Elevated CO2 alters grain quality of two bread wheat cultivars grown under different environmental conditions

• Two wheat cultivars were grown at elevated [CO2], under six environmental conditions.
• Elevated [CO2] effect on grain protein, Zn, Mg and Na were varied with environmental conditions.
• Found no clear relationship of relative effects of e[CO2] on grain protein with any of the environmental indices tested.
• Relative effects of e[CO2] on grain yield and protein were strongly correlated.
• These findings suggest that primary factor determine grain protein concentration under e[CO2] is yield dilution.

Bread wheat (Triticum aestivum L. cv. Yitpi and cv. Janz) was grown under field conditions in the Australian Grains Free-Air CO2 Enrichment (AGFACE) facility. Ambient [CO2] (a[CO2], ∼384 μmol mol−1) and elevated [CO2] (e[CO2], ∼550 μmol mol−1) were combined with two soil water levels (rain-fed and irrigated) and two times of sowing (TOS) in three consecutive years to provide six environments (2007-TOS1, 2007-TOS2, 2008-TOS1, 2008-TOS2, 2009-TOS1, 2009-TOS2). Grain samples were assessed for a range of physical, nutritional and dough rheological properties. The effect of e[CO2] on thousand grain weight (TGW) was significantly different in each growing environment: TGW was significantly increased under e[CO2] only at 2007-TOS2 (by 5%), 2009-TOS1 (by 5%) and 2009-TOS2 (by 15%) but not significantly changed under other conditions. The magnitude of reduction of grain protein concentration at e[CO2] differed among the growing environments but was highly correlated with the percentage yield stimulation under e[CO2] (r2 = 0.91) suggesting that grain protein concentration under e[CO2] was diluted by increased yield. Across all treatments, grain nutrient concentration was significantly reduced by e[CO2] for Fe (3.9%, 6.2%), Cu (2.2%, 3.4%), Zn (5.9%, 5.7%), Ca (5.6%, 7.3%), Mg (5.6%, 5.8%), Na (21.2%, 30.4%), S (4.4%, 4.4%), P (4.1%, 3.2%) in cv. Yitpi and Janz, respectively. Effects of e[CO2] on grain Zn, Mg and Na concentrations were dependent on the growing environment. Relative reduction of grain S, Fe, Mg, Zn, P at e[CO2] were significantly correlated with grain yield stimulation at e[CO2]. Reductions of these nutrients under e[CO2] were not fully explained by biomass dilution as the relationships differed for each nutrient. Under e[CO2], flour yield of cv. Janz was increased but that of cv. Yitpi was not changed. Even though grain protein concentrations of both cultivars were similar at e[CO2], bread volume as inferred indirectly by dough rheology parameters was 12% greater for cv. Janz (185 ± 5 cm3) than cv. Yitpi (162 ± 4 cm3) at e[CO2]. This disparity may be related to the compositional changes in wheat flour protein at e[CO2], suggesting that future breeding and adaptation strategies to improve the grain quality under e[CO2] should consider the prevailing hydro-thermal conditions.