In the mitochondrial CMSII mutant of Nicotiana sylvestris photosynthetic activity remains higher than in the WT under persisting mild water stress

► The CMSII mutant of N. sylvestris was compared to WT leaves at 80% leaf-RWC. ► Photosynthesis at mild water stress was higher in CMSII than in the WT leaves. ► Stomatal and mesophyll conductance declined in both genotypes. ► WT but not CMSII leaves were impaired in photorespiration. ► Defect of mitochondrial complex I improved acclimation to mild water stress.