Frost Induces Cold Acclimation and Peroxide-scavenging Systems Coupled with the Pentose Phosphate Cycle in Apple Twigs under Natural Conditions

The relationship between frost, cold acclimation, and peroxide-scavenging systems in the twigs of apple, Malus pumila MILL., was studied in the field under natural conditions. The resistance to freezing of twigs increased within 2 weeks after each of the first two frosts. In the living bark, the activities of enzymes in the peroxide-scavenging systems coupled with the pentose phosphate cycle increased immediately after each of the first two frosts, and the increases occurred considerable earlier than the development of increased resistance to freezing and simultaneously. In the xylem, although the profiles in activities of peroxide-scavenging systems that require reduced glutathione and ascorbate in response to frost differed from those in the living bark, the activities of these enzymes were significantly enhanced by exposure of twigs to frost; activities of glucose-6-phosphate dehydrogenase, hexokinase, glucosephosphate isomerase, glyceraldehyde-3-phosphate dehydrogenase and glutathione peroxidase only increased immediately after the first frost. The activities of glutathione reductase and ascorbate peroxidase increased immediately after both the first and the second frost. The increase in dehydroascorbate reductase activity was only observed after the second frost. By contrast, peroxidase and catalase activities were not affected by the first or the second frost. In the living bark and xylem, the levels of glucose-6-phosphate increased immediately after each frost. The levels of fructose-6-phosphate, glucose-1-phosphate and reduced glutathione after each frost showed an immediate, transient decrease and then increased before the cold acclimation, but the levels of oxidized glutathione appeared to be inversely related to the levels of these substrates. Frosts appeared to exert little effect on the levels of ascorbate. These results show that frost induces an increase in the activities of the peroxide-scavenging systems coupled with the pentose phosphate cycle, and these increases result in the acclimation of the trees to cold.