Alfalfa (Medicago sativa), a very important kind of forage, is widely planted. Studies recorded in the present paper on three alfalfa germplasm resources: Longdong (strong drought-resistant), Gongnong No1 (medium drought resistant), and BL-02-329 (weak drought resistant), were selected to explore the adaptive mechanism of alfalfa under drought stress. The main results of research are as follows:1. A study on active oxygen generation and clearing mechanism in leaves of the three alfalfa cultivars shows: Under drought stress, the leaf active oxygen generation rate, malondiatdehyde content, and damage of cell membrane increased. Compared to the weak drought-resistant alfalfa cultivar, leaf of the strong drought resistant alfalfa generated less active oxygen, and accumulated less malondiatdehyde, and its cell membrane was less damaged. At the same time, the leaf SOD, and POD activity enhanced, and the antioxidation substance, carotenoid and ascorbic acid, increased. Moreover, the endogenous coordination of enzyme system and non-enzyme system in the leaf of the strong drought-resistant alfalfa was well regulated.2. Results of the study on water metabolism and osmotic adjustment substance in the leaves of the three alfalfa cultivars show that, under drought stress, the leaf water potential and water relative content decreased, while the water saturation deficit and bound water content increased. The strong drought- resistant alfalfa boasted the ability to keep normal water state under drought stress, and to maintain greater extent of low water potential, limited extent of decreased water relative content and of water saturation deficit, while the ratio of bound water content to free water content in the leaves increased. At the same time, the content of proline and soluble sugar which are two main osmotic adjustment substances increased. More particularly, the proline content in the leaves of the strong drought-resistant alfalfa under drought stress rose a hundred times than before. On the 12 th day under drought stres
