Silicon (Si) has an important role in mitigating diverse biotic and abiotic stresses in plants, mainly via the silicification of plant tissues. Environmental changes such as atmospheric CO 2 concentrations may affect grass Si concentrations which, in turn, can alter herbivore performance. We recently demonstrated that preindustrial atmospheric CO 2 increased Si accumulation in Brachypodium distachyon grass, yet the patterns of Si deposition in leaves and whether this affects insect herbivore performance remains unknown. Moreover, it is unclear whether CO 2 -driven changes in Si accumulation are linked to changes in gas exchange (e.g. transpiration rates). We therefore investigated how pre-industrial (reduced; rCO 2 , 200 ppm), ambient (aCO 2 , 410 ppm) and elevated (eCO 2 , 640 ppm) CO 2 concentrations, in combination with Si-treatment (Si+ or Si−), affected Si accumulation in B.distachyon and its subsequent effect on the performance of the global insect pest, Helicoverpa armigera. rCO 2 increased Si concentrations by 29% and 36% compared to aCO 2 and eCO 2 respectively. These changes were not related to observed changes in gas exchange under different CO 2 regimes, however. The increased Si accumulation under rCO 2 decreased herbivore relative growth rate (RGR) by 120% relative to eCO 2, whereas rCO 2 caused herbivore RGR to decrease by 26% compared to eCO 2 . Si supplementation also increased the density of macrohairs, silica and prickle cells, which was associated with reduced herbivore performance.There was a negative correlation among macrohair density, silica cell density, prickle cell density and herbivore RGR under rCO 2 suggesting that these changes in leaf surface morphology were linked to reduced performance under this CO 2 regime. To our knowledge, this is the first study to demonstrate that increased Si accumulation under pre-industrial CO 2 reduces insect herbivore performance. Contrastingly, we found reduced Si accumulation under higher CO 2 , which suggests that some grasses may become more susceptible to insect herbivores under projected climate change scenarios.

1. Silicon (Si) is a beneficial nutrient that has been reported to ameliorate many abiotic and biotic stresses in plants, including insect herbivory. Insect herbivory has been shown to induce Si defences in plants, although the magnitude and nature of induction remain largely ambiguous. In particular, it is unclear whether herbivore induction of Si defences is confined to attacked tissues (local) or occurs elsewhere in the plant (systemic). 2. We grew cucumber, Cucumis sativus L. plants (var. Burpless F1 and Beit Alpha), an intermediate Si accumulator, hydroponically under Si-supplemented or Si-free conditions and measured the level of Si induction caused by a polyphagous chewing insect, the cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae). We also examined the impacts of Si on insect performance by conducting in vitro feeding assays on excised leaves (ex situ) and intact leaves on plants (in situ). 3. Herbivory significantly increased Si accumulation both locally in attacked leaves (21% increase in Beit Alpha and 17% in Burpless F1) and systemically in non-attacked leaves (19% increase in Beit Alpha and 10% in Burpless F1). Si supplementation significantly increased % foliar Si and C:N ratio, while significantly decreasing larval relative consumption (RC) and relative growth rate (RGR) in the in situ assays. In ex situ assays, however, Si only reduced larval RGR when fed on Beit Alpha plants. 4. Our results confirm that Si-based defences can also operate in moderate Si-accumulating plants and, for the first time, that insect herbivory induces systemic Si accumulation equivalently between plant varieties.

Many temperate grasses are both hyper‐accumulators of silicon (Si) and hosts of Epichloë fungal endophytes, functional traits which may alleviate environmental stresses such as herbivore attack. Si accumulation and endophyte infection may operate synergistically, but this has not been tested in a field setting, nor in the context of changing environmental conditions. Predicted increases in atmospheric CO2 concentrations can affect both Si accumulation and endophyte function, but these have not been studied in combination. We investigated how elevated atmospheric CO2 (eCO2), Si supplementation, endophyte‐presence and insect herbivory impacted plant growth, stoichiometry (C, N, P and Si), leaf gas exchange (rates of photosynthesis, stomatal conductance, transpiration rates) and endophyte production of anti‐herbivore defences (alkaloids) of an important pasture grass (tall fescue; Lolium arundinaceum) in the field. eCO2 and Si supplementation increased shoot biomass (+52% and +31%, respectively), whereas herbivory reduced shoot biomass by at least 35% and induced Si accumulation by 24%. Shoot Si concentrations, in contrast, decreased by 17%–21% under eCO2. Si supplementation and herbivory reduced shoot C concentrations. eCO2 reduced shoot N concentrations which led to increased shoot C:N ratios. Overall, shoot P concentrations were 26% lower in endophytic plants compared to non‐endophytic plants, potentially due to decreased mass flow (i.e. observed reductions in stomatal conductance and transpiration). Alkaloid production was not discernibly affected by any experimental treatment. The negative impacts of endophytes on P uptake were particularly strong under eCO2. We show that eCO2 and insect herbivory reduce and promote Si accumulation, respectively, incorporating some field conditions for the first time. This indicates that these drivers operate in a more realistic ecological context than previously demonstrated. Reduced uptake of P in endophytic plants may adversely affect plant productivity in the future, particularly if increased demand for P due to improved plant growth under eCO2 cannot be met. Read the free Plain Language Summary for this article on the Journal blog.