Mandarins have many unique flavonoids with documented health benefits and that help to prevent chronic human diseases. Flavonoids are difficult to measure and cannot be phenotyped without the use of specialized equipment; consequently, citrus breeders have not used flavonoid contents as selection criteria to develop cultivars with increased benefits for human health or increased tolerance to diseases. In this study, peel, pulp, and seed samples collected from many mandarin accessions and their hybrids were analyzed for the presence of selected flavonoids with documented human health benefits. A genome-wide association study (GWAS) was used to identify SNPs associated with biosynthesis of flavonoids in these mandarin accessions, and there were 420 significant SNPs were found to be associated with 28 compounds in peel, pulp, or seed samples. Four candidate genes involved in flavonoid biosynthesis were identified by enrichment analysis. SNPs that were found to be associated with compounds in pulp samples have the potential to be used as markers to select mandarins with improved phytonutrient content to benefit human health. Mandarin cultivars bred with increased flavonoid content may provide value to growers and consumers.

This study utilized the Fourier-based method to analyze the morphology of over 4000 leaves from more than 190 accessions selected from the Givaudan Citrus Variety Collection at the University of California Riverside, one of the world’s most diverse collections of citrus and closely related genera. Our analysis revealed significant variations in leaf morphology among the major citrus species groups, and hybrid varieties produced through breeding exhibited intermediate leaf morphology compared to their parental citrus species. We found a positive correlation between leaf area in native citrus species and temperature in lower tropical latitudes, while negative/positive associations between aspect ratio and temperature/rainfall were also observed, respectively. These results suggest that citrus leaves may have evolved into larger but thinner leaves to increase their photosynthetic capacity per unit area while maintaining water balance by reducing water loss through transpiration. Our analysis also indicates that the existing biodiversity observed in citrus species can be attributed to their migrations across the foothills of the Himalayas, southward to the islands of Indonesia, and northward to the islands of Japan. Our study supports the hypothesis that citrus species have adapted to warm areas lacking extreme daily and nightly temperatures, where a large number of species of origin are found. Overall, this study presents a promising approach to investigate the morphological variation in citrus leaves, which could potentially aid in the selection and breeding of citrus cultivars with superior physiological traits and deepen our understanding of citrus diversity.

Citrus fruits are valued not only for their nutritional benefits but also for their rich phytochemical content. Metabolomics has emerged as a comprehensive technique for assessing the chemical composition of fruits. The botanical connection between leaves, flowers, and fruits is reflected in both their structure and chemical composition, particularly in the flow of nutrients between plant organs. We introduced a new logarithm ratio-based approach to compare metabolite profiles between fruits and leaves. We hypothesize that this method allows for the analysis of multiple citrus metabolomic profiles to reveal known and novel correlation patterns, reflecting the dynamic connections between metabolic sources. To test this hypothesis, we leveraged comprehensive leaf metabolomic profiles from over 200 accessions in the Givaudan Citrus Variety Collection and reviewed published metabolomics data for fruits and juices of matching citrus types. By employing logarithm-transformed metabolic ratios within each dataset, we accounted for systematic differences across metabolomic platforms, achieving an unbiased analysis.