Leaf Si Concentration Research Articles

Extractable soil silicon in relation to sugarcane yield on mineral soil

Florida sugarcane production is mostly on organic soils, but the cultivation area on mineral soils is also expanding. Silicon (Si) is beneficial for sugarcane growth and development on weathered soil. However, the low solubility and availability of soluble Si is an issue in Florida’s mineral soils. This study aimed to assess soil Si extraction methods based on sugarcane yield response to calcium silicate application. It also examined the relationship between sugarcane yield with extracted plant-available soil Si, alongside plant Si concentrations. The experiment followed a randomized complete block design involving 2 fields, with 24 plots each, and 6 replications. Four rates of calcium silicate (0 control, 2.24, 4.48, and 6.67 Mg ha−1) were applied before planting sugarcane. Soil samples were collected prior to planting and after each harvest. Soil Si was extracted using 0.5 M acetic acid, 0.7 N NH4OAc, Mehlich 3, and 0.01 M CaCl2 extractants. Leaf samples were collected over a three-year sugarcane life cycle (from 2019 to 2022) to evaluate the relationship between soil Si and leaf Si concentrations. The relationships between parameters were analyzed using linear and nonlinear regression models. Results indicated that higher Si application rates increased Si concentrations in sugarcane leaves in both fields. However, only one field showed a yield response to Si, likely due to high pre-plant soil Si in the other field. The best extractant should correlate strongly with either yield or leaf Si content, but no single extractant showed a strong correlation with either, making it infeasible to identify the best extractant.

Nitrogen and Silicon Contribute to Wheat Defense's to Pyrenophora tritici-repentis, but in an Independent Manner.

Nitrogen (N) and silicon (Si) are mineral elements that have shown a reduction in the damage caused by tan spot (Pyrenophora tritici-repentis (Ptr)) in wheat. However, the effects of these elements were studied separately, and the N and Si interaction effect on wheat resistance to tan spot remains elusive. Histocytological and biochemical defense responses against Ptr in wheat leaves treated with Si (+Si) at low (LN) and high N (HN) inputs were investigated. Soil amendment with Si reduced the tan spot severity in 18% due to the increase in the leaf Si concentration (around 30%), but it was affected by the N level used. The superoxide dismutase (SOD) activity was higher in +Si plants and inoculated with Ptr, leading to early and higher H2O2 and callose accumulation in wheat leaf. Interestedly, phenylalanine ammonia-lyase (PAL) activity was induced by the Si supplying, being negatively affected by the HN rate. Meanwhile, catalase (CAT), and peroxidase (POX) activities showed differential response patterns according to the Si and N rates used. Tan spot severity was reduced by both elements, but their interaction does not evidence synergic effects in this disease's control. Wheat plants from -Si and HN and +Si and LN treatments recorded lower tan spot severity.

Effects of leaf silicon on drought performance of tropical tree seedlings.

Elevated leaf silicon (Si) concentrations improve drought resistance in cultivated plants, suggesting Si might also improve drought performance of wild species. Tropical tree species, for instance, take up substantial amounts of Si, and leaf Si varies markedly at local and regional scales, suggesting consequences for seedling drought resistance. Yet, whether elevated leaf Si improves seedling drought performance in tropical forests is unknown. To manipulate leaf Si concentrations, seedlings of seven tropical tree species were grown in Si-rich and -poor soil, before exposing them to drought in the forest understorey. Survival, growth and wilting were monitored. Elevated leaf Si did not improve drought survival and growth in any of the species. In one species, drought survival was reduced in seedlings previously grown in Si-rich soil, contrary to our expectation. Our results suggest that elevated leaf Si does not improve drought resistance of wild tropical tree species. Elevated leaf Si may even reduce drought performance, suggesting differences in soil conditions influencing leaf Si may contribute to soil-related variation of tropical seedling performance. Furthermore, our results are at odds with most studies on cultivated species and show that alleviative effects of Si in crops cannot be generalized to wild plants in natural systems.

