Physiological and biochemical changes in different drought-tolerant alfalfa (Medicago sativa L.) varieties under PEG-induced drought stress

Medicago sativa L. cv. Longzhong is a nutritious forage plant in dryland regions of the Loess Plateau with strong drought tolerance and broad adaptability. To understand the adaptation mechanism of alfalfa (M. sativa L. cv. Longzhong) to drought stress, growth, and physiological parameters including levels of chlorophyll content, osmotic adjustment, reactive oxygen species (ROS), and antioxidant enzymes and antioxidants were measured under simulated levels of drought (− 0.40, − 0.80, − 1.20, − 1.60, and − 2.00 MPa). The changes in M. sativa L. cv. Longzhong were compared with those of plants of M. sativa L. cv. Longdong control (Variety I) suited to moderate rainfall areas and M. sativa L. cv. Gannong No. 3 (Variety II) suited to irrigated areas. The results showed that root–shoot ratio, the chlorophyll (a + b) and osmolytes contents, the degree of lipid peroxidation and ROS production, and the levels of antioxidative enzymes and antioxidants increased significantly with increasing drought stress, whereas plant height, aboveground biomass, chlorophyll a/b ratio, leaf water potential (Ψ1), and relative water content (RWC) decreased in response to drought. The Longzhong variety responded early to beginning drought stress (between 0 and − 0.4 MPa) compared with the controls. Under drought stress (between − 0.4 and − 2.0 MPa), the Longzhong variety had significantly higher belowground biomass, root–shoot ratio, Ψ1, RWC, catalase (CAT) activity and reduced glutathione content than those of Varieties I and II, but hydrogen peroxide and hydroxyl free radical (OH·) contents were significantly lower. Step regression analysis showed that OH·, CAT, malondialdehyde, superoxide anion-free radical (O2·−), and superoxide dismutase of Longzhong had the most marked response to drought stress. In conclusion, the stronger drought tolerance of the Longzhong variety might be due to its higher water-holding capacity, root–shoot ratio, and ability to coordinate enzymatic and non-enzymatic antioxidant systems, which coordinate the peroxidation and oxidative systems.