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Leaf tolerance to low temperatures, as determined by vital stain uptake and chlorophyll a fluorescence, was compared for seedlings of three Yucca species native to the south-western United States: Yucca brevifolia, which is distributed throughout the Mojave Desert; Yucca schidigera, which occurs in both coastal and desert California; and Yucca whipplei, which is primarily coastal but occurs in portions of the Mojave Desert. Seedlings maintained at day/night glasshouse air temperatures of 40/25 degrees C or 20/5 degrees C, and under ambient (360 mu mol mol(-1)) or elevated (700 mu mol mol(-1)) levels of CO2 were compared to test the hypothesis that cold acclimation and freezing tolerance are enhanced by exposure to elevated CO2. Plants maintained at elevated CO2 had greater low-temperature tolerance compared to controls, yet a larger shift in survival was attributable to the downward shift in day/night temperatures. Low-temperature tolerance was similar to extreme minimum air temperatures for the collection sites averaged over the period 1961 to 1990. For seedlings exposed to elevated CO2, low-temperature tolerance was - 11.9 degrees C for Yucca brevifolia, - 9.6 degrees C for Y. schidigera, and - 13.5 degrees C for Y. whipplei. Elevated CO2 caused excitation energy transfer in Photosystem II (measured as F-V/F-M) to be maintained at lower temperatures for Yucca brevifolia and Y. whipplei. Phi PSII at low temperatures was increased due to elevated CO2 for Y. brevifolia only. The results suggest that survival during episodic sub-zero temperature events will be enhanced for seedlings of these three Yucca species in a future elevated CO2 environment. (C) 2000 Academic Press.