Reduced photorespiration and increased energy-use efficiency in young CO₂-enriched sorghum leaves

• 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, φPSll: 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.