Responses of Wild and Cultivated Tetraploid Wheat to CO ₂ Enrichment: Stomatal Optimization of Photosynthesis and Water Use Efficiency

The rising atmospheric concentration of carbon dioxide will influence the anatomical and physiological traits that regulate photosynthesis and water use efficiency (WUE) in crops. To assess how domestication will shape responses to long‐term [CO 2 ] enrichment, we compared wild tetraploid wheat Triticum turgidum ssp. dicoccoides and its cultivated derivative Triticum turgidum ssp. durum . Plants were grown under ambient (a[CO 2 ]: ~420 μmol mol −1 ) and naturally elevated (e[CO 2 ]: ~1000 μmol mol −1 ) [CO 2 ] at the Bossoleto CO 2 ‐degassing vent in central Italy. Stomatal density (SD), stomatal size (SS), stomatal kinetics, and photosynthetic parameters, the maximum Rubisco carboxylation rate ( V c max ), the maximum electron transport rate ( J max ), and the quantum yield of photosystem II (Φ PSII ), were analyzed. Growth in e[CO 2 ] reduced SD in both subspecies, indicating an adaptive adjustment to limit transpirational water loss. However, the two taxa differed markedly in SS and stomatal behaviour. Triticum dicoccoides showed a slight increase in SS and a pronounced increase in stomatal physiological responsiveness, whereas T. durum exhibited reduced SS and slower stomatal responses. These findings indicate distinct coordination between stomatal morphology and physiological control of gas exchange. Neither apparent V c max nor apparent J max differed significantly between treatments, suggesting limited biochemical acclimation, whereas both subspecies displayed a significant increase in Φ PSII under e[CO 2 ], implying improved photochemical efficiency. Wild T. dicoccoides exhibited higher stomatal responsiveness and greater WUE, while cultivated T. durum maintained more stable gas exchange and carbon assimilation. This suggests that the morphological and physiological traits conducive to high photosynthesis and stomatal optimization of WUE may be mutually exclusive.