Influence of cold treatment and photoperiod on flowering pathways of different-sized bulbs in Lilium×formolongi

•Flowering and quality in Lilium×formolongi bulbs are influenced by photoperiods, cold treatments, and developmental stages.•Small bulbs utilize the photoperiod pathway, while large bulbs, particularly after cold treatment, activate the vernalization pathway. Flowering is a pivotal horticultural trait influenced by temperature, photoperiod, and developmental stage. Lilium×formolongi is a perennial plant with high ornamental value and suitability as cut flowers. Besides seed propagation, it can also be propagated using bulbs. However, when using bulbs as propagules, research on influecing flowering is still not comprehensive enough. In this study, we investigated the flowering of Lilium×formolongi bulbs of different sizes induced by different temperature and photoperiod conditions, as well as the expression patterns of key genes in the photoperiod pathway and vernalization pathway during this process. Results highlighted the long-day photoperiod as a prerequisite for flowering, especially for small bulbs. Cold treatment enhanced the expression of photoperiodic genes and promoted flowering in small bulbs. Remarkably, when Lilium×formolongi flowered via the photoperiod pathway, a consistent FLAVIN-BINDING, KELCH REPEAT, F-BOX 1-GIGANTEA -CONSTANS-FLOWERING LOCUS T (GI-FKF1-CO-FT) flowering control module was upregulated. For large bulbs, the long-day condition and cold treatment were mutually substituted, combining cold treatment with long-day not only promoted flowering but also enhanced flowering quality. Without cold treatment, flowering was induced through the photoperiod pathway under long-day conditions. Conversely, when cold treatment was applied, the vernalization pathway play a predominant role irrespective of photoperiod, promoting flowering by inhibiting floral repressor genes such as SHORT VEGETATIVE PHASE (LfSVP), FRIGIDA (LfFRI), and VERNALIZATION2 (LfVRN2) to enhance LfVRN1 and LfFT1 expression. In conclusion, we have clarified the temperature and photoperiod conditions required for flowering in bulbs of different sizes of Lilium×formolongi, and it can be applied to production practices and the regulation of flowering periods.