Sugar Metabolism and Pineapple Flesh Translucency

Sugar accumulation and the activities of sugar metabolizing enzymes were related to the occurrence of pineapple (Ananas comosus (L.) Merr. ) flesh translucency. During early fruit development, glucose and fructose were the predominant sugars. Sucrose began to accumulate 6 weeks before harvest at a higher rate in the fruitlet than in the interfruitlet tissue. Electrolyte leakage from pineapple flesh increased rapidly from 6 weeks before harvest and paralleled sucrose accumulation. Sucrose synthase activity was high in young fruit flesh and declined with fruit development, while the activity of sucrose phosphate synthase was relatively low and constant throughout fruit development. The activities of acid invertase, neutral invertase, and cell-wall invertase (CWI) were high in the young fruit flesh and declined to very low levels 6 weeks before harvest when sucrose started to accumulate. CWI activity increased again, more in the fruitlet than in the interfruitlet tissue , 4 weeks before harvest. Removal of 1/3 of the plant leaves 3 weeks before harvest significantly reduced fruit flesh total solub le solids, CWI activity, and translucency incidence at harvest. The activity of CWI in translucent fruit flesh was significantly higher than in opaque fruit flesh at harvest. CWI activities in the basal section of pineapple flesh and in the fruitlet, where translucency first occurred, were higher than those in the apical section and in the interfruitlet tissue, respectively. Result s support the hypothesis that high CWI activity in pineapple flesh at the later stage of fruit development enhances sucrose unloading into the fruit flesh apoplast, leading to increased apoplastic solute concentration (decreased solute potential) and subsequent water movement into the apoplast. This, in turn, may reduce porosity and lead to increased fruit flesh translucency. susceptible to mechanical damage during harvest and postharvest handling. Postharvest peduncle leakage may lead to unsightly mold growth on the broken peduncle end and is correlated with translu- cency (Paull and Reyes, 1996). Pineapple translucency starts to occur 3 to 4 weeks before harvest in Hawaii and the incidence and severity increase with development. The more mature basal flesh usually is the first to show translucency (Paull and Reyes, 1996), suggesting that translucency is related to maturity and possibly to sugar accumulation, as basal tissue has 3% to 4% higher total soluble solids than the top (Sideris and Krauss, 1933), and the ratio of total soluble solids to acidity of pineapple juice increases with increasing translucency (Bowden, 1969). Sugar content is an important component of pineapple quality (Py et al., 1987). Even though the importance of sugar content in pineapple quality is recognized, our knowledge of carbohydrate metabolism in pineapples is limited. Different pathways are used for sugar accumulation in various fruit species (Hubbard et al., 1991), with different enzymes being involved, including acid invertase, sucrose synthase, and sucrose-phosphate synthase (Yamaki, 1995). In translucent pineapple flesh, the low porosity suggests that the intercellular free air spaces are filled with liquid due possibly to increased membrane permeability of fruit flesh cells or enhanced water movement into the apoplast caused by sugar-induced solute potential gradient between symplast and apoplast. Developmental changes in fruit sugar composition and the activities of sugar metabolizing enzymes in pineapples have not been reported previ- ously. In addition, the relationship between sugar accumulation and translucency occurrence is unknown. The objectives of this study were to determine the changes in the sugar composition and the activities of sugar metabolizing enzymes during pineapple develop- ment, and to describe the relationship between sugar accumulation and the occurrence of flesh translucency in mature pineapples.