Sugar metabolism

Sugar content plays an important role in the flavour characteristics and commercial assessment of pineapple fruit quality (Py et al., 1987). TSS, mainly sugars, are often used as an indicator of fruit maturity and quality (Paull, 1993). TSS can vary by 40 g l-1 from the more mature, sweeter basal tissue to the crown end of the fruit (Sideris and Krauss, 1933a; Miller and Hall, 1953), and decline only slightly after harvest (Paull and Rohrbach, 1982; Chen and Paull, 1995). Starch is not accumulated as the fruit ripens (Dull, 1971), though it is high during early fruit growth (Fig. 10.1B), which could explain the absence of dramatic changes in TSS postharvest.

The major sugars in mature fruit are sucrose, glucose and fructose (Gawler, 1962) and the peak in sucrose concentration is attained at full-yellow stage and then declines. Fruit sugars continued to increase through to senescence, unless the fruit is harvested (Kelly, 1911). Chen (1999) showed that total soluble sugar content is low during fruit growth and composed mainly of glucose and fructose. Glucose is at a slightly higher concentration than fructose during the early stages of fruit development. Sucrose accumulated rapidly 6 weeks before commercial harvest (Lodh et al., 1972; Py et al., 1987) and ultimately exceeds the glucose and fructose concentration (Chen, 1999). Fructose and glucose continue to increase postharvest (Singleton and Gortner, 1965; Tay, 1977). In addition, sucrose accumulated more in the fruitlet than in the interfruit-let tissue until the last 2 weeks of fruit development, when sucrose accumulation rate in the interfruitlet tissue was greater than in the fruitlet (Chen, 1999).

Three sugar metabolic enzymes (sucrose synthase, sucrose phosphate synthase and invertase) are thought to control sugar accumulation by fruit tissue. The activity of sucrose synthase (SS) is high in younger pineapple fruit and declines to a very low level 6 weeks before harvest, while the activ ity of sucrose phosphate synthase is relatively low and constant throughout fruit development (Chen, 1999). The activities of acid, neutral and cell-wall invertases are high in the younger fruit and decline to low levels 6 weeks before harvest, when sucrose starts to accumulate. The activity of cell-wall invertase (CWI) does increase 4 weeks before harvest, mainly in fruitlet tissue, while the activities of neutral invertase (NI) and acid invertase (AI) remain low, concomitant with the accumulation of sucrose, indicating that these enzymes may be a prerequisite for sucrose accumulation in pineapple fruit flesh. The high activity of CWI, favouring apoplastic phloem unloading, may play a role in sugar accumulation in pineapple fruit flesh at the later stages of fruit development (Chen, 1999).

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