Optimizing Light Quantity and Quality for Accelerating Flowering of Petunia with Associated Changes in FLOWERING LOCUS T Gene Expression

As plant factories with artificial lighting (PFALs) expand beyond leafy greens to include flowering crops, understanding the role of light quantity and quality in regulating plant development becomes increasingly important. This study investigated how far-red (FR) radiation and daily light integral (DLI) influences flowering time, plant morphology, and FLOWERING LOCUS T (FT) gene expression in petunia. Four facultative commercial cultivars and one obligate long-day model cultivar, ‘Mitchell Diploid’, were grown under two DLI conditions with a gradient of FR radiation. Increasing FR consistently accelerated flowering across both DLI conditions without reducing flower bud number, branch number, or shoot fresh weight at harvest. Higher DLI generally produced more compact plants by reducing plant height and canopy area, whereas increasing FR promoted stem elongation, particularly in ‘Mitchell Diploid’. Cultivar responses varied substantially, indicating that genotype is an important factor when applying FR-based lighting strategies. To explore potential molecular mechanisms associated with FR-induced flowering acceleration, the expression levels of five petunia homologs, PhFT1 to PhFT5, were analyzed across developmental stages under low and supplemental FR conditions. PhFT2-5 expression was generally associated with flower initiation, with PhFT2 showing the strongest positive relationship with flowering and responsiveness to supplemental FR, whereas PhFT1 showed a decreasing trend over time and was not positively associated with flowering. Overall, this study demonstrates that FR radiation can be used to accelerate petunia flowering in PFAL production without compromising key quality traits and suggests that specific FT homologs, particularly PhFT2, may contribute to FR-mediated flowering regulation.