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• During drought carbohydrates accumulate while monoterpenes remain constant. • During carbon limitation growth and carbohydrates decrease more than monoterpenes. • During drought newly-assimilated carbon is allocated to monoterpenes in balance with structural biomass and carbohydrates. • During carbon limitation newly-assimilated carbon is preferentially allocated to monoterpenes. Secondary metabolites play important roles in plant responses to environmental stress but may also represent a large carbon (C) cost, resulting in trade-offs with other C sinks like growth and storage. However, it remains uncertain how such trade-offs may vary with changes in resource availability including water and CO 2 availability. We conducted a glasshouse experiment with peppermint (Mentha x piperita L.) exposed to four treatments: control (sufficient irrigation and near-ambient CO 2), water deficit (50% irrigation and near-ambient CO 2), CO 2 limitation (sufficient irrigation and below-ambient CO 2) and the combination of water and CO 2 deficits. Continuous 13CO 2 labelling was used to trace allocation of newly-assimilated C. Concentrations of soluble sugars significantly increased under water deficit but decreased along with aboveground biomass under low CO 2 , while monoterpene concentrations remained relatively constant, independent of treatments. Under water deficit, there were no differences in allocation of new vs old C to monoterpenes production, structural growth and storage; plants grown under low CO 2 even invested proportionally more newly-assimilated C for monoterpenes production, suggesting a preferential allocation to defense at the expense of growth. We concluded that C allocation to monoterpenes is actively regulated in coordination with growth and storage under water and C stresses, consistent with an optimal defense strategy to protect young tissues. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]