Antibacterial, Antioxidant Potential, and Chemodiversity of <i>Aesculus indica</i> Bark Extracts Along the Garhwal Himalayan Altitudinal Gradient

The present study aimed to investigate the phytochemical diversity and biological activities of Aesculus indica along an altitudinal gradient in the Garhwal Himalaya encompassing Khirsu (1807 m), Adwani (2109 m), and Kanchula-Kharak (2637 m). Bark extracts were prepared using acetone, methanol, and distilled water. The acetone extract (AE) of Adwani exhibited the highest phenol (276.70 mg GAE/g), flavonoid (995.51 mg QE/g), and tannin content (857.08 mg TAE/g), while the highest flavonol (133.08 mg RE/g) was obtained from Khirsu. Antioxidant activity was evaluated via DPPH, ABTS, and FRAP. Adwani's AE showed the lowest DPPH IC₅₀ (8.90 µg/mL), Khirsu's methanol extract the highest ABTS inhibition (70.85%), and Kanchula-Kharak's AE the highest FRAP value (23.69 mmol Fe²⁺ E/mg). Antibacterial activity was tested against Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli. The AE of Kanchula-Kharak showed the largest inhibition against E. coli (27.33 mm), while Adwani's AE exhibited the lowest MIC (2.812 mg/mL) and MBC (3.75 mg/mL) against S. aureus. Secondary metabolite content, antioxidant, and antibacterial activity positively correlated with altitude (r = 0.96). GC-MS profiling revealed altitude-linked variation in compounds such as phenolic esters, hydrocarbons, phytosterols, triterpenoids, and fatty acid esters. This suggests that evaluating plants across elevations refines phytochemical source selection.