Transfer of iodine from soil to vegetables by applying exogenous iodine

Iodine deficiency disorders are one of the commonest preventable human health problems. Producing iodine-enriched crops could be an effective way to reduce their epidemicity in many regions. However, the actual knowledge on this issue is limited mostly to studies involving grain crops and inorganic iodine fertilizers such as I$^{-}$ and IO$_{3}^{-}$. Moreover, the translocation, transformation and distribution of iodine from soil to plants are not well understood. Here, we studied iodine transfer from soil to vegetables using both inorganic iodine (KI) and organic, seaweed iodine. Greenhouse culture experiments were undertaken to assess the absorption and accumulation of iodine by four vegetables: Chinese cabbage, lettuce, tomato and carrot. We also investigated the dynamic variation of exogenous iodine in soil by applying KI and a composite of seaweed and diatomite. Our results show first that iodine levels in vegetables increase with the increasing addition of iodine. Second, the iodine content in the edible portion ranks as follows: Chinese cabbage (high I) $>$ lettuce $>$ carrot $>$ tomato (low I). The iodine accumulation in the edible portion of the cabbage is thus 2.25 and 4.45 times higher than that of lettuce and carrot, respectively, and 19.67 times higher than that of tomato. In vegetable tissues the iodine distribution is ranked as: root (high I) $>$ leaf $>$ stem $>$ fruit (low I), except for carrot, where the average iodine level in the rhizome is 50% of the shoot. Third, vegetable growth is inhibited when the added iodine concentration is higher than 50 mg kg$^{-1}$. The order of tolerance against iodine toxicity is ranked as: carrot (high tolerance) $>$ Chinese cabbage $>$ lettuce $>$ tomato (low tolerance). Fourth, the seaweed composite iodine fertilizer demonstrates more potential of durability than KI. Indeed, when KI is added to the soil at 150 mg kg$^{-1}$, the biomass of cabbage, lettuce, tomato and carrot decreases by 34.8%, 41.3%, 46.8% and 17.9%, respectively. By comparison, the biomass decreases are lower, 16.6%, 22.9%, 23.4% and 9.7%, respectively, when applying the seaweed composite. Fifth, after harvest, the residual iodine in soil fertilized with KI is only 56% of the initial addition, which is less than that for seaweed composite. This study is of theoretical importance to understand iodine biogeochemistry and its transfer behavior, and also has practical implications for seeking effective alternatives of iodine biofortification to prevent iodine deficiency disorders