Trade-offs between biomass and bioactive compounds in Silybum marianum under elevated CO2 and water deficit across genotypes

Climate change, characterized by rising atmospheric CO 2 and increasing drought stress, significantly affects plant growth and metabolism. Milk thistle ( Silybum marianum ), valued for its silymarin-rich seeds, is an important medicinal plant sensitive to these environmental changes. This study evaluated four genotypes—Hungary, Isfahan, Omidiyeh, and Charam—with three soil moisture levels (well-watered, moderate, and severe water deficit stress) in a factorial design with four replications under two distinct CO 2 environments (ambient, 404 ± 24 μmol mol -1 CO 2 , and elevated, 702 ± 51 μmol mol -1 CO 2 ) imposed using two fixed open-top chambers (OTCs). Elevated CO 2 potentially enhanced growth and yield traits but was associated with declines in photosynthetic pigments and antioxidant enzyme activities. Genotype-specific responses were evident: Hungary and Isfahan showed the greatest seed weight, oil, and silymarin production under elevated CO 2 and in well-watered and moderate drought conditions; Charam maintained higher chlorophyll a and shoot biomass particularly under severe drought stress; Omidiyeh accumulated the most root biomass and proline, aiding drought tolerance under combined elevated CO 2 and severe drought stress. Drought stress increased total phenolics, flavonoids, and antioxidant activity but reduced oil content, silymarin yield, and photosystem II efficiency ( Fv/Fm ). Multivariate analysis highlighted Charam’s ability to sustain leaf water content and flavonoid production, while Omidiyeh demonstrated stronger antioxidant defenses under combined elevated CO 2 and drought. These genotype-specific adaptations reveal a growth-defense trade-off under elevated CO 2 and drought, offering promising targets for breeding milk thistle varieties that balance biomass, medicinal compounds, and stress tolerance in a changing climate. • Elevated CO 2 may promote biomass and yield while potentially reducing protective metabolites • Drought stress boosts antioxidant defenses and silymarin content • Genotypes vary in response to elevated CO 2 and drought, with distinct seed yield, chlorophyll, and proline accumulation • Elevated CO 2 appears to mitigate drought impact on seed weight