关键词: 苗期, 紫花苜蓿, 干旱胁迫, 适应机制, 活性氧产生与清除, 渗透调节, 内源激素, 光合生理

Abstract: 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 stress, the proline content of leaves of Longdong alfalfa reached the highest, and the soluble sugar content reduced. In the later drought stress period, the rich accumulation of proline content could offset the reduction of soluble sugar content.3. The net photosynthetic rate, transpiration rate, and stomatal conductance of leaves of the three alfalfa cultivars under drought stress decreased dramatically. A positive correlation existed between stomatal conductance and transpiration. Compared to the weak drought-resistant alfalfa under intense drought stress, the net photosynthetic rate of leaves of the strong drought-resistant alfalfa descended slowly. Stomatal restriction was the main factor causing the net photosynthetic rate to decrease in the leaves of the three alfalfa cultivars under slight drought stress, whereas under medium or serious drought stress, the non-stomatal restriction became the main factor inducing the net photosynthesis to decrease in the leaves of the three alfalfa cultivars.4. Measuring the chlorophyll fluorescence of leave of the three alfalfa cultivars under drought stress showed that their PSⅡFv/Fm and Fv/Fo decreased. However, the strong drought resistant alfalfa decreased less in Fv/Fm and Fv/Fo and its PSⅡ capability of light utility and potential activity was higher than those of the other two alfalfa cultivars. The three alfalfa Quenching(qP)of chlorophyll fluorescence decreased and Non-photochemical Quenching(qN)of chlorophyll fluorescence increased. What’s more, the fact that qP decreased less and qN increased more in the leaves of the strong drought-resistance alfalfa proved that the electron transport activity and photosynthesis apparatus damaged less the leaves of the strong drought-resistant alfalfa than the other two cultivars.5. The study on the endogenous hormones content in leaves of the three alfalfa cultivars shows: Under drought stress the content of ABA and IAA in the leaves of the three alfalfa cultivars increased and the content of GA3 and ZR decreased at different extents The general trend of coordination among the four endogenous hormones was closing the stomas and promoting the growth of roots.

Key words: Seedling stage, Alfalfa, Drought resistance, Adaptive mechanism, Active oxygen generation and clearing, Osmotic adjustment, Endogenous hormones, Photosynthetic physiology