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.