Three genotypes of wheat grown at two CO₂ concentrations were used in a drought experiment, where water was withheld from the pots at anthesis until stomatal conductance (g_s) dropped below 10% of the control and photosynthesis (A) approached zero. The genotypes had different leaf area (Gladius < LM19 < LM62) and while photosynthesis and shoot growth were boosted by elevated CO₂, the water use and drying rate were more determined by canopy size than by stomatal density and conductance. The genotypes responded differently regarding number of fertile tillers, seeds per spike and 1000 kernel weight and, surprisingly, the largest genotype (LM62) with high water use showed the lowest relative decrease in grain yield. The maximum photochemical efficiency of photosystem II (F_v/F_m) was only affected on the last day of the drought when the stomata were almost closed although some variation in A was still seen between the genotypes. A close correlation was found between F_v/F_m and % loss of grain yield. It indicates that the precise final physiological stress level measured by F_v/F_m at anthesis/early kernel filling could effectively predict percentage final yield loss, and LM62 was slightly less stressed than the other genotypes, due to only a small discrepancy in finalising the drying period. Therefore, F_v/F_m can be used as a proxy for estimating the yield performance of wheat after severe drought at anthesis.