Chemical diversity of native Australian fruits: The unique volatile chemotypes of five finger lime (Citrus australasica) cultivars

The finger lime (Citrus australasica F.Muell.) is one of six Citrus species endemic to Australia and has attracted recent interest as a potential commercial crop (Delort & Yuan, 2018). In contrast to most established crops, which show very low levels of intraspecific genetic diversity as an inadvertent result of selective breeding efforts, finger limes retain high levels of morphological diversity in the fruit size and colour (Agrifutures Australia, 2017). However, the extent of chemical diversity within this species is unknown. To fill this knowledge gap and aid in future commercialisation efforts for this species, we conducted a comparative analysis of the volatile constituents in five Australian finger lime cultivars. Gas chromatography coupled with single quadrupole mass spectrometry (GC-MS) revealed the presence of at least 113 volatile constituents in the peel extracts. Seventy five of these compounds were positively identified, including at least 10 compounds not previously reported from this species. A further 31 volatile compounds were tentatively identified. All finger lime cultivars had limonene as the most abundant volatile compound (ranging from 61.4-87.5% of the total volatile content), with the predominant chemotypes including limonene/γ-terpinene/citronellal, limonene/β-citronellol/citronellal, limonene/bicyclogermacrene/γ-terpinene and limonene/bicyclogermacrene/β-myrcene. Most of these chemotypes have not been reported in previous work on finger lime volatiles (Delort, et al. 2015). In addition to displaying highly distinctive chemotypes compared to the commercial Tahitian lime (Citrus × latifolia), there was a high level of inter-varietal differences, indicative of extensive chemodiversity within Citrus australasica as a species. Similarly, profiling of the phenolic acids via high-performance liquid chromatography (HPLC) revealed that this chemical diversity extended to the type and abundance of various phenolic acids and flavonoids in both the pulp and peel. Due to the unique organoleptic properties stemming from their volatile constituents, finger limes show promise for commercial development as a boutique food.