Respiratory metabolism during cold storage of apple fruit. I. Sucrose metabolism and glycolysis

This study provides the first report on the occurrence of the respiratory c limacteric during cold storage of apple fruit (Malus domestica Borkh. cv. R einette du Canada). The respiratory pattern at 4 degrees C was very similar to that observed during postharvest ripening at room temperature, except t hat shelf life was considerably extended and the onset of the climacteric d elayed. Increasing the calcium content of the apple fruit significantly red uced loss of firmness during cold storage, but showed no effect an respirat ion or on the other parameters determined. A gradual accumulation of solubl e sugars occurred during the first 60 days after harvest and,vas effectivel y completed before the climacteric peak was reached. This increase in sugar s correlated with an increase in the activity of sucrose-phosphate synthase (EC 2.4.1.14), and a marked change in the kinetic properties of the enzyme was observed after sucrose accumulation ceased. Changes in the hexose-phos phate pool and in glycolytic and gluconeogenic activities indicated an init ial increase in the gluconeogenic flow at early stages of the climacteric, followed by activation of glycolysis, with the carbon flow being most likel y regulated at the reversible phosphorylation of fructosed-6-phosphate to f ructose-1,6-bisphosphate (mostly via pyrophosphate:fructose-6-phosphate pho sphotransferase, EC 2.7.1.90) and at the pyruvate kinase (EC 2.7.1.40) step s. The results presented. indicate that the respiratory climacteric does no t occur to accommodate extra ATP requirements during sucrose synthesis nor can it be a consequence of an increased supply of respiratory substrate.