Two Arabidopsis lines, 10’4 and 3’1, with partial-repression, constitutively of mitochondrial pyruvate dehydrogenase kinase that could alter dark respiration (Rd) were grown on rockwool to reduce off-gassing from peat that interfered with assessment of Rd. At the rosette stage, Rd and photosynthesis (Pn) at high CO2 were greater than at ambient CO2, and Rd was greater for whole-plant than single leaf measurements due to the contribution of non-laminar tissues. However, whole-plant and leaf Rd and Pn were similar on a leaf-area-basis comparing mutants with controls. Whole-plant, analyses during reproductive stage showed that although, the inflorescence contributed as much as 90% of daily C-gain when the rosette leaves senesced, canopy-Pn on a surface-area-basis at each CO2 level remained similar to those at the rosette stage. At each CO2 level, the transgenic and control lines were similar indicating that the mutation resulted in no direct or indirect effect of Rd or Pn. / Ontario Graduate Scholarship, Green Crop Net Work