Rootstock-Induced Volatile Fingerprint Shifts in Citrus medica ‘Guangdong Xiangshui’ Leaves Revealed by HS-GC-IMS

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’.