Photosynthetic capacity of the inflorescence is a major contributor to daily-C-gain and the responsiveness of growth to elevated CO2 in Arabidopsis thaliana with repressed expression of mitochondrial-pyruvate-dehydrogenase-kinase

• The Arabidopsis inflorescence highly contributes to plant photosynthesis and C-gain.
• Whole-plant gas exchange on a surface-area basis remained constant during development.
• Photosynthesis, respiration, C-gain and water-use-efficiency increased at elevated CO2.
• mtPDHK-mutant growth increased more at elevated CO2 during inflorescence maturation.
• Whole-plant gas exchange phenotyping can assess environmental/genetic modifications.

The balance between whole-plant net-photosynthesis (Pn) and dark-respiration (Rd) determines daily-C-gain (dC) and thus growth. However, in Arabidopsis thaliana, a well-studied organism, the contribution of the inflorescence as a respiring sink and a photosynthetic source has never been quantified together in either ambient CO2 (AC) or elevated CO2 (EC) conditions. When sink-strength is altered by modifying mitochondrial pyruvate metabolism increases in productivity can occur in both AC and EC. Rd and Pn during canopy transition from its rosette-leaf form to the highly branched inflorescence stages were quantified. Diel patterns of leaf and whole-plant gas exchange were measured at four critical developmental stages for controls (wild-type (WT) and plasmid (pBI121)) and two transgenics (3′1 and 10′4) Arabidopsis lines having partial constitutive repression of mitochondrial pyruvate dehydrogenase kinase (mtPDHK) grown at AC and EC.In all lines the mature inflorescence structures, although having higher Rd than the rosette leaves, contributed over 90% of Pn and dC at both AC and EC. Expressed on a plant basis, Pn, Rd and dC were 700–900% higher and water-use-efficiency (WUE) was double when the inflorescence was mature as when the rosette-leaves alone sustained growth. On a dry-matter basis Pn, Rd and dC were 50–75% lower, but on a surface-area basis these traits were remarkably constant during development, reflecting the high photosynthetic strength and carbon-use-efficiency (CUE) of the inflorescence. All lines demonstrated greater Pn, Rd, dC and WUE under EC, however, at the rosette stage the controls responded more to EC, whereas, during the inflorescence 3rd-stage it was the transgenics that responded more to EC.These results underscore the need for measuring key phenotype traits such as whole-plant Pn, Rd and dC to assess how source and sink strengths change during the life-cycle, especially following genetic modification of a primary metabolic process.