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 Saskatche wan, Canada. Wintering terminal cladodes of O. fragilis and O. polyaca ntha survived -50 and -40 degrees C, respectively, without any apparen t tissue injury. Approximately 50% of the cladodes of O. fragilis coll ected on either 10 October or 10 December 1985 tolerated -70 degrees C and immersion in liquid nitrogen (-196 degrees C) following slow pref reezing to -40 degrees C. These are the lowest values reported for the freezing tolerance of cacti as determined by regrowth and tissue necr osis. Newly formed cladodes of the same species did not tolerate -3.5 degrees C in June, but freezing tolerance increased from -7 to -50 deg rees C from 6 September to 10 October. This increase in freezing toler ance 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 approximat ely 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 mi dsummer, the current year cladodes of O. fragilis tolerated a 60-min e xposure to 50 degrees C, while the previous year cladodes tolerated 55 degrees C without any visible symptoms of injury. Temperature extreme s of -30 and 51 degrees C were recorded at the microhabitats of O. fra gilis. This study demonstrates that this species can acclimate to tole rate both the high and low temperature extremes experienced in its nat ive habitat.