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 (aCO(2); similar to 410 ppm) and elevated CO2 (eCO(2); 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].