Up-regulation of photosynthesis and sucrose metabolism enzymes in young expanding leaves of sugarcane under elevated growth CO2

Midday leaf CO2 exchange rate (CER), concentration of chlorophyll (Chl) and soluble protein and activity of the primary enzymes involved in leaf photosynthesis and sucrose metabolism were determined during leaf ontogeny for sugarcane plants grown at ambient (360 μmol mol−1) and double-ambient (elevated, 720 μmol mol−1) CO2. Although leaf CER of both CO2 treatments increased and was highest at 14 days after leaf emergence (DALE), leaf CER of the elevated-CO2 plants, however, was 20, 7 and 10% greater than that of the ambient-CO2 plants at 7, 14 and 32 DALE, respectively. Elevated-CO2 plants also had up to 51% lower stomatal conductance and 39% less transpiration, which resulted in 26–52% greater water-use efficiency (WUE) than ambient-CO2 plants, during leaf growth and development. Concentrations of total Chl and soluble protein and activities of RuBP carboxylase-oxygenase (Rubisco), PEP carboxylase (PEPC), NADP-malate dehydrogenase (NADP-MDH), pyruvate Pi dikinase (PPDK) and sucrose-P synthase (SPS), expressed on a leaf area basis, generally followed leaf CER patterns during leaf ontogeny. For the elevated-CO2 plants, total Chl and soluble protein were 31 and 15% greater, and Rubisco, PPDK and NADP-MDH were up-regulated by 21, 117 and 174%, respectively, at 14 DALE, whereas PEPC and NADP-malic enzyme tended to be lower than or similar to the ambient-CO2 plants throughout leaf development. In addition, leaf SPS activity was increased by 13 and 37% and leaf sucrose concentration was 31 and 19% higher at 7 and 14 DALE, respectively, under elevated growth CO2. At final harvest, elevated growth [CO2] enhanced leaf area by 31%, leaf fresh weight by 13.5%, stem fresh weight by 55.5%, total above-ground plant fresh weight by 44%, and stem juice volume by 83%. The up-regulation of the key photosynthesis and sucrose metabolism enzymes at early stages of leaf development would indicate an acclimation to elevated growth [CO2] for the C4 sugarcane plant. An up-regulation of the enzymes, together with a reduction in leaf stomatal conductance and transpiration and an improvement in leaf WUE and plant water status, could lead to an enhancement in leaf area, plant biomass accumulation and sucrose production for the CO2-enriched sugarcane plants.