Elevated [CO2] negatively impacts C4 photosynthesis under heat and water stress without penalizing biomass.

Elevated [CO2] (eCO2) and water stress (WS) reduce leaf stomatal conductance (gs), which may affect leaf thermoregulation during heat waves (HS). Two sorghum lines, with different leaf width (LW) were grown in a glasshouse at mean day temperature of 30°C, under different [CO2] and watering levels, and subjected to HS (43°C) for six days at the start of the reproductive stage. We measured leaf photosynthetic and stomatal responses to light transients, before harvesting the plants. Photosynthesis at growth conditions (Agrowth) and biomass accumulation were enhanced by eCO2 under control conditions. HS increased gs, especially in wider leaves, and reduced the time-constant of stomatal opening (kopen) at aCO2 but not eCO2. However, HS reduced photosynthesis under WS and eCO2 due to increased leaf temperature and reduced evaporative cooling. eCO2 prevented the reduction of biomass under WS and HS, possibly due to improved plant and soil water status as a result of reduced gs. Our results suggest that the response of the C4 crop sorghum to future climate conditions depends on the trade-off between low gs needed for high water use efficiency and drought tolerance, and the high gs needed for improved thermoregulation and heat tolerance under an eCO2 future.