Wheat (Triticum aestivum L.) production may be affected by the future climate, but the impact of the combined increases in atmospheric CO2 concentration, temperature and incidence of drought that are predicted has not been evaluated. The combined effect of elevated CO2, high temperature and terminal drought on biomass accumulation and grain yield was evaluated in vigorous (38–19) and nonvigorous (Janz) wheat genotypes grown under elevated CO2 (700 µL L–1) combined with temperatures 2°C, 4°C and 6°C above the current ambient temperature. Terminal drought was induced in all combinations at anthesis in a split-plot design to test whether the effect of elevated CO2 combined with high temperature ameliorates the negative effects of terminal drought on biomass accumulation and grain yield. Biomass and grain yield were enhanced under elevated CO2 with 2°C above the ambient temperature, regardless of the watering regimen. The combinations of elevated CO2 plus 4°C or 6°C above the ambient temperature did not enhance biomass and grain yield, but tended to decrease them. The reductions in biomass and grain yield (45–50%) caused by terminal drought were less severe (21–28%) under elevated CO2 with 2°C above the ambient temperature. The amelioration resulted from a 63% increase in the rate of leaf net photosynthesis in 38–19 and a 39% increase in tillering and leaf area in Janz. The contrasting responses and phenological development of these two genotypes to the combination of elevated CO2, temperature and terminal drought, and the possible influences on their source–sink relationships are discussed.

Elevated CO2 often increases grain yield in wheat by enhancing grain number per ear, which can result from an increase in the potential number of florets or a reduction in the death of developed florets. The hypotheses that elevated CO2 reduces floret death rather than increases floret development, and that grain size in a genotype with more grains per unit area is limited by the rate of grain filling, were tested in a pair of sister lines contrasting in tillering capacity (restricted- vs. free-tillering). The hypotheses were tested under elevated CO2, combined with +3°C above ambient temperature and terminal drought, using specialized field tunnel houses. Elevated CO2 increased net leaf photosynthetic rates and likely the availability of carbon assimilates, which significantly reduced the rates of floret death and increased the potential number of grains at anthesis in both sister lines by an average of 42%. The restricted-tillering line had faster grain-filling rates than the free-tillering line because the free-tillering line had more grains to fill. Furthermore, grain-filling rates were faster under elevated CO2 and +3°C above ambient. Terminal drought reduced grain yield in both lines by 19%. Elevated CO2 alone increased the potential number of grains, but a trade-off in yield components limited grain yield in the free-tillering line. This emphasizes the need for breeding cultivars with a greater potential number of florets, since this was not affected by the predicted future climate variables.