Identification of novel germplasm and genetic loci for enhancing mineral element uptake in soybean

Bioavailability, uptake, and spatial distribution of essential and toxic mineral elements are pivotal factors that govern crop growth, development, and productivity. Soybean is a major leguminous crop globally, and yield losses are commonly attributed to various abiotic factors, including nutrient deficiencies or the influence of toxic minerals in the soil. Therefore, understanding the molecular basis of differential mineral element uptake, translocation, and accumulation in soybean is vital for developing improved cultivars. Here, we used portable X-ray fluorescence (p-XRF) for rapid and high-throughput mineral element profiling of a diverse set of soybean germplasm using leaves, stem, root, and seed tissues. Genome-wide association (GWAS) was performed on the element profiles of 219 soybean accessions, revealing lines with notable two to tenfold difference in various mineral nutrient uptake. These identified lines represent valuable genetic resources for germplasm development and gene discovery. The GWAS analysis pinpointed significant genomic loci and haplotypes associated with accumulation of aluminum (Al), silicon (Si), iron (Fe), and manganese (Mn). Remarkably, genes associated with the transport of solutes, metals, and ions have been pinpointed, indicating their potential involvement in the nutrient uptake and soybean improvement. To gain functional insights, Si deposition, allelic variants and expression of three Si transporter genes was studied in greater details. The high Si accumulating line PI548452 showed 12-fold increase in leaf Si content compared to low Si lines. Expression of HiSil2b and HiSil2c effluxer genes showed root and leaf specific expression, respectively, providing evidence for tissue specific Si transport and the basis for precise management of Si uptake to improve abiotic and biotic stresses. In summary, this study uncovered the novel accessions, haplotypes, allelic diversity, and the potential candidate genes underpinning the mineral nutrient accumulation in soybean.

Silicon dioxide nanoparticles enhance plant growth, photosynthetic performance, and antioxidants defence machinery through suppressing chromium uptake in Brassica napus L.

Chromium (Cr) is a highly toxic heavy metal that is extensively released into the soil and drastically reduces plant yield. Silicon nanoparticles (Si NPs) were chosen to mitigate Cr toxicity due to their ability to interact with heavy metals and reduce their uptake. This manuscript explores the mechanisms of Cr-induced toxicity and the potential of Si NPs to mitigate Cr toxicity by regulating photosynthesis, oxidative stress, and antioxidant defence, along with the role of transcription factors and heavy metal transporter genes in rapeseed (Brassica napus L.). Rapeseed plants were grown hydroponically and subjected to hexavalent Cr stress (50 and 100 μM) in the form of K2Cr2O7 solution. Si NPs were foliar sprayed at concentrations of 50, 100 and 150 μM. The findings showed that 100 μM Si NPs under 100 μM Cr stress significantly increased the leaf Si content by 169% while reducing Cr uptake by 92% and 76% in roots and leaves, respectively. The presence of Si NPs inside the plant leaf cells was confirmed by using energy-dispersive spectroscopy, inductively coupled plasma‒mass spectrometry, and confocal laser scanning microscopy. The study's findings showed that Cr had adverse effects on plant growth, photosynthetic gas exchange attributes, leaf mesophyll ultrastructure, PSII performance and the activity of enzymatic and nonenzymatic antioxidants. However, Si NPs minimized Cr-induced toxicity by reducing total Cr accumulation and decreasing oxidative damage, as evidenced by reduced ROS production (such as H2O2 and MDA) and increased enzymatic and nonenzymatic antioxidant activities in plants. Interestingly, Si NPs under Cr stress effectively increased the NPQ, ETR and QY of PSII, indicating a robust protective response of PSII against stress. Furthermore, the enhancement of Cr tolerance facilitated by Si NPs was linked to the upregulation of genes associated with antioxidant enzymes and transcription factors, alongside the concurrent reduction in metal transporter activity.

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.

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.

