Citric acid content is a critical quality determinant in citrus (Citrus spp.) fruits. Although arsenic (As) can effectively reduce citric acid content to improve citrus fruit quality, it can have adverse environmental effects. The discovery of nontoxic substitutes is hampered by the incomplete elucidation of the underlying mechanisms of As action in citrus fruits. Metabolic, transcriptomic, and physiological analyses were employed to investigate As action on citric acid accumulation to discover the mechanisms of As action in citrus. The enzyme activity related to citrate biosynthesis was not inhibited and the content of the involved metabolites was not reduced in As-treated fruits. However, the proton pump genes CitPH5 and CitPH1 control the vacuolar citric acid accumulation and transcription factor genes CitTT8 and CitMYB5, which regulate CitPH5 and CitPH1, were downregulated. The oxidative stress-response genes were upregulated in As-treated fruits. The reactive oxygen species (ROS) treatment also downregulated CitTT8 and CitMYB5 in juice cells. The mitochondrial ROS production rate increased in As-treated fruits. As^III was more potent in stimulating isolated mitochondria to overproduce ROS compared to As^V. Our results indicate that the As inhibition of citric acid accumulation may be primarily due to the transcriptional downregulation of CitPH5, CitPH1, CitTT8, and CitMYB5. As-induced oxidative stress signaling may operate upstream to downregulate these acid regulator genes. Mitochondrial thiol proteins may be the principal targets of As action in citrus fruits.

Citrus medica ‘Guangdong Xiangshui’ is a commercially valuable specialty citrus cultivar whose leaf volatile organic compounds (VOCs) hold promise for applications in food flavor development. Although rootstock genotype is known to influence secondary metabolism in scions, its specific regulatory effects on leaf VOCs in this cultivar remain poorly understood. In this study, headspace gas chromatography–ion mobility spectrometry (HS-GC-IMS) combined with multivariate statistical analysis was employed to profile leaf VOCs from plants grafted onto four different rootstocks, alongside own-rooted controls. For comparative purposes, leaf and pericarp VOC profiles were also analyzed under identical conditions. A total of 43 valid ion signals were detected, of which 27 neutral parent volatile compounds—predominantly aldehydes, alcohols, ketones, esters, and sulfur-containing compounds—were tentatively identified via database matching (match score > 80%). Distinct volatile fingerprints were observed among rootstock genotypes and propagation modes, and nine candidate differential compounds with variable importance in projection (VIP) > 1 were screened, demonstrating that HS-GC-IMS coupled with exploratory chemometric analysis enables preliminary discrimination of leaf samples across rootstock treatments. Comparative tissue analysis revealed pronounced metabolic divergence: leaves accumulated abundant C6 aldehydes and alcohols, indicative of lipoxygenase (LOX) pathway activity, whereas pericarp exhibited a higher proportion of terpenoids, consistent with terpene synthase (TPS) pathway dominance. However, these metabolic profiling results alone do not conclusively establish pathway-mediated regulatory mechanisms, necessitating further molecular and biochemical investigations. Collectively, this study provides foundational data for rootstock selection and the utilization of leaf aroma resources in C. medica ‘Guangdong Xiangshui’.