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The ability of seedlings to tolerate temperature extremes is important in determining the distribution of perennial plants in the arid south-western USA, and the manner in which elevated CO2 impacts the ability of plants to tolerate high temperatures is relatively unknown, Whereas the effects of chronic high temperature (30-38 degrees C) and elevated CO2 are comparatively well understood, little research has assessed plant performance in elevated CO2 during extreme (> 45 degrees C) temperature events, We exposed three species of Yucca to 360 and 700 mu mol CO, mol(-1) for 8 months, then 9 d of high temperature (up to 53 degrees C) to evaluate the impacts of elevated CO2 on the potential for photosynthetic function during external high temperature. Seedlings of a coastal C-3 species (Yucca whipplei), a desert C-3 species (Yucca brevifolia), and a desert CAM species (Yucca schidigera), were used to test for differences among functional groups. In general, Yuccas exposed to elevated CO2 showed decreases in carboxylation efficiency as compared with plants grown at ambient before the initiation of high temperature, The coastal species (Y. whipplei) showed significant reductions (33%) in CO2 saturated maximum assimilation rate (A(max)), but the desert species (Y. brevifolia and Y. schidigera) showed no such reductions in A(max). Stomatal conductance was lower in elevated CO2 as compared with ambient throughout the temperature event; however, there were species-specific differences over time. Elevated CO2 enhanced photosynthesis in Y. whipplei at high temperatures for a period of 4 d, but not for Y. brevifolia or Y. schidigera, Elevated CO2 offset photoinhibition (measured as F-v/F-m) in Y. whipplei as compared with ambient CO2, depending on exposure time to high temperature. Stable F-v/F-m in Y. whipplei occurred in parallel with increases in the quantum yield of photosystem II (Phi PSII) at high temperatures in elevated CO2. The value of Phi PSII remained constant or decreased with increasing temperature in all other treatment and species combinations, This suggests that the reductions in F-v/F-m resulted from thermal energy dissipation in the pigment bed for Y. brevifolia and Y. schidigera, The greater efficiency of photosystem II in Y. whipplei helped to maintain photosynthetic function at high temperatures in elevated CO2. These patterns are in contrast to the hypothesis that high temperatures in elevated CO2 would increase the potential for photoinhibition, Our results suggest that elevated CO2 may offset high-temperature stress in coastal Yucca, but not in those species native to drier systems, Therefore, in the case of Y. whipplei, elevated CO2 may allow plants to survive extreme temperature events, potentially relaxing the effects of high temperature on the establishment in novel habitats.