Silicon supply promotes differences in growth and C:N:P stoichiometry between bamboo and tree saplings

BackgroundSi can be important for the growth, functioning, and stoichiometric regulation of nutrients for high-Si-accumulating bamboo. However, other trees do not actively take up dissolved silicic acid [Si(OH)4] from the soil, likely because they have fewer or no specific Si transporters in their roots. It is unclear what causes differential growth and C:N:P stoichiometry between bamboo and other trees across levels of Si supply.ResultsSi supply increased the relative growth rate of height and basal diameter of bamboo saplings, likely by increasing its net photosynthetic rate and ratios of N:P. Moreover, a high concentration of Si supply decreased the ratio of C:Si in bamboo leaves due to a partial substitution of C with Si in organic compounds. We also found that there was a positive correlation between leaf Si concentration and its transpiration rate in tree saplings.ConclusionsWe demonstrated that Si supply can decrease the ratio of C:Si in bamboo leaves and increase the ratio of N:P without altering nutrient status or the N:P ratio of tree saplings. Our findings provide experimental data to assess the different responses between bamboo and other trees in terms of growth, photosynthesis, and C:N:P stoichiometry. These results have implications for assessing the growth and competition between high-Si-accumulating bamboo and other plants when Si availability is altered in ecosystems during bamboo expansion.

Silicon uptake and utilization on Panicum maximum grass modifies C:N:P stoichiometry under warming and soil water deficit

Future increases in air temperature are expected to increase the growth of Panicum maximum, but with climate change it will be common for the binomial increase in temperature and drought to occur. In this condition, plants may use strategies to adapt to this new scenario by favoring the uptake of beneficial elements such as Si and modifying plants nutritional status, but this needs to be tested. Thus, this research was conducted with the objective of evaluating the effect of warming and water restriction on Si uptake, accumulation, Si use efficiency, the C:Si, N:Si and P:Si stoichiometric ratios and their effects on growth of Panicum maximum plants grown under field conditions. The experiment was carried out in a 2 × 2 factorial scheme, with two temperature conditions (T): ambient temperature (aT), and high temperature (eT - increase of 2 °C above ambient temperature) combined with two levels of soil water availability (W), with water stress - non-irrigated (wS) and irrigated (sW), arranged in randomized blocks. Si concentration and accumulation are affected by temperature variations and soil water availability in different ways. Soil water restriction reduces leaf Si concentration, accumulation and increases the C, N and P stoichiometric ratios. Biomass production always improved for all growth cycles under warming and irrigated conditions, while, leaf C: Si, N: Si, and P: Si ratios decreased under non-irrigated and elevated temperature. It was revealed for the first time that the tolerance of Panicum maximum to warming and soil water restriction can be associated with Si uptake and changes in plant C:N:P stoichiometry.

Creeping Bentgrass Nutritional, Morphological, and Putting Green Performance Response to Ca/Mg-Silicate Slag Liming Agent

While not classified as an essential plant nutrient, silicon (Si) assimilation following exogenous Si application has enhanced the wear resistance of cool-season turfgrass. Given this beneficial supplementation of lignin by Si reported in epidermal tissue of monocotyledonous plants, our research objective was to quantify root morphology, vegetative nutrition and vigor, soil chemistry, and putting green performance in response to split applications of pelletized liming agents rich in Si and/or Ca and Mg. Field evaluation of granular liming agent treatment, 2441 kg (ha year)−1, was conducted on creeping bentgrass (Agrostis stolonifera L. cv. Penn G-2) putting green maintained in the Mid-Atlantic US. Pelletized Ca/Mg-SiO3 slag or dolomitic limestone treatments were conducted in frequent split applications and incorporated into the upper 5 cm of the rootzone. Measurements of canopy color and density, shoot growth as clipping yield, soil pH, Si and nutrient content of clippings, and soil extractable Si were performed each season. Cumulative Ca/Mg-SiO3 application (kg ha−1) increased mean acetic acid (HOAc) extractable Si by 35 to 60 mg kg−1 and leaf Si content by 1.0 to 1.5 mg g−1. However, neither putting green canopy quality, shoot nutrient concentration, 5 to 15 cm depth root length density nor ball roll distance was improved by liming agent treatment. Liming agent-treated or untreated plots showed statistical, yet inconsistent, differences in clipping yield 4, 14, 15, 16, and 17 months from initiation (MFI). This thorough shuffling of treatment rank, resulting in identical experiment-wide means precludes the expectation of dependably superior vigor by any.

