Injuries of Cold Acclimatized Poplar Twigs Resulting from Enzyme Inactivation and Substrate Depression during Frozen Storage at Ambient Temperatures for a Long Period

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Abstract
Wintering twigs of poplar could be stored in frozen state for about one year, but longer storage resulted in metabolic dysfunction and death of the twigs. Analyses of enzyme activities revealed that most of the activity of glucose-6-phosphate and 6-phosphogluconate dehydrogenases and glutathione reductase present normally in the twigs disappeared. Another four enzyme activities assayed also decreased to some extent. In the twigs, glucose 6-phosphate decreased to a level of 10−5 M and glutathione was present totally in the disulfide form. The extent of the decrease of ascorbate and ATP was considerably less than that of the above two. Peroxide levels rose slightly, but lipid hydroperoxide levels rose several fold. It is suggested that the reactions catalyzed by glucose-6-phosphate dehydrogenase and related ones are obligatory to the cells even in the frozen state and that the activity to supply glucose 6-phosphate, hence NADPH, is limited to a certain period of time. Beyond this capacity, a decrease in the ability to remove peroxide may also occur, due to a shortage of NADPH supply, resulting in inactivation of some enzymes and the formation of lipid hydroperoxide in cellular membranes.

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