Drought Stress Limits the Photosynthetic Benefit of Elevated CO2 in Chinese Fir Saplings via Stomatal Closure and Non-Stomatal Impairment

The frequency and magnitude of droughts are increasing concurrently with atmospheric CO2 concentration, with profound consequences for plant carbon assimilation. However, the interactions and the underlying physiological mechanism are still not fully understood. To fill the knowledge gap, we exposed Chinese fir (Cunninghamia lanceolata) saplings to two CO2 concentrations (400 and 800 ppm, representing ambient and elevated CO2) and two soil water regimes (70% and 40% field capacity; well-watered and drought-stressed conditions) in a factorial design. Net photosynthetic rate (An), chlorophyll fluorescence, photosynthetic pigments, oxidative stress indicators, and antioxidant enzyme activities were measured four times over a 45-day treatment period. Under well-watered conditions, elevated CO2 significantly increased An by 61.5%. However, drought stress substantially reduced An by 75.0% under ambient CO2 and by 75.6% under elevated CO2, whereas no statistically significant CO2-induced increase was detected under drought conditions. Furthermore, drought stress caused marked reductions in stomatal conductance, transpiration, chlorophyll content, and photosystem II (PSII)-related parameters, together with increased malondialdehyde, proline, and antioxidant enzyme activities. Variance partitioning analysis suggested that stomatal regulation (SR), photosynthetic capacity (PC), and stress response (STR) jointly explained 71% of the variation in An. Structural equation modeling further suggested that drought stress restricted the photosynthetic benefits of elevated CO2 primarily through stomatal closure, concurrently associated with stress-related declines in pigment stability and photochemical performance. These findings suggest that the carbon sink potential of Chinese fir plantations under future CO2-enriched climates may be strongly constrained by water deficits.