Growth-defence trade-off in rice: fast-growing and acquisitive genotypes have lower expression of genes involved in immunity.

Plant ecologists and molecular biologists have long considered the hypothesis of a trade-off between plant growth and defence separately. In particular, how genes thought to control the growth-defence trade-off at the molecular level relate to trait-based frameworks in functional ecology, such as the slow-fast plant economics spectrum, is unknown. We grew 49 phenotypically diverse rice genotypes in pots under optimal conditions and measured growth-related functional traits and the constitutive expression of 11 genes involved in plant defence. We also quantified the concentration of silicon (Si) in leaves to estimate silica-based defences. Rice genotypes were aligned along a slow-fast continuum, with slow-growing, late-flowering genotypes versus fast-growing, early-flowering genotypes. Leaf dry matter content and leaf Si concentrations were not aligned with this axis and negatively correlated with each other. Live-fast genotypes exhibited greater expression of OsNPR1, a regulator of the salicylic acid pathway that promotes plant defence while suppressing plant growth. These genotypes also exhibited greater expression of SPL7 and GH3.2, which are also involved in both stress resistance and growth. Our results do not support the hypothesis of a growth-defence trade-off when leaf Si and leaf dry matter content are considered, but they do when hormonal pathway genes are considered. We demonstrate the benefits of combining ecological and molecular approaches to elucidate the growth-defence trade-off, opening new avenues for plant breeding and crop science.

Silicon Sources and Bacterial Inoculants on Growth Parameters, Leaf Yield, Quality of Coriander (Coriandrum sativum L.) and Soil Adsorbed Silicon in Sandy Loam Soil

Aims: The objective of the present study was to investigate the effect of different silicon sources with and without microbial inoculants on growth parameters, leaf yield and quality of Coriander. Study Design: Factorial Completely Randomized Block Design (FCRD) was used in the present study with thirty-six treatments and three replications. Place and Duration of Study: The pot culture experiment was carried out in the Department of Soil Science and Agricultural Chemistry, Tamil Nadu Agricultural University, Coimbatore, India during January to March 2022. Methodology: A pot culture experiment was conducted with different silicon sources namely CaSiO3, MgSiO3 and Rice Husk Ash (RHA) at three levels (0, 50 and 100 mg Si kg-1) with and without two silicate solubilizing bacteria (SSB) and a zinc solubilizing bacterium (ZSB) at 2 kg ha-1 with coriander CO (CR) 4. The influence of the treatments on growth parameters, leaf Si content, leaf yield, total soluble sugar content of leaf and soil adsorbed Si were investigated at 25 and 50 DAS. Results: The results revealed a significant influence of Si sources with bacterial inoculants. The plant height and SPAD index values were the highest by the application of RHA­ @ 100 mg Si kg-­1 + SSB4 followed by CaSiO3 @ 100 mg Si kg-­1 + SSB4. All the sources increased the leaf Si as well as soil adsorbed Si and the effect was highly significant when Si sources were combined with SSB4 and other inoculants. Coriander leaf yield ranged from 4.32 to 8.24 g plant-1. The maximum leaf yield was recorded with the application of RHA @ 100 mg Si kg-­1 with SSB4 (8.24 g plant-1) followed by RHA @ 50 mg Si kg-­1 with SSB4 (7.67 g plant-1) and ­ CaSiO3 @ 100 mg Si kg-­1 with SSB4 (7.56 g plant-1). The effect of Si sources with bacterial inoculants on TSS content of leaf was significant with a maximum value of (38.23 mg g-1 DW) by RHA @ 100 mg Si kg-­1 + SSB4 followed by CaSiO3 @ 100 mg Si kg-­1 + SSB4 (36.10 mg g-1 DW). Conclusion: The current study revealed that the combined application of Si sources and microbial inoculants significantly enhanced the growth attributes, leaf Si, leaf yield, leaf quality of coriander and soil adsorbed Si. The leaf yield increase over control for RHA @ 100 mg Si kg-­1 with SSB4 was 88.92 per cent followed by RHA @ 50 mg Si kg-­1 with SSB4 (76.58%) and CaSiO3 @ 100 mg Si kg-­1 with SSB4 (73.82%). The research work needs further validation at the field level.

