Responses of the Physiological and Biochemical Characteristics of Alfalfa Roots with Different Root Collar Diameters to Low Temperatures Under Simulated Winter Irrigation

Abstract Background: Due to the frequent occurrence of extremely low temperatures in northern China, the problem of alfalfa mortality during overwintering requires urgent solutions to support the development of large-scale alfalfa production in the region. Understanding the effects of winter irrigation and the alfalfa root collar diameter on physiological and biochemical cold resistance mechanisms in alfalfa will be of great significance to finding such solutions.Methods: In this study, alfalfa roots with different root collar diameters (3.50±0.10 mm, 5.25±0.10 mm, 7.00±0.10 mm) were subjected to simulated winter irrigation (0 ml, 20 ml, 40 ml) and different low-temperature stress treatments (4°C, -10°C, -15°C, -20°C) in the laboratory. The relationships between alfalfa root collar activity, relative conductivity and the nonstructural carbon and nitrogen contents and antioxidant enzyme activity in the collar were explored.Results: Low-temperature stress was the most important factor causing significant changes in the physiological and biochemical indexes of the alfalfa root collar, followed by the root collar diameter. Low-temperature stress and the root collar diameter had significant effects on nonstructural carbon and nitrogen levels and enzyme activity (except POD activity) in alfalfa root collars, and winter irrigation had extremely significant effects on the nonstructural nitrogen contents of alfalfa root collars. The larger the diameter of the root collar was, the higher the soluble protein content and the stronger the SOD activity. Simulated winter irrigation increased the C/N ratio in the alfalfa root collars. The increases in nonstructural carbon content, C/N ratio and SOD activity in alfalfa root collars helped improve alfalfa cold resistance.Conclusions: Alfalfa resists low-temperature stress by adjusting the proportions of nonstructural carbon and nitrogen as well as the SOD activity in the root collar. Therefore, fertilization, early sowing and other management measures should be adopted to ensure healthy alfalfa growth and thereby increase the root collar diameter of alfalfa grown in cold regions in northern China. Moreover, timely supplementary irrigation to reach soil saturation in winter can play an important role in improving cold resistance in alfalfa.