The present study was carried out to test the hypothesis thatelevated atmospheric CO2 (Ca) will alleviate over-excitationof the C4 photosynthetic apparatus and decrease non-photochemicalquenching (NPQ) during periods of limited water availability. Chlorophyll a fluorescencewas monitored in Sorghum bicolor plants grown under a free-aircarbon-dioxide enrichment (FACE) by water-stress (Dry) experiment.Under Dry conditions elevated Ca increased the quantum yield ofphotosystem II (φPSII) throughout the day throughincreases in both photochemical quenching coefficient (qp)and the efficiency with which absorbed quanta are transferred toopen PSII reaction centres (Fv′/Fm′).However, in the well-watered plants (Wets) FACE enhanced φPSIIonly at midday and was entirely attributed to changes in Fv′/Fm′. Underfield conditions, decreases in φPSII under Dry treatmentsand ambient Ca corresponded to increases in NPQ but the de-epoxidation stateof the xanthophyll pool (DPS) showed no effects. Water-stress didnot lead to long-term damage to the photosynthetic apparatus asindicated by φPSII and carbon assimilation measuredafter removal of stress conditions. We conclude that elevated Caenhances photochemical light energy usage in C4 photosynthesisduring drought and/or midday conditions. Additionally,NPQ protects against photo-inhibition and photodamage. However,NPQ and the xanthophyll cycle were affected differently by elevatedCa and water-stress.