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

To determine the response of C-4 plants to elevated CO2 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 CO2. The effect of free-air CO2 enrich ment (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 (phi CO2) and photosystem II (phi PSII), 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, phi PSII : phi CO2 was high at low atmospheric CO2 concentration (Ca) due to overcycling of the C-4 pump and increased leakiness. As Ca increased, phi PSII : phi CO2 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 CO2 leakage from bundle sheath cells, and by decreasing overcycling of the C-4 pump.