Effects of extreme high temperature, drought and elevated CO2 on photosynthesis of the Mojave Desert evergreen shrub, Larrea tridentata

The interaction of extreme temperature events with future atmospheric CO2 c oncentrations may have strong impacts on physiological performance of deser t shrub seedlings, which during the critical establishment phase often endu re temperature extremes in conjunction with pronounced drought. To evaluate the interaction of drought and CO2 on photosynthesis during heat stress, o ne-year-old Larrea tridentata [DC] Cov. seedlings were exposed to nine days of heat with midday air temperature maxima reaching 53 degrees C under thr ee atmospheric CO2 concentrations (360, 550 and 700 mu mol mol(-1)) and two water regimes (well-watered and droughted). Photosynthetic gas exchange, c hlorophyll fluorescence and water potential responses were measured prior t o, during and one week following the high temperature stress event. Heat st ress markedly decreased net photosynthetic rate (A(net)), stomatal conducta nce (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 plan ts. In droughted L. tridentata, A(net) was ca. 2x (in 550 mu mol mol(-1) CO 2) to 3x (in 700 mu mol mol(-1) CO2) higher than in ambient-CO2-grown plant s, while g(s) and F-v/F-m were similar and low in all CO2 treatments. Follo wing heat stress, g(s) in all well-watered plants rose dramatically, exceed ing pre-stress levels by up to 100%. In droughted plants, g(s) and A(net) r ose 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 t o A(net) and g(s) values similar to pre-heat following several days of reco very. Extreme heat diminished the photosynthetic down-regulation response t o growth at elevated CO2 under well-watered conditions, similar to the acti on of drought. Ambient-CO2-grown L. tridentata did not show significant rec overy of photosynthetic capacity (A(max) and CE) after alleviation of tempe rature stress, especially when exposed to drought, while plants exposed to elevated CO2 appeared to be unaffected. These findings suggest that elevate d CO2 could promote photosynthetic activity during critical periods of seed ling establishment, and enhance the potential for L. tridentata to survive extreme high temperature events.