Nitrogen nutrition and aspects of root growth and function of two wheat cultivars under elevated [CO₂]

Nitrogen (N) input into food production is environmentally sensitive and economically significant, making efficient N use an important goal in agricultural practice and in plant breeding. In cereals, grain N concentration [N] is an important component of grain quality and nutritional value. Increasing atmospheric CO₂ concentration [CO₂] will not only stimulate growth and yield but also diminish grain [N], raising concerns about product quality and nutritional value. In this study we investigated how differences in root structure and function and agronomic properties between two bread wheat (Triticum aestivum L.) cultivars affect N uptake and allocation to grains in a low rainfall environment, and whether such differences can indicate strategies to mitigate grain [N] decreases under increased [CO₂]. Two cultivars (‘Silverstar’ and ‘Yitpi’) were chosen for their similar phenology and yields, but ‘Silverstar’ often has lower grain [N]. A glasshouse experiment showed contrasting root structure and function strategies between the two cultivars in response to soil N: ‘Yitpi’ but not ‘Silverstar’ responded to lower soil N with increased root growth, whereas ‘Silverstar’ but not ‘Yitpi’ showed increased N uptake per unit root mass in response to lower N. When grown in the Australian Grains Free Air CO₂ Enrichment facility over multiple seasons both cultivars produced similar yields, but ‘Silverstar’ had consistently lower grain [N], smaller grains and greater harvest index. In situ N uptake measurements with ¹⁵N label showed that wheat roots can take up nitrate, ammonium and glutamine, and also confirmed differences in uptake strategies between cultivars: ‘Silverstar’ roots had significantly greater uptake capacity than ‘Yitpi’ roots for ammonium. Whilst these results suggest that different strategies in response to variations in soil N supply could be related to grain N outcomes at this field site, there was no interaction with atmospheric [CO₂] for any of the measured parameters. Regardless of cultivar, elevated [CO₂] (550 μmol mol⁻¹) increased yields and decreased grain [N], but did not affect root uptake capacities for either N form. Contrasting root uptake strategies seem unrelated to grain [N] decrease under elevated [CO₂], at least for this site.