Citrus grandis ‘Tomentosa’, commonly known as ‘Huajuhong’ pummelo (HJH), is used in traditional Chinese medicine and can moisten the lungs, resolve phlegm, and relieve coughs. A spontaneous bud mutant, named R-HJH, had a visually attractive phenotype with red albedo tissue and red juice sacs. In this study, the content and composition of carotenoids were investigated and compared between R-HJH and wild-type HJH using HPLC–MS analysis. The total carotenoids in the albedo tissue and juice sacs of R-HJH were 4.03- and 2.89-fold greater than those in HJH, respectively. The massive accumulation of carotenoids, including lycopene, β-carotene and phytoene, led to the attractive red color of R-HJH. However, the contents of flavones, coumarins and most volatile components (mainly D-limonene and γ-terpinene) were clearly reduced in R-HJH compared with wild-type HJH. To identify the molecular basis of carotenoid accumulation in R-HJH, RNA-Seq transcriptome sequencing was performed. Among 3948 differentially expressed genes (DEGs), the increased upstream synthesis genes (phytoene synthase gene, PSY) and decreased downstream genes (β-carotene hydroxylase gene, CHYB and carotenoid cleavage dioxygenase gene, CCD7) might be the key factors that account for the high level of carotenoids in R-HJH. These results will be beneficial for determining the molecular mechanism of carotenoid accumulation and metabolism in pummelo.

Sunburn is a major abiotic stress in fruit crops, leading to significant economic losses due to reduced fruit quality and marketability. Wampee (Clausena lansium), a tropical specialty fruit with growing industrial interest, is particularly vulnerable to sunburn-induced peel browning during ripening. To elucidate the underlying mechanisms, we conducted comprehensive field surveys and integrated physiological, biochemical, metabolomic, and transcriptomic analyses. Our integrated analyses suggest that strong light exposure is a key environmental trigger for sunburn, while shading treatments effectively mitigated damage despite elevated bag temperatures. Physiological assessments revealed that sunburn progression involved reactive oxygen species (ROS) accumulation, membrane lipid peroxidation, and dynamic changes in antioxidant enzymes (SOD, CAT, APX) and osmotic regulators. Metabolomic analysis identified 277 differentially accumulated metabolites, with significant enrichment of phenylpropanoid and flavonoid compounds such as coniferol, dihydrokaempferol, and naringenin-under severe stress. Transcriptomic data further highlighted coordinated upregulation of phenylpropanoid biosynthesis, flavonoid biosynthesis, and heat stress response pathways. We identified 28 structural genes and 56 transcription factors (including WRKY28, bHLH30, and HSFC1) as central regulators. Integrative analysis suggested that WRKY and bHLH transcription factors are associated with the upregulation of key genes (e.g., PAL and 4CL) and the accumulation of antioxidant metabolites, implying a potential role in improving photo-thermal tolerance. This study provides valuable physiological and molecular insights into sunburn response in wampee fruit and offers potential strategies for breeding resistant varieties and developing postharvest management practices to enhance fruit quality in industrial cultivation.