Elevated atmospheric CO2changes defence allocation in wheat but herbivore resistance persists

Abstract

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 thePoaceae. 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 pestHelicoverpa armigerapersisted; relative growth rates (RGRs) were reduced by at least 33% on Si-supplied plants, irrespective of CO2levels. 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 ofH. armigeraunder both CO2regimes, suggesting that shifts in defence allocation under future climate change may not compromise herbivore resistance in wheat.