The mutation in wheat increases grain size under ambient and elevated CO but not grain yield due to trade-off between grain size and grain number.

Grain size is potentially yield determining in wheat, controlled by the ubiquitin pathway and negatively regulated by ubiquitin receptor DA1. We analyzed whether increased thousand grain weight in wheat mutant is translated into higher grain yield and whether additional carbon provided by elevated (e)CO can be better used by the , displaying higher grain sink strength and size. Yield-related, biomass, grain quality traits, and grain dimensions were analyzed by two-factorial mixed-model analysis, regarding genotype and eCO. increased grain size but reduced spikes and grains per plant, grains per spike, and spikelets per spike, independent of eCO treatment, leaving total grain yield unchanged. eCO increased yield and grain number additively and independently of but did not overcome the trade-off between grain size and number observed for . eCO but not impaired grain quality, strongly decreasing concentrations of several macroelement and microelement. In conclusion, intrinsic stimulation of grain sink strength and grain size, achieved by , is not benefitting total yield unless trade-offs between grain size and numbers can be overcome. The results reveal interactions of yield components in -wheat under ambient and eCO, thereby uncovering limitations enhancing wheat yield potential.