WATER RELATIONS AND LOW-TEMPERATURE ACCLIMATION FOR CACTUS SPECIES VARYING IN FREEZING TOLERANCE

Opuntia ficus-indica and Opuntia streptacantha are widely cultivated c acti that can tolerate temperatures no lower than -10 degrees C, where as Opuntia humifusa, which is native to southern Canada and the easter n United States, can tolerate -24 degrees C. As day/night air temperat ures were decreased from 30/20 to 10/0 degrees C, the osmotic pressure increased 0.10 MPa for O. ficus-indica and O. streptacantha but 0.38 MPa for O. humifusa. The increases in osmotic pressures were due mostl y to the synthesis of fructose, glucose, and sucrose. In addition, O. humifusa produced a substantial amount of mannitol during exposure to low temperatures. Substantial accumulation of sugars and mannitol in c ells of O. humifusa may help prevent intracellular freeze dehydration and ice formation as well as provide noncolligative protection to its membranes. Mucilage was slightly higher in all three species at the lo wer temperatures. Extracellular nucleation of ice occurred closer to t he equilibrium freezing temperature for plants at 10/0 degrees C compa red with 30/20 degrees C, which could make the cellular dehydration mo re gradual and, thus, less damaging. Results from nuclear magnetic res onance indicated a restricted mobility of intracellular water at the l ower temperatures, especially for O. humifusa, which is consistent wit h its lower water content and higher levels of low molecular weight so lutes.