Metabolic divergence between garlic and speedwell enables concurrent selenium biofortification and weed suppression through foliar spraying

Selenium (Se) is essential for human health, and agronomic biofortification of food crops offers a direct strategy to address global dietary Se deficiency. Plants exhibit pronounced interspecific variation in Se sensitivity, yet whether this differential susceptibility enables concurrent crop biofortification and weed suppression remains unexplored. Garlic (Allium sativum L.) possesses exceptional Se-accumulating capacity due to its sulfur-rich metabolism, whereas Veronica didyma—a pernicious speedwell that heavily infests garlic fields and requires repeated herbicide sprayings – lacks this specialization. Here we demonstrate that foliar Na2SeO3 spraying elicits a sharply divergent hormetic response between these species. At 225 g hm−2, Se significantly promoted garlic growth. Chlorophyll increased 24.01%, photosynthetic parameters (Pn, Tr, and Gs) enhanced by 29.45–39.77%, and Ci reduced by 23.10%. Antioxidant enzyme activities rose, proteins, sugars, amino acids, and glutathione were markedly elevated, while malondialdehyde (MDA) and proline decreased by 21.08% and 25.94%. In contrast, the same treatment induced phytotoxicity in the co-cultivated speedwell. Biomass was inhibited, chlorophyll declined by 20.24%, and photosynthetic parameters (Pn, Tr, and Gs) significantly decreased, while Ci significantly increased. Antioxidant enzymes were suppressed, proline and MDA increased by 20.46% and 21.86%. The tissue Se concentration in speedwell and garlic increased significantly with increasing foliar spray concentration. Higher concentrations (450–675 g hm−2) could induce oxidative stress responses in both plants. Our findings establish a discrete Se spraying window where the element simultaneously acts as a biofortifying biostimulant for garlic and a selective phytotoxic agent against speedwell, eliminating separate herbicide applications and transforming Se fertilization into an integrated crop management tool.