Morphological and physiological adaptation of Caragana species in the Inner Mongolia Plateau

The morphological structure, osmotic adjustment, stomatal regulation and antioxidative enzymes of mesophilous, xerophilous and super- xerophilous species of Caragana were studied, in order to understand their ecological adaptation mechanisms. The results showed that flat leaves with few green villi were common in mesophilous species, tile-shaped leaves with offwhite villi were common in xerophilous species and tile-shaped or tube-shaped leaves with densely vertical or fallen silky villi were common in super-xerophilous species. Leaf thickness was greatest in super-xerophilous species, less in xerophilous species, and least in mesophilous species. Leaf area, leaf biomass, and specific leaf area (SLA) were highest for mesophilous species, with xerophilous species having lower values, and super-xerophilous species having the lowest values. The thickness-area ratio and length-width ratio in super-xerophilous species and xerophilous species were greater than those in mesophilous species. With these morphological features, the water retention capability was highest in super-xerophilous species, and lower and lowest in xerophilous and mesophilous species, respectively; conversely, light use capability was highest in mesophilous species, followed by xerophilous and super-xerophilous species, respectively. The osmotic adjustment substances content, cytoplasmic ion concentration and osmotic potential of super- xerophilous species were the highest, with those of xerophilous species being intermediate and those of mesophilous species being the lowest. The most obvious differences in osmotic adjustment substances content were that the soluble sugar and inorganic ion contents decreased substantially from super- xerophilous to xerophilous and then to mesophilous species. The leaf total and free water content, leaf water potential, as well as stomatal conductance were highest in mesophilous species, intermediate in xerophilous species and lowest in super- xerophilous species, whereas the bound water content, ratio between bound and free water, peroxidase (POD) and superoxide dismutase (SOD) activity were in the reverse order. Catalase (CAT) activity was highest in xerophilous, intermediate in mesophilous, and lowest in super- xerophilous species. These ecophysiological characteristics resulted in different drought resistance ability and metabolism intensity among Caragana species. Super-xerophilous species had the strongest drought resistance, while xerophilous species ha