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

Summary To determine the response of C 4 plants to elevated CO 2 it is necessary to establish whether young leaves have a fully developed C 4 photosynthetic apparatus, and whether photosynthesis in these leaves is responsive to elevated CO 2 . The effect of free‐air CO 2 enrichment (FACE) on the photosynthetic development of the C 4 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 (φCO 2 ) 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 CO 2 concentrations were exhibited. In these leaves, φPSII : φCO 2 was high at low atmospheric CO 2 concentration (Ca) due to overcycling of the C 4 pump and increased leakiness. As Ca increased, φPSII : φCO 2 decreased as a greater proportion of energy derived from linear electron transfer was used by the C 3 cycle. The stimulation of C 4 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 CO 2 leakage from bundle sheath cells, and by decreasing overcycling of the C 4 pump.