Mitigating low-temperature stress in alfalfa by postponing phosphorus application and remodeling of antioxidant activities and carbon-nitrogen metabolism

Low-temperature stress has become a major limiting factor for the sustainable production of forage crops and animal husbandry. This experimental study evaluated the effects of optimizing phosphorus application on the antioxidant properties and carbon-nitrogen metabolism physiology of alfalfa (<i>Medicago sativa</i> L.) under LT stress, aiming to provide a reference for efficient stress-resistant alfalfa production. In this study, the LT tolerant cultivar 'Caoyuan' (CY) and LT sensitive cultivar 'Xinmu' (XM) were used as plant materials, and the physiological changes of alfalfa plants under natural temperature (NT) and LT were compared under traditional phosphorus application (R1) and postponing phosphorus application (R2) treatments. The results showed that LT stress increased the accumulation of malondialdehyde (MDA) in alfalfa plants and inhibited root activity, carbon metabolism, and photosynthesis in both cultivars. The negative impacts of LT are more prevalent in XM than in CY. The postponing phosphorus application treatments enhanced root vitality as compared to the traditional phosphorus application treatments and accumulated more soluble sugar (5.6-11.2%), sucrose (8.5-14.0%), proline (7.5-11.7%), and soluble protein (8.3-11.7%) by increasing the enzyme activities related to carbon-nitrogen metabolism. Under postponing phosphorus application treatments, the enzymatic activities of antioxidants and regulation of osmotic sub-stances significantly increased in the leaves, MDA contents were decreased by 4.6-7.6%, and chlorophyll contents were increased by 4.8-8.6%, the net photosynthetic rate in alfalfa leaves increased by 5.1-7.5%. Besides, plant dry weight, root dry weight, and plant phosphorus concentration increased by 5.8-16.9%, 7.8-21.0%, and 5.1-9.9% under postponing phosphorus application treatments. In summary, split-phosphorus fertilization improved the nutrient absorption capacity of alfalfa roots compared to traditional phosphorus application treatments under LT stress. Moreover, it improved the carbon-nitrogen metabolism physiology and photosynthetic production capacity of the alfalfa plants, thus reducing the adverse effects of LT stress on the growth and development of alfalfa.