Plant growth regulators enhance growth and photosynthetic performance in endangered Firmiana Kwangsiensis seedlings

Firmiana kwangsiensis, a species endemic to China's Karst region, is listed as one of China's 120 key protected wild plants with extremely small populations. Climate change and human activities have severely restricted the survival and distribution of wild F. kwangsiensis communities, making urgent rescue and conservation measures imperative. To elucidate the growth-promoting mechanisms of plant growth regulators (PGRs) on F. kwangsiensis seedlings, a foliar spray experiment was conducted, and morphological, leaf, stem, and photosynthetic traits were measured. The results showed: (1) PGRs significantly enhanced morphological, stem, and photosynthetic indices, improved leaf traits, and promoted seedling growth and development; (2) Tight linkages existed between photosynthetic and growth traits in seedlings, with net photosynthetic rate (Pn) and chlorophyll content (measured as SPAD values) identified as key drivers of growth; (3) PGRs improved seedling quality by enhancing photosynthetic-growth interactions; (4) PGRs exhibited a "low-concentration promotion and high-concentration inhibition" effect, with highest growth observed under 100 mg·L^{- 1} NAA treatment, followed by 200 mg·L^{- 1} IAA and 50 mg·L^{- 1} SA. This study provides novel insights for the conservation and management of F. kwangsiensis.