Grasses are hyper-accumulators of silicon (Si), which they acquire from the soil and deposit in tissues to resist environmental stresses. Given the high metabolic costs of herbivore defensive chemicals and structural constituents (e.g. cellulose), grasses may substitute Si for these components when carbon is limited. Indeed, high Si uptake grasses evolved in the Miocene when atmospheric CO 2 concentration was much lower than present levels. It is, however, unknown how pre-industrial CO 2 concentrations affect Si accumulation in grasses. Using Brachypodium distachyon , we hydroponically manipulated Si-supply (0.0, 0.5, 1, 1.5, 2 mM) and grew plants under Miocene (200 ppm) and Anthropocene levels of CO 2 comprising ambient (410 ppm) and elevated (640 ppm) CO 2 concentrations. We showed that regardless of Si treatments, the Miocene CO 2 levels increased foliar Si concentrations by 47% and 56% relative to plants grown under ambient and elevated CO 2 , respectively. This is owing to higher accumulation overall, but also the reallocation of Si from the roots into the shoots. Our results suggest that grasses may accumulate high Si concentrations in foliage when carbon is less available (i.e. pre-industrial CO 2 levels) but this is likely to decline under future climate change scenarios, potentially leaving grasses more susceptible to environmental stresses.

Predicting how plants allocate to different anti-herbivore defences in response to elevated carbon dioxide (CO 2 ) 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 CO 2 (e[CO 2 ]) 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[CO 2 ], resistance to the global pest Helicoverpa armigera persisted; relative growth rates (RGRs) were reduced by at least 33% on Si-supplied plants, irrespective of CO 2 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[CO 2 ] reduces silicification and causes wheat to increase concentrations of phenolics. However, Si supply, at all levels, suppressed the growth of H. armigera under both CO 2 regimes, suggesting that shifts in defence allocation under future climate change may not compromise herbivore resistance in wheat.

Plants are often sequentially attacked by multiple herbivores; feeding by one herbivore can alter host plant quality that affects the performance of subsequent herbivores. Previous studies suggest that silicon (Si) is a highly inducible defence in grasses (Poaceae) following herbivory, so could mediate such temporally separated insect herbivore interactions. Elevated atmospheric CO2 concentrations (eCO2), in contrast, often reduce Si accumulation, which potentially weakens this interaction. We examined the effects of prior feeding by Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) on the performance of crickets, Acheta domesticus (L.) (Orthoptera: Gryllidae), subsequently feeding on the same plant. We used Brachypodium distachyon (L.) P. Beauv. grass supplemented or non‐supplemented with Si and grown in the glasshouse maintained at ambient (aCO2) or eCO2 concentrations. As hypothesized, prior feeding by caterpillars induced Si accumulation. Despite eCO2 reducing Si accumulation, initial herbivore induction of Si negated the effects of eCO2. Both, previous caterpillar herbivory and Si supplementation reduced cricket performance. Si induction by successive herbivory was additive. Plant biomass was similar in plants attacked by caterpillars alone or both herbivores, suggesting that initial Si induction by caterpillars deterred feeding by crickets. Our results demonstrate that Si induction by one herbivore negatively impacts successive herbivores, as has been demonstrated for secondary metabolite‐mediated interactions. Uniquely, however, Si induction is an irreversible defence and potentially a stronger, or perhaps longer‐lasting mediator of such herbivore interactions in some plant taxa.