Folic acid coordinates metabolomic and transcriptomic reprogramming to enhance growth plasticity, photosynthetic efficiency, flavonoid biosynthesis, and glycosylation in dendrobium officinale

• FA markedly promotes vigorous new stem development in D. officinale. • FA improves photosynthetic efficiency, increasing Fv/Fm, Y(II), and ETR while reducing Y(NPQ). • FA enhances antioxidant capacity, elevating DPPH and ABTS radical scavenging activity. • FA substantially increases the accumulation of TFC, TPC, and amino acids. • FA promotes flavonoid biosynthesis by upregulating precursor availability and the expression of CHS and GTs. Folic acid (FA) plays a critical role in plant nutrient metabolism and stress responses; however, its regulatory functions in medicinal orchids, particularly regarding secondary metabolite biosynthesis and photosynthetic adaptation, remain poorly understood. Dendrobium officinale , a threatened medicinal orchid valued for its bioactive compounds, faces cultivation challenges due to environmental sensitivity and limited metabolite accumulation. Here, we investigated the effects of foliar FA application on growth, photosynthetic efficiency, antioxidant capacity, and metabolic reprogramming in D. officinale . Plants were sprayed weekly with 5 mg·L⁻¹ FA or distilled water (control) for 16 weeks. Growth parameters, chlorophyll fluorescence, antioxidant activity, and integrated metabolomic and transcriptomic profiles were then assessed. FA treatment promoted uniform bud distribution, increased stem elongation by 24.6%, and improved post-harvest regeneration capacity, collectively indicating enhanced growth plasticity. Under low-light conditions, FA improved photosynthetic performance, elevating PSII maximum quantum yield (Fv/Fm), actual photochemical efficiency (Y(II)), and electron transport rate (ETR), while reducing regulated energy dissipation (Y(NPQ)). Antioxidant capacity was significantly enhanced, with DPPH and ABTS radical scavenging activities increasing by 15.6% and 14.9%, respectively. Metabolomic analysis revealed a 61.5% increase in total flavonoid content, driven by enhanced precursor accumulation (phenylalanine/tyrosine), upregulation of key biosynthetic enzymes (e.g., chalcone synthase), and selective induction of glycosyltransferases that facilitated flavonoid glycosylation. Transcriptomic analysis further confirmed enrichment of flavonoid biosynthetic pathways and differential expression of glycosyltransferase genes. Overall, FA functions as a multifunctional regulator, coordinating photosynthetic adaptation and secondary metabolism to enhance both stress resilience and medicinal quality in D. officinale . These findings provide a promising strategy for optimizing the cultivation of shade-adapted medicinal plants.