The Application of Melatonin and Organic Waste Derived from Vitamin C Industry Effectively Promotes Seed Germination and Seedling Growth of Cotton in Saline–Alkali Soil

Saline–alkali stress severely affects plant growth and productivity. Although melatonin can promote seed germination as a growth regulator, it cannot address the weak seedling growth caused by insufficient organic nutrients in saline–alkali soil. The RAE (residue after evaporation, an industrial waste from the industrial production of vitamin C) can enhance plant salt tolerance by stimulating vitamin C (ASA) synthesis and contains abundant small molecular organic acids. We hypothesized that the combined application of melatonin and RAE might synergistically enhance cotton germination and seedling growth. The cotton seeds used in this study were “Xin Lu Zhong No. 87”; a Petri dish simulation experiment and a pot experiment were conducted in 2023. Four treatments were set: control (CK), melatonin (MT), RAE (RAE), and the combined application of MT and RAE (MR). Compared to CK, MT significantly increased the germination rate of cotton seed (194.4%), while RAE significantly enhanced the underground biomass of cotton seedlings (40.3%) and ASA content (203.8%). Compared to MT and R, the combined application of melatonin and RAE significantly increased the ASA content (54.5%, 29.6%) in roots, superoxide dismutase (SOD) activity (220.3%, 89.6%) in roots, catalase (CAT) activity (15.8%, 97.5%) in leaves on the 15th day, soil cation exchange capacity (CEC) (57.2%, 9.7%), and total fresh weight (20.8%, 33.8%). Collectively, these findings indicate that the synergistic effect under the combined use of melatonin and RAE promotes cotton seed germination and seedling growth, offering a novel technical solution for salt–alkali soil cotton cultivation along with an innovative approach for the resource utilization of RAE.