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 (CO 2 ) 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 CO 2 (e[CO 2 ]) 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[CO 2 ], resistance to the global pest Helicoverpa armigera persisted; relative growth rates (RGRs) were reduced by at least 33% on Si-supplied plants, irrespective of CO 2 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[CO 2 ] reduces silicification and causes wheat to increase concentrations of phenolics. However, Si supply, at all levels, suppressed the growth of H. armigera under both CO 2 regimes, suggesting that shifts in defence allocation under future climate change may not compromise herbivore resistance in wheat.

Elevated atmospheric CO2 suppresses silicon accumulation and exacerbates endophyte reductions in plant phosphorus

“…With greater C availability under eCO 2 , this may be less advantageous and plants may, for example, produce more C‐based phenolic defences rather than accumulate Si (e.g. Johnson et al, 2022).…”

“…Herbivory, for example, is widely reported to rapidly increase Si uptake as a form of induced defence (Massey et al, 2007; Waterman et al, 2021). There is emerging evidence that eCO 2 reduces Si accumulation in many grass species due, in part, to concurrent increases in C concentrations (Biru et al, 2021; Johnson et al, 2022; Johnson & Hartley, 2018). The negative relationship between C and Si in plants may be due to “stoichiometric dilution” whereby an increase in one element, by definition, necessitates lower levels of other constituents (Hodson & Guppy, 2022; Quigley et al, 2020).…”

Contrasting impacts of herbivore induction and elevated atmospheric CO2 on silicon defences and consequences for subsequent herbivores

“…These effects are likely due to carbon (C) being more available under eCO 2 and it being less advantageous to accumulate Si (Johnson & Hartley, 2018). Several studies report negative correlations between Si and C concentrations, including structural carbon (e.g., cellulose and lignin) (Raven, 1983; Schoelynck et al, 2010; Klotzbucher et al, 2018) and defensive compounds (e.g., phenolics) (Cooke & Leishman, 2012; Frew et al, 2016; Johnson & Hartley, 2018; Johnson et al, 2022).…”

The Reciprocal Effect of Elevated CO2 and Drought on Wheat-Aphid Interaction System

“…Elevated CO 2 changes the primary and secondary metabolism of plants by stimulating the photosynthetic rate (Zavala et al, 2017 ), which not only resulted in increased carbon:nitrogen ratio, but also changed the insect resistance in plants (Zavala et al, 2013 ; Sun et al, 2015 , 2016 ; Guo et al, 2017 ; Johnson et al, 2022 ). Meanwhile, drought stress drives plants to develop a number of physiological responses as well, such as closing stoma, increasing carbohydrate and amino acid accumulation, and activating phytohormone signaling pathways (Mcdowell et al, 2008 ; Lee and Luan, 2012 ; Danquah et al, 2014 ; Ullah et al, 2018 ; Xie et al, 2021 ), which subsequently lead to the change in plant nutritional quality (water content and C/N ratio) as well as plant chemical defenses to insects (English-Loeb et al, 1997 ; Guo et al, 2016 ; Castagneyroi et al, 2018 ; Suarez-Vidal et al, 2019 ).…”