The atmospheric CO2 concentration ([CO2]) is rapidly increasing, and this may have substantial impact on how plants allocate metabolic resources. A thorough understanding of allocation priorities can be achieved by modifying [CO2] over a large gradient, including low [CO2], thereby altering plant carbon (C) availability. Such information is of critical importance for understanding plant responses to global environmental change. We quantified the percentage of daytime whole-plant net assimilation (A) allocated to night-time respiration (R), structural growth (SG), nonstructural carbohydrates (NSC) and secondary metabolites (SMs) during 8 weeks of vegetative growth in winter wheat (Triticum aestivum) growing at low, ambient and elevated [CO2] (170, 390 and 680 ppm). R/A remained relatively constant over a large gradient of [CO2]. However, with increasing C availability, the fraction of assimilation allocated to biomass (SG + NSC + SMs), in particular NSC and SMs, increased. At low [CO2], biomass and NSC increased in leaves but decreased in stems and roots, which may help plants achieve a functional equilibrium, that is, overcome the most severe resource limitation. These results reveal that increasing C availability from rising [CO2] releases allocation constraints, thereby allowing greater investment into long-term survival in the form of NSC and SMs.