Elevated carbon-dioxide effects on wheat grain quality differed under contrasting nitrogen and phosphorus fertiliser supply

Atmospheric carbon-dioxide concentration ([CO₂]) is increasing rapidly, but its interactions with nitrogen (N) and phosphorus (P) fertiliser on wheat grain quality are not well understood. We investigated the effects of ambient CO₂ (aCO₂; ∼410 ppm) and elevated CO₂ (eCO₂; 760 ppm) on crop harvest index (CHI), nutrient harvest index (NuHI), shoot macro-nutrient content and grain macro-nutrient concentration of wheat grown under two contrasting amounts of N (0.5 and 6 mol m⁻³ NO₃⁻ N) and P (10 and 250 mmol P m⁻³) fertiliser supply (low and optimum, respectively). Our results highlighted interactions between [CO₂] and N and P fertiliser supply for the shoot biomass at anthesis and straw biomass at harvest maturity. This was because biomass yield did not respond to CO₂ level when fertiliser was deficient. However, shoot and straw yield increased (10.0–-34.0%) with increasing [CO₂] at optimum fertiliser rates. Across experiments, grain yield increased (15.6%) with increasing [CO₂], which resulted in grain nutrient concentration decreasing (3.0–-13.0%) with increasing [CO₂]. This was attributed to nutrient 'dilution' due to increased carbohydrate content in the grain. Overall, fertiliser supply impacted crop responses more than CO₂ treatments, and the impact was greater under N than P deficiency. This was reflected through conservative values for CHI, thousand grain weight and NuHIs suggesting plants allocated biomass and nutrients at similar rates for vegetative and reproductive organs independent of [CO₂].