Sugar metabolism and pineapple flesh translucency

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.