• sorghum (Sorghum bicolor);
  • C4 photosynthesis;
  • free-air CO2 enrichment (FACE);
  • elevated CO2;
  • photorespiration;
  • energy efficiency


  •  To determine the response of C4 plants to elevated CO2 it is necessary to establish whether young leaves have a fully developed C4 photosynthetic apparatus, and whether photosynthesis in these leaves is responsive to elevated CO2.
  •  The effect of free-air CO2 enrichment (FACE) on the photosynthetic development of the C4 crop Sorghum bicolor was monitored. Simultaneous measurements of chlorophyll a fluorescence and carbon assimilation were made to determine energy utilization, quantum yields of carbon fixation (φCO2) and photosystem II (φPSII), as well as photorespiration.
  •  Assimilation in the second leaf of FACE plants was 37% higher than in control plants and lower apparent rates of photorespiration at growth CO2 concentrations were exhibited. In these leaves, φPSII : φCO2 was high at low atmospheric CO2 concentration (Ca) due to overcycling of the C4 pump and increased leakiness. As Ca increased, φPSII : φCO2 decreased as a greater proportion of energy derived from linear electron transfer was used by the C3 cycle.
  •  The stimulation of C4 photosynthesis at elevated Ca in young leaves was partially due to suppressed photorespiration. Additionally, elevated Ca enhanced energy-use efficiency in young leaves, possibly by decreasing CO2 leakage from bundle sheath cells, and by decreasing overcycling of the C4 pump.