Silicon and elevated CO2 act as parallel drivers of wheat growth and oxidative status, with divergent hormone signalling

Background and Aims Rising atmospheric CO₂ and silicon supply each alter cereal physiology, but their combined action in wheat ( Triticum aestivum L. cv. Omer) has not been resolved across growth, oxidative status, hormone signalling, metabolism, and stoichiometry. We aimed to define the combined state elevated CO₂ and silicon produce in wheat, and where the two converge, diverge, or interact. Methods Wheat was grown under a factorial combination of ambient or elevated CO₂ and with or without added silicon, replicated across two vegetative experiments, with a maturity experiment for yield and grain nitrogen and a separate imposed stress experiment. Growth, root architecture, oxidative-stress markers, phytohormones, metabolite profiles, mineral nutrients, and leaf and grain nitrogen were measured. Results Both factors increased plant size, root development, and spike mass but the combination rarely exceeding the better single factor. Oxidative damage fell under silicon at ambient CO₂ but not in elevated CO ₂ . Hormone signalling diverged: elevated CO₂ raised salicylic acid roughly two-fold, whereas silicon lowered salicylic acid and raised jasmonic acid, an induction that elevated CO₂ largely abolished. The metabolome shifted to a carbon-rich state under elevated CO₂, and both factors lowered leaf nitrogen concentration while uptake was maintained, indicating dilution by growth. Conclusions Elevated CO₂ and silicon act as parallel drivers that converge on growth and redox but oppose one another in hormone signalling. Silicon’s benefit was largest at ambient CO₂ and small once CO₂ was elevated, bearing on its value as an input under rising CO₂.