Longan is an ancient and popular plant with highly nutritious fruit. The pulp is the main part of the fruit which is consumed. However, the potentials of other longan fruit parts such as the peel and the seed as reservoirs of important bioactive components have not yet fully been explored. This study aimed at exploring the diversity of primary and secondary metabolites in two commercial longan varieties, i.e., Shixia and Chuliang, concerning fruit, pulp, peel, and seed based on a widely targeted metabolomics approach. A total of 706 metabolites were identified grouped into eight diverse classes of metabolites, including flavonoids, amino acids and its derivatives, alkaloids, tannins, phenolic acids, organic acids, nucleotides and derivatives, lipids, and lignans. Chuliang peel showed a higher accumulation of metabolites viz. flavonoids, alkaloids, and tannins, while Shixia seeds showed abundance in amino acids, organic acid, nucleic acids and its derivatives. Chuliang pulp showed relatively higher accumulation of lipids when compared with other fruit parts. Differential metabolome profiling between fruit parts indicates the prevalence of key metabolites, i.e., O-Phosphocholine, betaine, piperidine, L-Arginine, and L-Phenylalanine, in seed and peel of the longan fruit. The findings in this research provide a new glance on the nutritional value and biochemical basis of longan fruit pharmaceutical value, with emphasis on valorization of longan peel and seed tissues for their pharmaceutical properties and nutritional values.

Longan (Dimocarpus longan L.) is one of the most important tropical and subtropical fruits in the world. Longan fruit has high nutritional and medical value, and is regarded as a treasure among fruits. Since it was first reported that potassium chlorate (KClO3) could be successfully applied to promote flowering in longan, this compound has been widely used in the production of on-season and off-season longan fruits. KClO3 has thus played a great role in promoting the development of the longan industry. In this review, we summarize the application methods, influencing factors, and physiological and molecular mechanisms associated with KClO3-mediated induction of longan flowering. It can be deduced that leaves may play a crucial role in the transport of and response to KClO3. Leaves supply carbon and nitrogen nutrition, and hormone and signaling molecules needed for the differentiation of apical buds. Moreover, cytokinins may be crucial for KClO3-mediated induction of longan flowering. More effort should be focused on studying the molecular mechanisms underlying this process. This will not only help us to better understand floral induction by KClO3 in longan but also enrich our understanding of flowering regulation mechanisms in woody plants.