Effect of silicon and fungicide on photosynthetic responses in barley leaves challenged by Bipolaris sorokiniana

The effect of silicon (Si) amendment in the soil, combined with fungicide application 7 or 15 days before plant inoculation, was evaluated on barley plants challenged by Bipolaris sorokiniana. For this, the disease control, the gas exchange parameters, and the concentration of photosynthetic pigments were evaluated on two barley cultivars (AnaG01 and BRS Cauê). The higher spot blotch severity occurred on plants not supplied with Si and without fungicide application. The disease reduced the net assimilation of CO2 (A), the stomatal conductance (gs), internal CO2 concentration (Ci) and transpiration rate (E). The concentration of photosynthetic pigments was significantly altered in inoculated leaves. The plants supplied with Si showed an increase in leaf Si concentration, which was associated with a reduction of up to 15% in spot blotch, and increase in A, gs, Ci, and E, and concentration of pigments (Chlorophylls a and b, and carotenes). The fungicide reduced disease intensity and increased A and gs, and the concentration of pigments. Higher control of the disease by fungicide occurred on plants supplied with Si, especially when application occurred at 15 day before inoculation. In conclusion, the association of Si and fungicide provided a greater reduction in spot blotch and maintained a better physiological state of the plant when affected by B. sorokiniana.

Uptake of silicon in barley under contrasting drought regimes

PurposeSilicon (Si) accumulation in plant tissues plays a vital role in alleviating biotic and abiotic stresses, including drought. Temperate regions are predicted to experience reductions in the quantity and frequency of rainfall events, potentially impacting plant Si uptake via the transpiration stream. Despite the importance for predicting plant responses to Si amendments, the effects of changes in rainfall patterns on Si uptake in cereals have not been characterised.MethodsFive watering regimes were applied based on predicted precipitation scenarios, varying the quantity of water delivered (ambient, 40% or 60% reduction) and watering frequency (40% reduction in quantity, applied 50% or 25% of ambient frequency), and the effects on growth and leaf Si concentrations of a barley landrace and cultivar were determined.ResultsReductions in the quantity of water reduced plant growth and yield, whereas reducing the watering frequency had little impact on growth, and in some cases partially ameliorated the negative effects of drought. Reductions in quantity of water lowered leaf Si concentrations in both the cultivar and landrace, although this effect was alleviated under the drought/deluge watering regime. The landrace had greater leaf Si concentration than the cultivar regardless of watering regime, and under ambient watering deposited Si in all cells between trichomes, whereas the cultivar exhibited gaps in Si deposition.ConclusionThe impact of future reductions in rainfall on barley productivity will depend upon how the water is delivered, with drought/deluge events likely to have smaller effects on yield and on Si uptake than continuous drought.

Elevated atmospheric CO2 changes defence allocation in wheat but herbivore resistance persists.

Predicting how plants allocate to different anti-herbivore defences in response to elevated carbon dioxide (CO2) concentrations is important for understanding future patterns of crop susceptibility to herbivory. Theories of defence allocation, especially in the context of environmental change, largely overlook the role of silicon (Si), despite it being the major anti-herbivore defence in the Poaceae. We demonstrated that elevated levels of atmospheric CO2 (e[CO2]) promoted plant growth by 33% and caused wheat (Triticum aestivum) to switch from Si (-19%) to phenolic (+44%) defences. Despite the lower levels of Si under e[CO2], resistance to the global pest Helicoverpa armigera persisted; relative growth rates (RGRs) were reduced by at least 33% on Si-supplied plants, irrespective of CO2 levels. RGR was negatively correlated with leaf Si concentrations. Mandible wear was c. 30% higher when feeding on Si-supplemented plants compared to those feeding on plants with no Si supply. We conclude that higher carbon availability under e[CO2] reduces silicification and causes wheat to increase concentrations of phenolics. However, Si supply, at all levels, suppressed the growth of H. armigera under both CO2 regimes, suggesting that shifts in defence allocation under future climate change may not compromise herbivore resistance in wheat.

