Changes in the metabolic activities of peroxide-producing systems and peroxide-scavenging systems after freezing and thawing in flower buds of the apple, Malus pumila Mill., were studied with special reference to freezing injury. In flower buds of the ‘McIntosh’ apple that were frozen below lethal temperatures, the activity of NADH-Cyt c reductase (EC 1.6.99.3), one of the enzymes in the electron-transport chains that are related to the peroxide-producing systems, decreased slightly, while that of Cyt c oxidase (EC 1.9.3.1) hardly changed. By contrast, the activities of glucose-6-phosphate dehydrogenase (EC 1.1.1.49), dehydroascorbate reductase (EC 1.8.5.1) and ascorbate peroxidase (EC 1.11.1.11), which are involved in the peroxide-scavenging systems, decreased to very low levels. The activity of glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.12) also decreased markedly. However, little change was observed in the activities of hexokinase (EC 2.7.1.1), glucosephosphate isomerase (EC 5.3.1.9), glutathione reductase (EC 1.6.4.2) and glutathione peroxidase (EC 1.11.1.9). Examination of substrates involved in the peroxide-scavenging systems revealed that the levels of glucose-6-phosphate and fructoses-phosphate decreased to approximately 10−4 to 10−5 M and 10−5 M, respectively, and the levels of GSH decreased to about 10−5 M or became barely detectable. A decrease in the levels of GSSG also occurred while levels of ascorbate rose slightly. Similar results were observed with flower buds from ‘Starking Delicious’ and ‘Jonathan’ apple trees. These results suggest that the freezing injury to apple flower-buds is closely related to the collapse of the peroxide-scavenging systems that are coupled with the pentose phosphate cycle. The results also suggest that the dysfunction of these peroxide-scavenging systems is caused by H2O2, which may be produced during freezing and thawing.
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Kuroda et al. (1992) studied this question.