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 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 pre‐industrial 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.

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