Reproductive biology, parthenocarpy, and fruit quality traits of the citrus hybrid 'Faustrime' (Citrus australasica × [C. aurantifolia × Fortunella japonica])

'Faustrime' is a hybrid between Australian finger lime, also known as caviar lime (Citrus australasica), and 'Eustis' limequat (C. aurantiifolia × Fortunella japonica), originally developed in California. Despite its growing commercial interest, this genotype has not yet been comprehensively characterised from morphological and agronomic perspectives. In the present study, we assessed flowering intensity, fruit set, parthenocarpic ability, and fruit development. Notably, 'Faustrime' requires a lower thermal accumulation to induce flowering through CiFT3 gene expression than sweet orange (Citrus sinensis [L.] Osbeck), and it displays a marked capacity for summer flowering. Regarding fruit set, the hybrid achieves an optimal crop load due to its high parthenocarpic ability, which exceeds that of both sweet orange and its parental species, C. australasica. Consistent with this behaviour, 'Faustrime' synthesises higher levels of auxin and bioactive gibberellins (GA1, GA4, and GA7) than its progenitors. In addition, this hybrid exhibits high nutritional value and distinctive organoleptic characteristics. Its pollen grains are pentacolpate, and both the ovary and fruit are oval in shape. The fruit contains small, spherical juice vesicles with a slightly acidic taste and a crunchy texture, which are highly appreciated in haute cuisine. Furthermore, the flavedo shows a high concentration of essential oils, mainly D-limonene (52%), γ-terpinene (12%), citronellal (15%), and δ-3-carene (5%), together with n-alkanes and coumarins, compounds valued in the pharmaceutical, perfumery, and cosmetic industries. Overall, these characteristics underline the potential of 'Faustrime' as a novel citrus genotype with agronomically relevant traits and prospective economic viability.