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ISHS Acta Horticulturae 398: Postharvest Physiology of Fruits

PHYSIOLOGY AND METABOLISM OF FRUIT DEVELOPMENT - BIOCHEMISTRY OF SUGAR METABOLISM AND COMPARTMENTATION IN FRUITS -

Author:   S. Yamaki
DOI:   10.17660/ActaHortic.1995.398.11
Abstract:
The carbohydrates assimilated in leaves are loaded into the fruit and finally accumulate in the vacuoles. Sugar metabolism is therefore closely associated with the various steps in the movement of assimilate. In this paper, sugar alcohol and sugar metabolism for sucrose accumulation in Rosaceae fruits is compared with those in other fruits.

Sorbitol is synthesized through the catalysis of sorbitol -6-P dehydrogenase (S6PDH) and acid phosphatase specific to sorbitol-6-P (S6P-APH). Sorbitol-6-P (S6P) produced from glucose-6-P (G6P) via S6PDH is hydrolyzed by S6P-APH. Sorbitol loaded in the fruit is then metabolized by NAD-dependent sorbitol dehydrogenase (NAD-SDH) and sorbitol oxidase (SOX). NAD-SDH converts sorbitol to fructose, while SOX converts sorbitol to glucose. Sucrose loaded in the fruit is also converted to glucose and fructose by acid invertase (AIV). Therefore, fructose and glucose accumulate mainly in the cytosol.

With fruit development, sucrose is resynthesized from the pool of fructose and glucose. There exists different pathways for sucrose accumulation among fruit species, namely an acid invertase type (AIV-type), sucrose-P synthase type (SPS-type), sucrose synthase type (SS-type) and sucrose synthase /sucrose-P synthase type (SS/SPS-type). These sugars finally accumulate in the vacuole. However, sugars sequestered in the vacuole easily efflux into the cytosol and the free space due to the leakiness of membrane stimulated by fruit maturation and ripening. Thus, sugar accumulate not only in the vacuoles, but also in the free space during fruit maturation.

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