Increase in foliar silicon content reduces defoliation by Spodoptera frugiperda (Smith) (Lepidoptera: Noctuidae) in maize

The effect of silicon (Si) on fitness and leaf consumption of the fall armyworm (FAW), Spodoptera frugiperda (Smith), has been observed in the laboratory, but the effects of Si on damage or defoliation reduction have not yet been determined under field conditions. Here, we evaluated the effects of amendment of soil with Si on defoliation by FAW and crop yield in maize. Two field experiments were carried out, with four doses of Si and a control (no Si amendment). Experiment #1 evaluated 150, 300, 450, and 600 kg of total Si·ha-1, with natural or manual infestation with FAW eggs, while Experiment #2 tested 600, 800, 1,000 and 1,200 kg of total Si·ha-1, with natural or manual infestation with FAW larvae. Defoliation reduction was observed with Si increasing in both Experiments #1 and #2. However, reduction was not observed in all evaluations, and it was dependent of infestation (natural and manual) and the time since manual infestation. Reduction in defoliation began at 600 kg Si·ha-1. In contrast, plant Si content increased linearly with increasing soil Si application. No effect of Si application was observed on maize kernel yield. The increase in leaf Si content reduced FAW defoliation, but it did not affect maize yield from 600 to 1,200 kg·ha-1 of Si. Amendment of soil with Si can be used as a strategy to optimize integrated pest management of FAW in maize.

Siliceous and non‐nutritious: Nitrogen limitation increases anti‐herbivore silicon defences in a model grass

Abstract Silicon (Si) accumulation alleviates a diverse array of environmental stresses in many plants, including conferring physical resistance against insect herbivores. It has been hypothesised that grasses, in particular, utilise ‘low metabolic cost’ Si for structural and defensive roles under nutrient limitation. While carbon (C) concentrations often negatively correlate with Si concentrations, the relationship between nitrogen (N) status and Si is more variable. Moreover, the impacts of N limitation on constitutive physical Si defences (e.g. silica and prickle cells) against herbivores are unknown. We determined how N limitation affected Si deposition in the model grass Brachypodium distachyon and how changes in these constitutive defences impacted insect herbivore Helicoverpa armigera growth rates. We used scanning electron microscopy (SEM) and energy dispersive X‐ray spectrometry in conjunction with X‐ray mapping (XRM) to quantify physical structures on leaves and determine Si deposition patterns. We also determined how N limitation and Si supply impacted the jasmonic acid (JA) pathway, the master regulator of induced defences against arthropod herbivores. N limitation reduced shoot growth by over 40%, but increased root mass (+21%), leaf Si concentrations (+50%) and the density of silica (+28%) and flattened prickle (+76%) cells. Energy‐dispersive X‐ray spectroscopy and XRM established that Si was being deposited in these structures, together with hooked prickle cells and macro‐hairs. Herbivore relative growth rates (RGR) were more than 115% lower in Si‐supplied plants compared to plants without Si supply and negatively correlated with leaf Si concentration and silica cell density. RGR was further reduced by N limitation and positively correlated with leaf N concentrations. Increases in JA concentrations following induction of the JA pathway were at least doubled by N limitation. Synthesis. Si accumulation and deposition were highly regulated by N availability, with N limitation promoting both constitutive Si physical defences and induction of the JA defensive pathway, in line with the resource availability hypothesis. These results indicate that grasses use ‘low‐cost Si’ when resources are limited and suggest that plant productivity may benefit from optimising conventional fertilisers and Si fertilisation.

