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Predicting how plants allocate to different anti-herbivore defences in response to elevated carbon dioxide (CO2) 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 CO2 (e[CO2]) 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[CO2], resistance to the global pest Helicoverpa armigera persisted; relative growth rates (RGRs) were reduced by at least 33% on Si-supplied plants, irrespective of CO2 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[CO2] reduces silicification and causes wheat to increase concentrations of phenolics. However, Si supply, at all levels, suppressed the growth of H. armigera under both CO2 regimes, suggesting that shifts in defence allocation under future climate change may not compromise herbivore resistance in wheat. [ABSTRACT FROM AUTHOR]

Silicon (Si) can adversely affect insect herbivores, particularly in plants that evolved the ability to accumulate large quantities of Si. Very rapid herbivore‐induced accumulation of Si has recently been demonstrated, but the level of protection against herbivory this affords plants remains unknown. Brachypodium distachyon, a model Si hyperaccumulating grass, was exposed to the chewing herbivore, Helicoverpa armigera, and grown under three conditions: supplied Si over 34 d (+Si), not supplied Si (−Si), or supplied Si once herbivory began (−Si → +Si). We evaluated the effectiveness of each Si treatment at reducing herbivore performance and measured Si‐based defenses and phenolics (another form of defense often reduced by Si). Although Si concentrations remained lower, within 72 h of exposure to Si, −Si → +Si plants were as resistant to herbivory as +Si plants. Both +Si and −Si → +Si treatments reduced herbivore damage and growth, and increased mandible wear compared to −Si. After 6 h, herbivory increased filled Si cell density in −Si → +Si plants, and within 24 h, −Si → +Si plants reached similar filled Si cell densities to +Si plants, although decreased phenolics only occurred in +Si plants. We demonstrate that plants with short‐term Si exposure can rapidly accumulate Si‐based antiherbivore defenses as effectively as plants with long‐term exposure. [ABSTRACT FROM AUTHOR]

Altered precipitation patterns due to climate change are likely to impose water‐deficit stress in plants resulting in changes to specific leaf mass, leaf water content and chemical defences that may impact herbivorous arthropods. Grasses, in particular, accumulate large concentrations of silicon (Si) which provides physical defence against herbivores. Although Si uptake by plants may be affected by water availability, very few studies have investigated the combined effect of water‐deficit stress and Si on insect herbivore performance.We grew tall fescue Festuca arundinacea Schreb. hydroponically, with and without Si, and half of the plants were treated with 20% polyethylene glycol (PEG) to impose osmotic stress. In all, 11 leaf traits (physiological, chemical and structural) were measured, silicified phytoliths on the leaf surface were visualised using scanning electron microscopy (SEM) in conjunction with X‐ray mapping, and plants were exposed to a chewing insect herbivore [Helicoverpa armigera Hübner (Lepidoptera: Noctuidae)].Although osmotic stress was associated with changes to leaf physiological and chemical traits, including increased specific leaf mass, decreased leaf relative water content and increased leaf nitrogen (N), there was no significant effect on H. armigera relative growth rate (RGR). However, Si reduced RGR of H. armigera by 80%–98% while generating few changes to physiological and chemical leaf traits. Instead, the decline in RGR with Si was associated with changes to leaf structural traits, in particular, a greater density of silicified phytoliths on the leaf surface.Comparison of effect sizes indicated that leaf traits were primarily affected by osmotic stress but not Si, and that herbivore RGR was strongly negatively affected by Si but not osmotic stress. There was no interactive effect between the osmotic stress and Si treatments on H. armigera RGR or plant traits except for leaf nitrogen and phenolic concentrations. This study provides further support that Si may prove to be beneficial to plants against chewing insect pests and remains robust regardless of water‐deficit stress conditions. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]