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.

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