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Whether environmental conditions influence the sensitivity to extreme temperatures, which determines where species occur naturally or can be cultivated, was investigated for Nopalea cochenillifera and Opuntia robusta. These cacti tolerated high-temperatures, as LT50 (the temperature killing 50% of the cells, based on uptake of the vital stain neutral red) averaged 57 degrees C for stem chlorenchyma cells and root cortical cells for plants at day/night air temperatures of 25/20 degrees C the plants were relatively susceptible to low temperatures, for which LT50 averaged -7 degrees C. As air temperatures were reduced by 15 degrees C (1015 degrees C), LT50 for low temperatures decreased 1.1 degrees C for stems and roots; as they were raised by 20 degrees C (45/40 degrees C), LT50 for high temperatures increased 4.2 degrees C for stems and 3.4 degrees C for roots. This acclimation leads to advantageous seasonal adjustments to ambient temperatures. Variations of the total daily photosynthetic flux density incident on the opaque stems for 35 days did not influence the tolerance to extreme temperatures, nor did shading the plants by 90%, indicating no effect of locally adverse photosynthetic conditions on temperature sensitivity. In contrast, the low-temperatures tolerated decreased 2.1 degrees C for stems and 1.3 degrees C for roots during 100 days of drought; concomitantly, the high temperatures tolerated increased 2.5 degrees C for stems, although roots were unaffected. The increased low-temperature tolerance during drought was accompanied by a decreased stem water potential and water content. (c) 2007 Elsevier Ltd. All rights reserved.