558 J. AMER. SOC. HORT. SCI. 125(5):558–562. 2000. J. AMER. SOC. HORT. SCI. 125(5):558–562. 2000. Sugar Metabolism and Pineapple Flesh Translucency Ching-Cheng Chen 1 and Robert E. Paull 2 Department of Tropical Plant and Soil Sciences, University of Hawaii at Manoa, College of Tropical Agriculture and Human Resources, 3190 Maile Way, Honolulu, HI 96822 ADDITIONAL INDEX WORDS. Ananas comosus, plant development, enzymes ABSTRACT. 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 soluble 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. Results 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. Materials and Methods PLANT AND FRUIT MATERIALS. The translucent susceptible ‘Smooth Cayenne’ pineapple (clone 3621) from the Dole Food Co. plantation on the island of Oahu in Hawaii was used in all experiments. The Received for publication 3 Aug. 1999. Accepted for publication 11 Mar. 2000. College of Tropical Agriculture and Human Resources Journal Series no. 4465. We thank Dole Food Co., Honolulu, Hawaii for pineapple fruit samples, and Nancy Chen, Gail Uruu, and Ted Goo for laboratory assistance. This study was funded by the Hawaii Department of Agriculture Grant no. 43182. This research is a portion of a dissertation submitted by Ching-Cheng Chen for the PhD degree in horticulture. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. 1 PhD fellow. 2 Professor, to whom reprint requests should be addressed; e-mail: paull@hawaii.edu. Pineapple (Ananas comosus) is one of the most economically important crops in tropical and subtropical areas. Pineapple is a perennial herbaceous monocotyledonous plant that produces a single syncarpous fruit at the apex of the stem by fusion of a number of berrylike fruitlets, called eyes, that develop from the flowers. In ‘Smooth Cayenne’, the most widely grown cultivar, there are 100 to 200 flowers per inflorescence, which are arranged in a compact spiral cluster. Flowering occurs in acropetal sequence with one to several flowers opening each day (Okimoto, 1948). Ovaries of adjacent flowers, developing into fruitlets, are separated by the noncarpellary parenchymatous tissue, consisting largely of sepal and bract bases (Okimoto, 1948), called ‘interfruitlet tissue’. Flower initiation takes place at the terminal axis of the stem and is induced naturally by short days and cool nights, or can be forced with chemicals, such as ethephon (2-chloroethylphosphonic acid). Chemi- cal forcing of flowering is done commercially at any time of year when the plants are of sufficient size. The days from forcing to harvest varies with solar radiation and temperature and takes from 6 to 7 months (3 to 4 months from the end of flowering to harvest) in Hawaii (Nakasone and Paull, 1998). Translucency is a physiological disorder of pineapple flesh. The affected flesh has low porosity and a water soaked appearance, due to the intercellular free spaces being filled with liquid (Sideris and Krauss, 1933). Highly translucent fruit has flat and overripe off flavors, and significantly lower edible quality (Bowden, 1967). Translucent fruit are prone to preharvest sunburn (Keetch, 1977) and are extremely fragile (Py et al., 1987), making these fruit