Freezing and heat tolerance of Opuntia cacti native to the Canadian prairie provinces

Seasonal changes in the freezing tolerance in situ were determined for Opuntia fragilis and Opuntia polyacantha native to southern Saskatchewan, Canada. Wintering terminal cladodes of O. fragilis and O. polyacantha survived −50 and −40 °C, respectively, without any apparent tissue injury. Approximately 50% of the cladodes of O. fragilis collected on either 10 October or 10 December 1985 tolerated −70 °C and immersion in liquid nitrogen (−196 °C) following slow prefreezing to −40 °C. These are the lowest values reported for the freezing tolerance of cacti as determined by regrowth and tissue necrosis. Newly formed cladodes of the same species did not tolerate −3.5 °C in June, but freezing tolerance increased from −7 to −50 °C from 6 September to 10 October. This increase in freezing tolerance was accompanied by a decrease in water content from 5.4 to 2.6 g H2O/g dry wt. and the cladodes were shrunken and reduced by approximately 30% in diameter. Although Opuntia is a succulent plant that avoids drought by storing water in the cladodes, during cold acclimation O. fragilis appears to shift to a more drought-tolerating strategy. In midsummer, the current year cladodes of O. fragilis tolerated a 60-min exposure to 50 °C, while the previous year cladodes tolerated 55 °C without any visible symptoms of injury. Temperature extremes of −30 and 51 °C were recorded at the microhabitats of O. fragilis. This study demonstrates that this species can acclimate to tolerate both the high and low temperature extremes experienced in its native habitat. Keywords: cacti, cold acclimation, freezing tolerance, heat tolerance, Opuntia, water relations.