Abstract
Cladodes (stem segments) of platyopuntias generally lose water as ambient temperatures decrease, even when the plants are in wet soil. Two weeks after shifting plants from day/night air temperatures of 30<sup>⚬</sup>C/20<sup>⚬</sup>C to 10<sup>⚬</sup>C/1<sup>⚬</sup>C, cladode thickness decreased 15% for Opuntia ficus-indica and 25% for Opuntia polyacantha. During this period, daily transpirational water loss gradually decreased, but root water uptake from the soil immediately decreased about 90%. As the temperature was decreased from 30<sup>⚬</sup>C to 0<sup>⚬</sup>C, the root hydraulic conductivity (L<sub>p</sub>) for excised root segments also decreased about 90%, with larger fractional decreases occurring at lower temperatures. The large decrease in L<sub>p</sub> at the lower temperatures was chiefly responsible for the accompanying cladode desiccation. Such net loss of cladode water is apparently essential for certain cacti to survive seasonally cold periods in the field.
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