Leaf morphology, functional trait and altitude response in perennial vetch (Vicia unijuga A. Braun), alfalfa (Medicago sativa L.) and sainfoin (Onobrychis viciifolia Scop.).

Species have plasticity across altitude gradients in leaf morphology and function, and their response to high altitude conditions was mainly reflected in leaf cell metabolism and gas exchange. Leaf morphological and functional adaptation to altitude has received research attention in recent years, but there are no studies for forage legumes. Here we report differences in 39 leaf morphology and functional traits of three leguminous forages (alfalfa, sainfoin and perennial vetch) at three sites in Gansu Province, China, ranging from 1768 to 3074 m altitude to provide information for potential use in breeding programmes. With increasing altitude, plant water status increased, reflecting increase in soil water content and decreased average temperature, which lead to leaf intercellular CO concentration. Stomatal conductance and evapotranspiration increased significantly but water-use efficiency decreased. At high altitude, Φ decreased but non-photochemical quenching and chlorophyll a:b ratio increased while spongy mesophyll tissue and leaf thickness increased. These changes may be due to UV or low-temperature damage of leaf protein and metabolic cost of plant protection or defence responses. Contrary to many other studies, leaf mass per area decreased significantly at higher altitude. This was consistent with predictions under the worldwide leaf economic spectrum on the basis that soil nutrients increased with increasing altitude. The key species differences were more irregularly shaped epidermal cells and larger stomatal size in perennial vetch compared to alfalfa or sainfoin that enhanced gas exchange and photosynthesis by generating mechanical force, increasing guard cell turgor, and promoting stomatal operation. The lower adaxial stomatal density also enhanced water-use efficiency. These adaptations might confer perennial vetch an advantage in environments with extreme diurnal temperature fluctuation or in frigid conditions.