The composition and content of phenolic acids and flavonoids among the different varieties, development stages, and tissues of Chinese jujube (Ziziphus jujuba Mill.) were systematically examined using ultra-high-performance liquid chromatography to provide a reference for the evaluation and selection of high-value resources. Five key results were identified: (1) Overall, 13 different phenolic acids and flavonoids were detected from among the 20 excellent jujube varieties tested, of which 12 were from the fruits, 11 from the leaves, and 10 from the stems. Seven phenolic acids and flavonoids, including (+)-catechin, rutin, quercetin, luteolin, spinosin, gallic acid, and chlorogenic acid, were detected in all tissues. (2) The total and individual phenolic acids and flavonoids contents significantly decreased during fruit development in Ziziphus jujuba cv.Hupingzao. (3) The total phenolic acids and flavonoids content was the highest in the leaves of Ziziphus jujuba cv.Hupingzao, followed by the stems and fruits with significant differences among the content of these tissues. The main composition of the tissues also differed, with quercetin and rutin present in the leaves; (+)-catechin and rutin in the stems; and (+)-catechin, epicatechin, and rutin in the fruits. (4) The total content of phenolic acid and flavonoid ranged from 359.38 to 1041.33 μg/g FW across all examined varieties, with Ziziphus jujuba cv.Jishanbanzao having the highest content, and (+)-catechin as the main composition in all 20 varieties, followed by epicatechin, rutin, and quercetin. (5) Principal component analysis showed that (+)-catechin, epicatechin, gallic acid, and rutin contributed to the first two principal components for each variety. Together, these findings will assist with varietal selection when developing phenolic acids and f lavonoids functional products.

Introduction: Jujube is an important economic forest tree whose fruit is rich in alkaloids. Chinese jujube (Ziziphus jujuba Mill.) and sour jujube (Ziziphus spinosa Hu.) are the two most important species of the jujube genus. However, the mechanisms underlying the synthesis and metabolism of alkaloids in jujube fruits remain poorly understood.Methods: In this study, the fruits of Ziziphus jujuba ‘Hupingzao’ and Ziziphus spinosa ‘Taigusuanzao’ in different harvest stages were used as test materials, we first integrated widely targeted metabolomics and transcriptomics analyses to elucidate the metabolism of alkaloids of jujube fruits.Results: In the metabolomics analysis, 44 alkaloid metabolites were identified in 4 samples, 3 of which were unique to sour jujube fruit. The differential alkaloid metabolites (DAMs) were more accumulated in sour jujube than in Chinese jujube; further, they were more accumulated in the white ripening stage than in the red stage. DAMs were annotated to 12 metabolic pathways. Additionally, transcriptomics data revealed 259 differentially expressed genes (DEGs) involved in alkaloid synthesis and metabolism. By mapping the regulatory networks of DAMs and DEGs, we screened out important metabolites and 11 candidate genes.Discussion: This study preliminarily elucidated the molecular mechanism of jujube alkaloid synthesis. The candidate genes regulated the synthesis of key alkaloid metabolites, but the specific regulation mechanism is unclear. Taken together, our results provide insights into the metabolic networks of alkaloid synthesis in Chinese jujube and sour jujube fruits at different harvest stages, thereby providing a theoretical reference for further research on the regulatory mechanism of jujube alkaloids and their development and utilization.

Chinese jujube (Ziziphus jujuba Mill.) originated in the Yellow River basin (YRB) of the Shanxi–Shaanxi region. The genomic C-value is a crucial indicator for plant breeding and germplasm evaluation. In this study, we used flow cytometry to determine the genomic C-values of jujube germplasms in the YRB of the Shanxi–Shaanxi region and evaluated their differences in different sub-regions. Of the 29 sub-regions, the highest and lowest variations were in Linxian and Xiaxian, respectively. The difference between jujube germplasms was highly significant (F = 14.89, p < 0.0001) in Linxian. Cluster analysis showed that both cluster 2 and 4 belonged to Linxian, which were clearly separated from other taxa but were cross-distributed in them. Linxian County is an important gene exchange center in the YRB of the Shanxi–Shaanxi region. Principal component analysis showed that cluster 1 had low genomic C-values and single-fruit weights and cluster 2 had high genomic C-values and vitamin C contents. The genomic C-value was correlated with single-fruit weight and vitamin C content. In addition, the genomic C-value was used to predict fruit agronomic traits, providing a reference for shortening the breeding cycle and genetic diversity-related studies of jujube germplasm.

Flavonoids are an important type of bioactive substance contained in jujubes. Flavonol synthase (FLS) is a key enzyme for the synthesis of flavonoids such as flavonols and anthocyanins. To study the biological functions of FLS in jujubes, we cloned the ZjFLS gene; analyzed its physicochemical properties and evolutionary relationships; and then conducted an expression characteristic analysis, subcellular localization, prokaryotic expression and heterologous overexpression in Arabidopsis thaliana. The results showed that the length of ZjFLS is 951 bp, and it encodes 316 amino acids. A sequence analysis revealed that ZjFLS exhibited a high degree of conservation in evolution. The results of a qRT-PCR analysis indicated that the ZjFLS gene could be expressed in different tissues of jujube: the expression level was the highest in the leaves, followed by the flowers, and the lowest was in the fruits. Within these expression levels, it was higher in young leaves than in mature leaves and higher in the white-ripe-stage fruits than in the semi-red-stage fruits. Subcellular localization indicated that the ZjFLS gene was located in the nucleus, cytoplasmic matrix, and cytoplasmic membrane. Our research findings show that the ZjFLS protein can be induced and obtained in the prokaryotic expression system and successfully purified. It mainly exists in the form of inclusion bodies and has a relatively low content in the soluble supernatant. The total flavonoid content of Arabidopsis thaliana strains with a heterologous overexpression of the ZjFLS gene was significantly higher than that of the wild type, confirming that the ZjFLS gene can promote the biosynthesis of flavonoid substances.