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 ap paratus, and whether photosynthesis in these leaves is responsive to elevat ed CO2. The effect of free-air CO2 enrich ment (FACE) on the photosynthetic develop ment of the C-4 crop Sorghum bicolor was monitored. Simultaneous measuremen ts of chlorophyll a fluorescence and carbon assimilation were made to deter mine energy utilization, quantum yields of carbon fixation (phi CO2) and ph otosystem II (phi PSII), as well as photorespiration. Assimilation in the second leaf of FACE plants was 37% higher than in contr ol plants and lower apparent rates of photorespiration at growth CO2 concen trations were exhibited. In these leaves, phi PSII : phi CO2 was high at lo w 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 gr eater proportion of energy derived from linear electron transfer was used b y the C-3 cycle. The stimulation of C-4 photosynthesis at elevated Ca in young leaves was pa rtially due to suppressed photorespiration. Additionally, elevated Ca enhan ced energy-use efficiency in young leaves, possibly by decreasing CO2 leaka ge from bundle sheath cells, and by decreasing overcycling of the C-4 pump.