Effects of iodinated copper-based foliar fertilizers on iodine content, yield and quality of lettuce (

Milk, dairy products, and certain marine foods are important dietary sources of iodine for humans. However, iodine intake is strongly influenced by dietary habits, local food availability, and iodine fortification policies, and ensuring adequate iodine intake remains a challenge in populations with low consumption of dairy products or seafood or in regions where salt iodization is limited. Under these circumstances, iodine-biofortified vegetables may serve as an important dietary source of iodine, and fertilization is an effective strategy to enhance their iodine content. In this study, using lettuce as a test crop, we conducted experiments over two growing seasons to evaluate an improved iodinated copper-based foliar fertilizer derived from traditional Bordeaux mixture (BDM), with treatments differing in iodine form and concentration. The results showed that the iodinated copper-based foliar fertilizers reduced copper concentrations in shoots and roots by 57.6%-91.57% and 61.56%-91.24%, respectively, compared with BDM, thereby reducing copper accumulation in plants and the potential risk of soil contamination. Compared with deionized water (CK), iodine application with copper-based fertilizers significantly increased iodine levels in lettuce tissues. Application of 1 g/L and 2 g/L KI copper-based foliar fertilizer increased shoot iodine content by 99.91% and 191.95%, respectively, and root iodine content by 180.66% and 211.41%, respectively. In another growing season, shoot iodine content increased by 24.71- to 66.20-fold, while root iodine content increased by 1.52- to 2.50-fold. In terms of quality, 1 g/L KI copper-based foliar fertilization increased vitamin C content by 33.19%, whereas 2 g/L reduced nitrate content by 39.08%; soluble sugar content increased by 34.41% and 39.27%. However, high iodine concentrations exerted a suppressive effect on yield, with high-concentration iodine treatments consistently reducing yield by approximately 13.85%-35.84% relative to CK across the two growing seasons. Overall, iodinated copper-based foliar fertilization effectively enhanced iodine enrichment in lettuce while simultaneously reducing copper accumulation, suggesting its potential as a safer alternative to traditional BDM for iodine biofortification. Among the tested treatments, 2 g/L KI copper-based foliar fertilization produced the greatest improvements in iodine enrichment and quality traits, although this was accompanied by a reduction in yield, indicating that the trade-off between nutritional enhancement and yield penalty should be carefully considered when determining the optimal application rate.