Seedling age determines transplantation success, growth and physio-biochemical performance of Arnebia benthamii (Wall. ex G. Don) I.M. Johnst. under polyhouse-controlled low-altitude conditions

Himalayan mountains host unique alpine species adapted to extreme conditions. Arnebia benthamii , as a characteristic alpine species, constitutes an integral component of the Himalayan Mountain ecosystem. Yet, overexploitation in its native habitat has caused population decline, necessitating conservation through cultivation at lower altitudes. The study evaluated age-dependent transplantation success and post-transplant physiological performance of A. benthamii seedlings transferred from high-altitude (2646.32 ± 9 m a.m.s.l.) to a low-altitude polyhouse conditions (1,530 m a.m.s.l.). Seedlings aged 1 (1 M), 3 (3 M), and 6 (6 M) months were assessed for survival, growth (rhizome and shoot development), and biochemical parameters (total phenolics, antioxidant activity, and chlorophyll content). Transplantation success, assessed as survival percentage, was recorded 30 days post-transplantation, while growth, physiological and biochemical parameters were measured six months after transplantation. Results showed that 6-month-old seedlings had the highest survival rate (83.33%) and significantly ( p < 0.05) superior shoot length, chlorophyll b, total chlorophyll, and carotenoid content, along with significantly higher total phenolic content and DPPH radical scavenging activity. These findings demonstrate that transplantation success in A. benthamii is strongly age-dependent, driven by progressively enhanced physiological and biochemical resilience in older seedlings. Among the age groups evaluated, 6 M seedlings demonstrated the most reliable establishment success and are recommended for nursery-level propagation and transplantation of A. benthamii . Polyhouse-based cultivation represents a scalable and practical model for sustainable propagation and ex-situ conservation of this threatened Himalayan medicinal species. By linking seedling age with adaptive responses under altered microclimatic conditions, the study offers a practical strategy for large- scale propagation, cultivation and opportunity to increase the economy of local farmers.