Combined Effect of Water and Salt Stress on Growth and Physiological Traits of Alfalfa at Branching Stage

【Objective】 Alfalfa is sensitive to water stress at branching stage, but how a simultaneous salinity stress affects this sensitivity is not well understood. The objective of this paper is to address this issue. 【Method】 Pot experiments were conducted in 2018—2019 at the Shengmu High-tech Economic Park, in Dengkou County, Inner Mongolia. It consisted of three water treatments with the soil water content kept at 55%~70% (W1), 75%~85% (W2), and 85%~100% (W3) of the field capacity, respectively. Each water treatment had four salinity stresses with the soil salt content set at 1.0 (S0), 1.5 (S1), 3.0 (S2) and 4.5(S3) g/kg, respectively. In each treatment, we measured osmotic substances, antioxidant enzymatic activities, above- and below-ground biomass at early flowering stage. 【Result】 Water and salinity stresses reduced water consumption and aboveground biomass more considerably when they worked collectively than working separately. When the water stress was minor, however, imposing a slight salinity stress with soil salt content ≤3 g/kg can increase the contents of proline and Na+, boosted peroxidase (POD) activity, and reduced superoxide dismutase (SOD). This alleviated oxidative damage of the water stress, thereby promoting crop growth. Compared with S0+W1, S2+W1 increased the contents of proline and Na+, and boosted POD activity, by 74.28%, 17.28% and 34.60%, respectively, while reducing SOD activity by 9.87%. These led to an increase in water consumption and aboveground biomass by 6.48% and 2.17%, respectively. 【Conclusion】 When water stress is not severe, imposing a slight salinity stress can improve the tolerance of alfalfa to water and salinity stress at branching stage. For all treatments we studied, keeping soil salt content in the range of 2~3 g/kg and soil water content in the range of 70%~85% of the field water capacity was optimal to improve resistance of the crop to both salinity and water stresses.