Leaf silicon accumulation rates in relation to light environment and shoot growth rates in paper mulberry (Broussonetia papyrifera, Moraceae).

While increasing numbers of studies report wide variations of leaf silicon (Si) accumulation among plant species, within-species variations of leaf Si accumulation have scarcely been examined for tree species. As in crop plants, environmental factors that affect transpiration rates may influence passive transpiration-dependent transport of Si uptake in trees. Here, we tested a hypothesis that leaf Si accumulation rate should be higher in shoots that receive more light and thus achieve faster growth, using Broussonetia papyrifera, a pioneer tree species with successive leaf production and Si accumulation with leaf age. We marked individual leaves weekly throughout the growing season (June-September), and measured Si concentration and light availability in relation to the chronosequence of leaf age in September. In shoots that continued growing and successively produced leaves throughout the growing season, leaf Si content increased linearly with leaf age. In support of our hypothesis, leaf Si accumulation rate varied widely among shoots with positive correlations with shoot growth and light availability. In conclusion, both leaf age and microenvironment affect within-species variations in leaf Si concentration of this species, a moderate Si accumulator.

Silicon-Mediated Enhancement of Herbivore Resistance in Agricultural Crops.

Silicon (Si) is a beneficial mineral that enhances plant protection against abiotic and biotic stresses, including insect herbivores. Si increases mechanical and biochemical defenses in a variety of plant species. However, the use of Si in agriculture remains poorly adopted despite its widely documented benefits in plant health. In this study, we tested the effect of Si supplementation on the induction of plant resistance against a chewing herbivore in crops with differential ability to accumulate this element. Our model system comprised the generalist herbivore fall armyworm (FAW) Spodoptera frugiperda and three economically important plant species with differential ability to uptake silicon: tomato (non-Si accumulator), soybean, and maize (Si-accumulators). We investigated the effects of Si supply and insect herbivory on the induction of physical and biochemical plant defenses, and herbivore growth using potted plants in greenhouse conditions. Herbivory and Si supply increased peroxidase (POX) activity and trichome density in tomato, and the concentration of phenolics in soybean. Si supplementation increased leaf Si concentration in all plants. Previous herbivory affected FAW larval weight gain in all plants tested, and the Si treatment further reduced weight gain of larvae fed on Si accumulator plants. Notably, our results strongly suggest that non-glandular trichomes are important reservoirs of Si in maize and may increase plant resistance to chewing herbivores. We conclude that Si offers transient resistance to FAW in soybean, and a more lasting resistance in maize. Si supply is a promising strategy in management programs of chewing herbivores in Si-accumulator plants.

Soil and climate affect foliar silicification patterns and silica-cellulose balance in sugarcane (Saccharum officinarum)

Silicon (Si) has beneficial effects in a variety of plant species and environments. Soil and climate affect silica accumulation in given plant species, but their roles on foliar silicification patterns and balance between silica and C-rich biopolymers as structural components is poorly known. We studied silica deposition in situ in sugarcane leaves collected in three tropical environments differing in soil and climate. Plant silica deposits were physically extracted from leaves through wet digestion. Leaves were observed and mapped for Si by ESEM-EDX. The C-rich biopolymers in leaves were determined by the Van Soest method. Silicon accumulation in the leaves was related to bioavailable Si in soil and plant transpiration. Epidermal silica deposits were either limited to silica cells as dumbbell-shaped phytoliths, or expanded to long and short cells arranged in prominent veins fully silicified, depending on whether the leaf Si concentration was lowest or highest. The size of silica deposits increased with increasing leaf Si through an increasing number of conjoined silicified cells. Leaf ash-free cellulose and Si concentrations were negatively correlated. Soil and climate impact markedly the magnitude of foliar silicification, with possibly significant impact on mechanical properties and Si-related plant functions. Environmental conditions further impact the counterbalance between silica and cellulose as leaf structural components via different levels of Si accumulation.