Photosystem II energy use, non-photochemical quenching and the xanthophyll cycle in Sorghum bicolor grown under drought and free-air CO2 enrichment (FACE) conditions

The present study was carried out to test the hypothesis that elevated atmospheric CO2 (Ca) will alleviate over-excitation of the C-4 photosynthetic apparatus and decrease non-photochemical quenching (NPQ) during periods of limited water availability. Chlorophyll a fluorescence was monitored in Sorghum bicolor plants grown under a free-air carbon-dioxide enrichment (FACE) by water-stress (Dry) experiment. Under Dry conditions elevated Ca increased the quantum yield of photosystem II (phiPSII) throughout the day through increases in both photochemical quenching coefficient (q(p)) and the efficiency with which absorbed quanta are transferred to open PSII reaction centres (F-v'/F-m'). However, in the well-watered plants (Wets) FACE enhanced phiPSII only at midday and was entirely attributed to changes in F-v'/F-m'. Under field conditions, decreases in phiPSII under Dry treatments and ambient Ca corresponded to increases in NPQ but the de-epoxidation state of the xanthophyll pool (DPS) showed no effects. Water-stress did not lead to long-term damage to the photosynthetic apparatus as indicated by phiPSII and carbon assimilation measured after removal of stress conditions. We conclude that elevated Ca enhances photochemical light energy usage in C-4 photosynthesis during drought and/or midday conditions. Additionally, NPQ protects against photo-inhibition and photodamage. However, NPQ and the xanthophyll cycle were affected differently by elevated Ca and water-stress.