Goldstein G., Nobel P. S.
Department of Biology and Laboratory of Biomedical and Environmental Sciences, University of California, Los Angeles, California 90024.
Plant Physiol. 1994 Feb;104(2):675-681. doi: 10.1104/pp.104.2.675.
Opuntia ficus-indica and Opuntia streptacantha are widely cultivated cacti that can tolerate temperatures no lower than -10[deg]C, whereas Opuntia humifusa, which is native to southern Canada and the eastern United States, can tolerate -24[deg]C. As day/night air temperatures were decreased from 30/20 to 10/0[deg]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 mostly 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 cells 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 lower temperatures. Extracellular nucleation of ice occurred closer to the equilibrium freezing temperature for plants at 10/0[deg]C compared with 30/20[deg]C, which could make the cellular dehydration more gradual and, thus, less damaging. Results from nuclear magnetic resonance indicated a restricted mobility of intracellular water at the lower temperatures, especially for O. humifusa, which is consistent with its lower water content and higher levels of low molecular weight solutes.
仙人掌和刺梨是广泛种植的仙人掌，能耐受不低于-10℃的温度，而原产于加拿大南部和美国东部的平卧仙人掌能耐受-24℃。当昼夜气温从30/20℃降至10/0℃时，仙人掌和刺梨的渗透压增加了0.10兆帕，而平卧仙人掌的渗透压增加了0.38兆帕。渗透压的增加主要是由于果糖、葡萄糖和蔗糖的合成。此外，平卧仙人掌在低温暴露期间产生了大量的甘露醇。平卧仙人掌细胞中大量积累的糖分和甘露醇可能有助于防止细胞内冷冻脱水和结冰，并为其膜提供非依数性保护。在较低温度下，所有三个物种的黏液含量都略高。与30/20℃相比，10/0℃时植物的细胞外结冰更接近平衡冻结温度，这可能使细胞脱水更加缓慢，从而减少损害。核磁共振结果表明，在较低温度下，细胞内水的流动性受限，尤其是平卧仙人掌，这与其较低的含水量和较高水平的低分子量溶质一致。
