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The objective of this study was to investigate the response of leaf gas exchange of tomato plant to progressive drought stress under ambient (a[CO2], 400 ppm) and elevated (e[CO2], 800 ppm) atmospheric CO2 concentration. The fraction of transpirable soil water (FTSW) was used to evaluate soil water status in the pots. The results showed that stomatal conductance (g(s)) and transpiration rate (T-r) were significantly lower while the net photosynthetic rate (A(n)) was significantly higher in plants grown under e[CO2] than those under a[CO2] at onset of drought stress. Along with soil drying, the FTSW thresholds at which g(s) and A(n) started to decrease were significantly lower in plants grown under e[CO2] as compared to plants grown under a[CO2]. The intrinsic water use efficiency and instantaneous water use efficiency of plants grown under e[CO2] was significantly higher than those under a[CO2]. Under e[CO2], the drought-stressed plants had greater leaf area, dry matter and water use efficiency than those grown under a[CO2]. e[CO2] notably enhanced shoot C concentration while decreased shoot N concentration hereby increased the C:N ratio. With the decrease of FTSW, the concentration of abscisic acid in leaf ([ABA](leaf) ) and xylem sap ([ABA](xylem)) increased exponentially. When FTSW > 0.2, under both CO2 environments, g, decreased linearly with increasing [ABA](xleaf) and [ABA]xylem ; and similar slopes but different intercepts were noticed for the regression lines, indicating that the responsiveness of g, to ABA was unaffected by CO2. In conclusion, CO2 elevation retarded the response of leaf gas exchange to progressive soil drying in tomato plants. This result provides novel knowledge for more precise prediction of plant response to drought stress in a future CO2 enriched environment.