Brassinolide and Sucrose Enhance the Nutritional Quality of Kale Sprouts by Modulating Sugar Metabolism, Redox Homeostasis, and Secondary Metabolism

Kale ( Brassica oleracea var. acephala ) is a natural product rich in bioactive substances and a good source of functional compounds. This study evaluated the individual and combined effects of exogenous brassinolide (BR, 0.1 mg L −1 ) and sucrose (Suc, 34.2 mg L −1 ) on photosynthesis, sugar metabolism, the AsA–GSH cycle, nitrite accumulation, phytochemical composition, and nutritional quality of kale sprouts. Compared with the control and single treatments, combined BR and Suc application markedly increased chlorophyll and carotenoid levels, chlorophyll fluorescence parameters (qL, Y(II), F v / F m ), sugar metabolism components (fructose, INVA, INVN, SSc, SSs), AsA–GSH cycle indicators (GSH, AsA/DHA), antioxidant enzyme activities (SOD, POD), and beneficial phytochemicals and nutrients (PAL, flavonoids, riboflavin, soluble sugars, cellulose, soluble protein, free amino acids). Conversely, it decreased levels of APX, DHAR, SPS, sucrose, glucose, AsA, DHA, and anthocyanins. Correlation analysis further identified key interactions between sugar metabolism and the AsA–GSH cycle, including sucrose with GSH and GSH/GSSG, GSSG with MDHAR, and fructose and INVA with AsA/DHA. These results indicate that BR and Suc co‐treatment improves kale sprout growth and quality by elevating photosynthetic performance, antioxidant capacity, and the accumulation of health‐promoting compounds. This studyprovides guidance for the combined application evaluation of plant hormones and carbon sources.