Anti‐herbivore silicon defences in a model grass are greatest under Miocene levels of atmospheric CO ₂

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