The grain protein yield of barley under future drought is modified by the joint action of elevated CO2 and temperature.

Nitrogen is the pivotal macronutrient for grain protein synthesis, which is important in human nutrition and the establishment of derived products. However, nitrogen metabolism in plants is likely to be susceptible to abiotic factors such as those derived from climate change: drought, elevated [CO2], and temperature. How the triple interaction of these factors will affect nitrogen metabolism and grain quality of cereals is unknown. This study aimed to determine the response of nitrogen metabolism in barley-one of the temperate cereals most tolerant to abiotic stresses-to the triple interaction during its whole life span. Our results pointed out a growth stage-dependent response on final nitrogen status. At the vegetative stage, the nitrogen assimilation capacity was boosted and matched with the biomass gain without altering the nitrogen status. However, at the anthesis and maturity stages, the nitrogen status of plants and grains was reduced. A non-overlapping effect of biomass dilution and lower mass flow is highlighted, while lower photorespiration activity cannot be completely ruled out. The elevated [CO2] is the main driver regulating nitrogen metabolism at the physiological level under future drought conditions, whereas elevated temperature hampers grain formation.