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

Elevated [CO2] (eCO(2)) and water stress reduce leaf stomatal conductance (g(s)), which may affect leaf thermoregulation during heat waves (heat stress). Two sorghum lines, with different leaf width were grown in a glasshouse at a mean day temperature of 30 degrees C, under different [CO2] and watering levels, and subjected to heat stress (43 degrees C) for 6 d 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 (A(growth)) and biomass accumulation were enhanced by eCO(2) under control conditions. Heat stress increased g(s), especially in wider leaves, and reduced the time constant of stomatal opening (k(open)) at ambient [CO2] but not eCO(2). However, heat stress reduced photosynthesis under water stress and eCO(2) due to increased leaf temperature and reduced evaporative cooling. eCO(2) prevented the reduction of biomass under both water and heat stress, possibly due to improved plant and soil water status as a result of reduced g(s). Our results suggest that the response of the C-4 crop sorghum to future climate conditions depends on the trade-off between low g(s) needed for high water use efficiency and drought tolerance, and the high g(s) needed for improved thermoregulation and heat tolerance under an eCO(2) future. Elevated [CO2] reduced thermoregulation of sorghum leaves due to lower stomatal conductance, and exacerbated photosynthetic depression under water and heat stress.