Iodine has long been recognized as a vital nutrient for human and animal health. For decades, research has been conducted on iodine biofortification – that is, the process of enriching the iodine content of food crops. These studies have shown that exogenous applications of iodine (usually in the form of KI and KIO3) can increase the concentration of iodine in plant tissues, which is beneficial for human health. Research on the nutritional role of iodine in the plant itself is sparse. Although it is not yet considered an essential nutrient, there is evidence that suggests iodine plays a functional role in plants and may be as important as many other micronutrients. Because iodine is found in trace amounts in the soil, water, and atmosphere, a plant cannot be grown in a truly iodine-free environment. This has precluded precise studies of iodine’s role as an essential micronutrient.

Plants can absorb iodine through their roots or from the atmosphere through their stomata. Iodine uptake is primarily passive although active transport is possible. Iodine can be translocated through the xylem. Little is known about the chemical forms of iodine inside plant tissues, but iodide (I-) seems to be most common. Plants can convert iodine to methyl iodide, which will evaporate back into the atmosphere, or store iodine in proteins through organification.

Low concentrations of iodine are associated with beneficial effects on plant growth. Applications of iodine have been demonstrated to cause early flowering, bring an increase in biomass and seed production, and upregulate the expression of several genes involved in plant defense. This suggests that proper iodine nutrition may protect against both biotic and abiotic stress. Iodine seems to have a positive effect on plant growth in concentrations similar to other plant micronutrients, suggesting that iodine may play a similar role in plant health. Studies have shown that providing plants with iodine through irrigation (fertigation) is a much more effective means of increasing plant iodine content than foliar applications or fertilizing the soil with iodine before planting. High concentrations of iodine can cause toxic effects, such as biomass reduction, leaf chlorosis, and ultimately leaf necrosis and plant death.