THE EFFECT OF HYPOXIA ON THE CONTROL OF CARBOHYDRATE-METABOLISM IN RIPENING BANANAS

The aim of this work was to determine the effects of hypoxia on the major fluxes of carbohydrate metabolism in climacteric fruit of banana (Musa cavendishii Lamb ex Paxton). Hands of bananas, untreated with ethylene, were allowed to ripen in air at 21 degrees C in the dark. When the climacteric began, fruit were transferred to 15 or 10% oxygen and were analysed once the climacteric peak had been reached 8-12 h later. The rates of starch breakdown, sucrose, glucose and fructose accumulation, and CO2 production were determined, as were the contents of hexose monophosphates, adenylates and pyruvate. In addition, the detailed distribution of label was determined after supplying [U-C-14]-, [1-C-14]-, [3,4-C-14]- and [6-C-14]glucose, and [U-C-14]glycerol to cores of tissue under hypoxia. The data were used to estimate the major fluxes of carbohydrate metabolism. There was a reduction in the rate of respiration. The ATP/ADP ratio was unaffected but there was a significant increase in the content of AMP. In 15% oxygen only minor changes in fluxes were observed. In 10% oxygen starch breakdown was reduced and starch synthesis was not detected. The rate of sucrose synthesis decreased, as did the rate of re-entry of hexose sugars into the hexose monophosphate pool. There was a large increase in both the glycolytic flux and in the flux from triose phosphates to hexose monophosphates. It is argued that the increase in these fluxes is due to activation of pyrophosphate: fructose-6-phosphate l-phosphotransferase, and that this enzyme has an important role in hypoxia. The results are discussed in relation to our understanding of the control of carbohydrate metabolism in hypoxia.