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

Climate change, characterized by rising atmospheric CO2 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 CO2 environments (ambient, 404 ± 24 μmol mol-1 CO2, and elevated, 702 ± 51 μmol mol-1 CO2) imposed using two fixed open-top chambers (OTCs). Elevated CO2 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 CO2 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 CO2 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 CO2 and drought. These genotype-specific adaptations reveal a growth-defense trade-off under elevated CO2 and drought, offering promising targets for breeding milk thistle varieties that balance biomass, medicinal compounds, and stress tolerance in a changing climate.