Growth and Quality of Purple Mustard Sprouts as Affected by Combined Light Quality and Sodium Selenite

Mustard (Brassica juncea) sprouts are appreciated for their high levels of bioactive constituents, but how light quality and exogenous selenium interactively regulate their growth and nutritional properties is still poorly understood. In this study, we evaluated the individual and combined effects of red (R), white (W) and blue (B) light together with 100 μmol·L−1 selenium solution (SeR, SeW, SeB) on sprouts growth, health-promoting compounds, and metabolic profiles. Under single light quality treatments, red light favored growth, white light promoted photosynthetic pigment accumulation, and blue light most effectively enhanced antioxidant compounds and glucosinolates. Combined treatments further improved both yield and quality. SeR showed the greatest gains in growth indices; SeW achieved the highest total chlorophyll, total carotenoids, and total glucosinolates (the latter reaching 108.18 μmol g−1 DW), and SeB attained the highest total selenium, soluble sugar, soluble protein, anthocyanins, ascorbic acid, flavonoids, total phenolics, and antioxidant capacity. Non-targeted metabolomics identified 1208 differential metabolites, with KEGG enrichment revealing that the phenylpropanoid biosynthesis pathway was significantly enriched across all selenium-treated groups. These findings demonstrate that targeted combinations of light quality and exogenous selenium can selectively enhance the growth or nutritional quality of mustard sprouts, offering practical guidance for their controlled-environment production.