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The interaction of extreme temperature events with future atmospheric CO2 concentrations may have strong impacts on physiological performance of desert shrub seedlings, which during the critical establishment phase often endure temperature extremes in conjunction with pronounced drought. To evaluate the interaction of drought and CO2 on photosynthesis during heat stress, one-year-old Larrea tridentata [DC] Cov. seedlings were exposed to nine days of heat with midday air temperature maxima reaching 53 degrees C under three atmospheric CO2 concentrations (360, 550 and 700 mu mol mol(-1)) and two water regimes (well-watered and droughted). Photosynthetic gas exchange, chlorophyll fluorescence and water potential responses were measured prior to, during and one week following the high temperature stress event. Heat stress markedly decreased net photosynthetic rate (A(net)), stomatal conductance (g(s)), and the photochemical efficiency of photosystem II (F-v/F-m) in all plants except for well-watered L. tridentata grown in 700 mu mol mol(-1) CO2. A(net) and g(s) remained similar to pre-stress levels in these plants. In droughted L. tridentata, A(net) was ca. 2x (in 550 mu mol mol(-1) CO2) to 3x (in 700 mu mol mol(-1) CO2) higher than in ambient-CO2-grown plants, while g(s) and F-v/F-m were similar and low in all CO2 treatments. Following heat stress, g(s) in all well-watered plants rose dramatically, exceeding pre-stress levels by up to 100%. In droughted plants, g(s) and A(net) rose only in plants grown at elevated CO2 following release from heat. This recovery response was strongest at 700 mu mol mol(-1) CO2, which returned to A(net) and g(s) values similar to pre-heat following several days of recovery. Extreme heat diminished the photosynthetic down-regulation response to growth at elevated CO2 under well-watered conditions, similar to the action of drought. Ambient-CO2-grown L. tridentata did not show significant recovery of photosynthetic capacity (A(max) and CE) after alleviation of temperature stress, especially when exposed to drought, while plants exposed to elevated CO2 appeared to be unaffected. These findings suggest that elevated CO2 could promote photosynthetic activity during critical periods of seedling establishment, and enhance the potential for L. tridentata to survive extreme high temperature events.