The effects of depleted, current and elevated growth [CO2] in wheat are modulated by water availability

• We have shown a photosynthetic acclimation of plants exposed to 260 and 700 ppm of CO2.
• Photosynthetic acclimation was caused by changes in Rubisco protein and N content.
• Absence of effects on biomass production at depleted and elevated [CO2].
• Photosynthetic acclimation is modulated by water availability.

Drought is the main constraint on wheat yield in Mediterranean conditions. The photosynthesis, chlorophyll fluorescence and plant growth parameters of durum wheat (Triticum turgidum, L. var. durum) were compared at three [CO2] (i.e., depleted 260 ppm, current 400 ppm and elevated 700 ppm) in plants subjected to two water regimes (i.e., well-watered WW, and mild water stress by drought or water deficit WS), during pre-anthesis, post-anthesis and the end of grain filling. We showed that [CO2] effects on plants are modulated by water availability. Plants at depleted [CO2] showed photosynthetic acclimation (i.e., up-regulation) and reduced plant biomass and Harvest Index, but depleted [CO2] combined with WS has a more negative impact on plants with decreases in C assimilation and biomass. Plants at elevated [CO2] had decreased plant growth and photosynthesis in response to a down-regulation mechanism resulting from a decrease in Rubisco and N content, but plants exposed to a combination of elevated [CO2] and WS were the most negatively affected (e.g., on plant biomass).