Isotope labeling reveals contribution of newly fixed carbon to carbon storage and monoterpenes production under water deficit and carbon limitation

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 CO2 availability. We conducted a glasshouse experiment with peppermint (Mentha x piperita L.) exposed to four treatments: control (sufficient irrigation and near-ambient CO2), water deficit (50% irrigation and near-ambient CO2), CO2 limitation (sufficient irrigation and below-ambient CO2) and the combination of water and CO2 deficits. Continuous 13CO2 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 CO2, 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 CO2 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.