Altitudinal variation in leaf morphology and functional traits of sea-buckthorn (Hippophae rhamnoides) in Gilgit region, Pakistan

Sea-buckthorn is a multi-purpose plant that provides food, feed, fuel, and medicine. Climate conditions affect its adaptability more than the terrain characteristics. Limited research has focused on its leaf functional traits important for climate adaptation. This study characterized seventy sea-buckthorn accessions from five locations in Gilgit region of northern Pakistan (2444-3172 m.a.s.l) for leaf-functional traits (leaf angle, hairs, rolling, groove, and wettability - summarized as leaf traits and physiological traits including stomatal conductance (gs), leaf relative water content (RWC), transpiration (E), water use efficiency (WUE) and photosynthesis (A)). Leaf surface structures were observed under a scanning electron microscope. Majority of accessions at Misgar (higher altitude) showing 0-20% inward leaf rolling, medium groove, and semi-erect leaf angle had higher E, gs, and lower WUE, RWC, moderate A, and hydrophilic leaf surface. Comparatively, most of accessions at middle altitudes (Passu) indicated adaptive leaf characters i.e., semi-droopy leaves, light leaf groove, 20-40% leaf rolling, moderate trichome density with hydrophobicity (> 90°) and high drop rolling efficiency (< 15°), and had higher A (8.9 ± 1.4 µmol CO₂ m^-2 s^-1), WUE (4.8 ± 0.4 mmol CO₂ mol^-1 H₂O), and fruiting (~ 67%). Moreover, the positive association of WUE with fruit yield indicated high photosynthetic productivity of such accessions. Umbrella-shaped peltate leaf trichomes were 151-175 μm long, with higher densities on abaxial surface, appressed to the epidermis and two layers with overlapping rays-shields. Stomatal density was higher on the abaxial surface, mostly covered by the trichomes. This study provides theoretical backgrounds of an ideotype suited for climate resilience supporting sea buckthorn germplasm conservation.