Water transport, vascular bundle integrity, and enzymatic activity in rambutan pericarp and spine browning

Postharvest rambutan (Nephelium lappaceum L.) rapidly loses its visual quality owing to accelerated water loss and browning of the spines and pericarp. Notably, the water loss and browning of the rambutan spines progress significantly quicker than that of the pericarp. To investigate the impact of the loss of water on the structural changes of the cell induced by enzymatic browning, the water status and distribution of rambutan peel during storage were analyzed. In addition, the ultrastructure of the pericarp and spines was observed. The spines lost significantly more water and browned more intensively compared to the pericarp. The low field NMR images suggested that the supply of water from the pericarp to the spines ceased during storage. Ultrastructural observations showed that the cell structure of vessels in the xylem was destroyed, and the phloem disappeared in the spines during storage. This probably terminated the hydraulic connectivity between the pericarp and spines. The severe water loss in the spines destroyed the plasma and vesicle membranes and the mitochondrial structure. Compared to the pericarp, the spines had higher levels of activities of polyphenol oxidase, peroxidase, and phenylalanine ammonia lyase during storage, which was accompanied by a more rapid decrease in the phenolics and a quicker accumulation of o-quinones. These results suggested that severe water loss in the spines may be associated with disruption of the vascular bundles, which leads to an insufficient water supply from the pericarp. The severe water loss further caused a breakdown of cellular compartmentalization, which facilitated contact between the enzymatic related to browning and the phenolics. Thus, this exacerbated browning in the spines.