for a better development of the whole plant than in the absence of Si. The development of silicified trichomes in durum wheat depends primarily on the availability of Si in soil and is not an adaptation to water stress.

Silicon-based plant defences, tooth wear and voles

Plant–herbivore interactions are hypothesized to drive vole population cycles through the grazing-induced production of phytoliths in leaves. Phytoliths act as mechanical defences because they deter herbivory and lower growth rates in mammals. However, how phytoliths impair herbivore performance is still unknown. Here, we tested whether the amount of phytoliths changes tooth wear patterns. If confirmed, abrasion from phytoliths could play a role in population crashes. We applied dental microwear texture analysis (DMTA) to laboratory and wild voles. Lab voles were fed two pelleted diets with differing amounts of silicon, which produced similar dental textures. This was most probably due to the loss of food mechanical properties through pelletization and/or the small difference in silicon concentration between diets. Wild voles were trapped in Poland during spring and summer, and every year across a population cycle. In spring, voles feed on silica-rich monocotyledons, while in the summer they also include silica-depleted dicotyledons. This was reflected in the results; the amount of silica therefore leaves a traceable record in the dental microwear texture of voles. Furthermore, voles from different phases of population cycles have different microwear textures. We tentatively propose that these differences result from grazing-induced phytolith concentrations. We hypothesize that the high amount of phytoliths in response to intense grazing in peak years may result in malocclusion and other dental abnormalities, which would explain how these silicon-based plant defences help provoke population crashes. DMTA could then be used to reconstruct vole population dynamics using teeth from pellets or palaeontological material.

Defending the leaf surface: intra- and inter-specific differences in silicon deposition in grasses in response to damage and silicon supply

Understanding interactions between grasses and their herbivores is central to the conservation of species-rich grasslands and the protection of our most important crops against pests. Grasses employ a range of defenses against their natural enemies; silicon-based defenses have been shown to be one of the most effective. Silicon (Si) is laid down on the leaf surface as spines and other sharp bodies, known as phytoliths, making grasses abrasive and their foliage indigestible to herbivores. Previous studies on Si defenses found that closely related species may have similar levels of Si in the leaves but differ markedly in abrasiveness. Here we show how the number, shape and distribution of Si-rich phytoliths and spines differ within and between different grass species and demonstrate that species also differ in their ability to change the deposition and distribution of these defenses in response to damage or increases in Si supply. Specifically, we tested the response of two genotypes of Festuca arundinacea known to differ in their surface texture and three different grass species (F. ovina, F. rubra, and Deschampsia cespitosa) differing in their abrasiveness to combined manipulation of leaf damage and Si supply. F. arundinacea plants with a harsh leaf surface had higher Si content and more spines on their leaf surface than soft varieties. F. ovina and D. cespitosa plants increased their leaf Si concentration and produced an increase in the number of leaf spines and phytoliths on the leaf surface in response to Si addition. F rubra also increased leaf Si content in response to treatments, particularly in damaged leaves, but did not deposit this in the form of spines or increased densities of phytoliths. We discuss how the form in which grasses deposit Si may affect their anti-herbivore characteristics and consider the ecological and agricultural implications of the differences in allocation to Si-based defenses between grass species.