Browning Mechanism of Water Convolvulus (Ipomoea aquativa Forsk.) Stored at Low Temperature

Abstract

The young leaves and stems of water convolvulus, a leafy vegetable that originates in tropical regions, showed browning as a symptom of chilling injury at temperatures below 9 °C. The main compound associated with browning in this vegetable was found to be chlorogenic acid, synthesized by caffeic acid:CoA ligase (CL) and caffeoyl CoA:quinate hydroxycinnamoyl transferase (CQT). The activity of CQT was higher at 1 °C than at 20 °C. Three phenolic fractions, free, esterified and bound, were extracted from water convolvulus. At 1 °C, a part of the bound form was transformed into a free reactive form. Electron-dense deposits produced by action of polyphenoloxidase on DOPA were observed with electron microscopy. These deposits were only seen in grana-and stomathylakoids. Chloroplast structure in the mesophyll cells was destroyed before the appearance of chilling injury. Three fractions: F-I, chloroplast rich precipitate (300~3,000Xg); F-II, a precipitate (3,000~100,000Xg) and F-III, supernatant (100,000Xg) were obtained by centrifugation. The activities of CQT and PPO in F-I increased, reaching a maximum just before the occurrence of chilling injury.A great deal of attention has been focused on browning associated with chilling injury of fruits and vegetables during low temperature storage. In water convolvulus, an important leafy vegetable located in southeast Asia and southern Japan, browning was observed in the young leaves and stems during low temperature storage. In higher plants, phenolic acids consist of free and bound compounds and the free form is more reactive in browning than the bound one (7, 2). Shomer et al. (3) reported that polyphenoloxidase (PPO) is localized in the chloroplast thylakoid membrane in olive fruit. The objectives of our studies were to identify the levels and precursors of browning, changes in organization of the cells, the cellular localization of CQT and PPO in water convolvulus stored at low temperature, and to examine the mechanism of browning associated with chilling injury.

Influence of packaging and storage conditions on biochemical quality and enzymatic activity in relation to shelf life enhancement of fresh basil leaf

“…Catechol oxidase can enhance POD-mediated browning or act independently. Catechol oxidase also uses phenolic compounds as a substrate and its activity has been correlated with chilling injury symptoms in some leafy vegetables (Ose et al 1995). Maintenance of membrane integrity during storage has been considered important which changes with the change in temperature.…”