The harsh natural environment on the Qinghai–Tibetan Plateau has a detrimental effect on the growth of vegetation. Elymus dahuricus, a widely distributed perennial herb on the Qinghai–Tibetan Plateau, is highly adapted to alpine regions. To unveil the mechanism of E. dahuricus adaptation to high altitude on the Qinghai–Tibetan Plateau, the relative photosynthetic characteristics and physiological indexes of wild E. dahuricus collected from different elevations in Huangzhong County and Minhe County of Qinghai Province were investigated. Increases in the maximum photochemical efficiency (Fv/Fm), potential activity (Fv/Fo), non-photochemical quenching (NPQ), total carotenoids content (Car), chlorophyll a to chlorophyll b ratio (Chl a/b) and total carotenoids to chlorophyll ratio (Car/Chl) were accompanied by decreases in photochemical quenching coefficient (qP), effective photochemical quantum yield of PSII (Y(II)) and chlorophyll a (Chl a) and chlorophyll b (Chl b) contents. Increases in Fv/Fm and Fv/F0 with altitude indicate that the photosynthetic capacity can be maintained with increases in altitude. As a photoprotective mechanism, decreases in chlorophyll content could lead to a reduction in the absorption of high energy light by leaves; therefore, the photosynthetic capacity of E. dahuricus was maintained on the Qinghai–Tibetan Plateau. Furthermore, the increasing malondialdehyde content along altitudinal gradients indicated that the alpine environments had an adverse effect on E. dahuricus growth. The increase in superoxide dismutase, peroxidase and catalase activities reflected a higher reactive oxygen species scavenging capacity, and the increases in soluble sugar and proline contents increased the osmotic adjustment substance contents, suggesting that the reactive oxygen species scavenging ability and osmotic pressure regulation play roles in maintaining the normal growth of E. dahuricus. In conclusion, the high altitude on the Qinghai–Tibetan Plateau negatively affected E. dahuricus growth, and the adaptation mechanism and survival strategies of E. dahuricus were ascribed to the comprehensive effects of photosynthetic capacity, reactive oxygen species scavenging and osmotic adjustments.
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