Grain Quality and Cropping System Changes from Reduced Wheat Nitrogen Content Under Elevated CO2.

Increasing atmospheric CO₂ concentration causes fundamental changes to plant physiology, including reductions in leaf nitrogen (N), grain protein and micronutrient contents. These changes impact both crop production and food quality. In the Australian Grains Free Air CO₂ Enrichment (AGFACE) facility in Horsham, Victoria Australia, nine years of measurements have encompassed a range of abiotic and biotic environments and cultivars to better understand the impacts of these changes on food production, principally in wheat. Under elevated CO₂ (eCO₂) results showed that reduced leaf N concentrations caused increased leaf feeding by the aphid host for barley yellow dwarf virus, potentially increasing virus spread by infected aphids and lowering yields. Lower N levels in wheat straw from eCO₂ caused C:N ratios to increase by 8% across all cultivars and environments, although under low soil N, C:N of straw increased by over 60%. This may increase immobilization of soil N after straw incorporation, leading to greater requirement for more fertilizer inputs. Fe and Zn grain concentrations were reduced by 10 and 5%, respectively, important for human nutrition where deficiency occurs. Mean grain protein concentration under eCO₂ reduced by 6% across all environments, leading to 35% downgrading of wheat grain from one quality grade to the next lower grade, which could impact prices paid to farmers. Bread quality is reduced through changes in factors such as dough extensibility but even after allowing for reduction of grain protein due to eCO₂, there are additional variety-dependent effects on other grain quality parameters. Reversing the reduction in plant N and grain protein and micronutrients may only be possible through breeding for increased nitrogen efficiency and biofortification. But, selecting cultivars for baking quality at a given protein content may be a more effective measure of breeding success. This would also address the needs of those where declines in grain protein would have a detrimental nutritional impact.