In Vitro Multiplication, Antioxidant Activity, and Phytochemical Profiling of Wild and In Vitro-Cultured Plants of <i>Kaempferia larsenii</i> Sirirugsa—A Rare Plant Species in Thailand

<i>Kaempferia larsenii</i> Sirirugsa, a rare species in Thailand belonging to the Zingiberaceae family, is known for its effective pharmaceutical properties. However, its slow natural growth and threats from overharvesting and habitat encroachment pose significant challenges. This study investigated the effects of plant growth regulators (PGRs) on the plant regeneration, transplantation success, phytochemical profiling, and antioxidant properties of wild and in vitro-cultured plants. Plantlets (~1 cm long) were cultivated for 8 weeks in different types of MS media (solid, liquid, and liquid-over-solid) combined with various PGRs (BA, kinetin, TDZ, NAA, and IAA). Solid MS medium enriched with 2 mg/L BA, 3 mg/L TDZ, and 0.2 mg/L NAA produced the highest number of shoots (13.10 shoots/explant). By comparison, liquid MS medium containing 1 mg/L BA and 0.5 mg/L IAA also promoted high shoot production (4.70 shoots/explant). The strongest root induction (16.90 roots/explant) was achieved using a liquid MS medium supplemented with 2 mg/L BA, 2 mg/L kinetin, and 1 mg/L NAA. Sandy soil as a planting material yielded the highest survival rate (100%) during transplantation. The total phenolic content (TPC) and total flavonoid content (TFC) were higher in mother plants than in in vitro-cultured plants. The addition of PGRs significantly enhanced the production of secondary metabolites. The leaves of <i>K. larsenii</i> exhibited superior antioxidant properties compared to other organs under both growing conditions. Cinnamic acid was identified as abundant in in vitro-cultured plants via HPLC analysis. FTIR analysis revealed functional groups associated with phenolic acids and flavonoids, which are useful for phytochemical screening and antioxidant evaluation. This research highlighted the potential of biotechnology as a crucial strategy for conserving <i>K. larsenii</i> and demonstrated its sustainable application in the medical and cosmetics industries.