Carbohydrate metabolism during postharvest ripening in kiwifruit

Mature fruit (kiwifruit) of Actinidia deliciosa var. deliciosa (A. Chev.), (C.F.) Liang and Ferguson cv. Haywood (Chinese gooseberry) were harvested and allowed to ripen in the dark at 20° C. Changes were recorded in metabolites, starch and sugars, adenine nucleotides, respiration, and sucrose and glycolytic enzymes during the initiation of starch degradation, net starch-to-sucrose conversion and the respiratory climacteric. The conversion of starch to sucrose was not accompanied by a consistent increase in hexose-phosphates, and UDP-glucose declined. The activity of sucrose phosphate synthase (SPS) measured with saturating substrate rose soon after harvesting and long before net sucrose synthesis commenced. The onset of sugar accumulation correlated with an increase in SPS activity measured with limiting substrates. Throughout ripening, until sucrose accumulation ceased, feeding [(14)C] glucose led to labelling of sucrose and fructose, providing evidence for a cycle of sucrose synthesis and degradation. It is suggested that activation of SPS, amplified by futile cycles, may regulate the conversion of starch to sugars. The respiratory climacteric was delayed, compared with net starchsugar interconversion, and was accompanied by a general decline of pyruvate and all the glycolytic intermediates except fructose-1,6-bisphosphate. The ATP/ ADP ratio was maintained or even increased. It is argued that the respiratory climacteric cannot be simply a consequence of increased availability of respiratory substrate during starch-sugar conversion, nor can it result from an increased demand for ATP during this process.