## 1 Introduction  
Iodine is an essential trace element for humans. Iodine deficit can cause iodine deficiency disorders (IDD), which, according to some estimates, affect around two billion people [1]. Recommended daily iodine intake levels are 90 \(\upmu\)g for preschool children (0 to 59 months), 120 \(\upmu\)g for children 6 to 12 years old, 150 \(\upmu\)g for adults over 12 years old, and 200 \(\upmu\)g for pregnant and nursing women, according to the World Health Organization (WHO), the United Nations Children's Fund (UNICEF), and the International Council for Control of Iodine Deficiency Disorders (ICCIDD) [2]. Some experts believe that doses of 1-2 mg per day are probably safe for most people, although not all [3; 4]. A worldwide initiative in 1920 led to the universal iodization of table salt, which led to a significant increase in iodine intake. Unfortunately, inorganic iodine salts have a disadvantage in that up to 20% can be lost through volatilization, on top of the percentages that are lost depending on the type of cooking used [5; 6; 7]. The WHO recommends that salt consumption not surpass more than 5 g per day, equivalent to >2 g of sodium per day, as high salt intake combined with insufficient potassium intake (<3.5 g per day) can provoke high blood pressure. Because of that, the WHO aims to reduce salt intake by 30% in the worlds' population by 2025 [8]. Thus, alternative options for adequate iodine intake while reducing salt consumption are necessary. The iodine supplementation of foods or the biofortification of plant-based foods would be one alternative for maintaining iodine intake while reducing table salt consumption. The advantage of biofortification is that the iodine is found in organic forms that have greater bioavailability and stability against volatilization than the inorganic salt forms. Another advantage of biofortification is that it can be applied to frequently consumed or traditional crops in certain regions [9]. Iodine is not considered an essential nutrient for terrestrial plants; however, it plays an important part of algae metabolism [9]. The chemical forms of iodine most commonly used for crop biofortification are potassium iodide and potassium iodate. In lettuce, biofortification with different doses of iodide (I\({}^{-}\)) and iodate (IO\({}_{3}\)\({}^{-}\)) cause changes in biomass, enzymatic activity, and antioxidant activity, suggesting they might cause toxicity in plants [10; 11; 12; 13; 14]. The type of iodine application, either sprayed on the foliage or applied directly to the growth substrate or soil, can affect the final concentrations of iodine in the edible parts of the plants. Some studies have found foliar iodine applications to be more effective than soil applications [11; 15]. However, foliar applications are reportedly less effective for biofortifying fruits or seeds, supposedly due to lower iodine mobility through phloem when applied via foliar spray [9; 16; 17]. When iodine is applied to soil or substrate, its bioavailability depends on the volatilization rate and its interaction with other soil components like metal oxides or hydroxides. For example, Fe(OH)\({}_{3}\), Al(OH)\({}_{3}\), and MnO\({}_{2}\), can play an important role in the determination of iodine behavior in soil, both through inorganic iodine adsorption and iodine oxidation [18; 19; 20]. Chitosan (Cs) is a biodegradable, biological polymer that functions as a metal and trace metal complex agent. Chitosan also possesses a wide variety of properties: it is a plant elicitor as it induces phytoalexin production; it protects plants from pathogen diseases as it inhibits microbial growth; it activates MAP-kinases and provokes chromatin alterations; and it participates in the synthesis of alkaloids and plant growth regulators. It is also biodegradable, biocompatible, induces several plant defense genes, and induces proteinase inhibitors. Responses to its application will depend on the concentration, plant species, and growth phase [21; 22; 23; 24].