## 4 Discussion
### Enzymatic and Antioxidant Activity  
There were no significant differences observed in enzymatic activity after any treatments. Once again, it was demonstrated that the applied doses of iodine did not induce a defensive response in lettuce. There are high positive correlations (R \(\geq\) 0.5) between enzymatic activities of GSH-GPX, CAT-GPX, and CAT-GSH that we believe have to do with maintaining a balance at the cellular level. There are no high and significant correlations between enzyme and antioxidant activities, with the exception of SOD and wall-linked phenols where there is a low but significant correlation. The latter we can attribute it to the fact that SOD heads the first line of defense of enzymatic activity and its correlation with phenols linked to the wall, we perceive it as a redox or neutralization mechanism against high concentration IO\({}_{3}\)- salts. Conversely, some authors have reported increases in enzymatic activity following biofortification with iodine. During soilless cultivation of lettuce, SOD was reduced after application of I\({}^{-}\) at 20, 40, and 80 \(\upmu\)M, while IO\({}_{3}\)\({}^{-}\) led to its increase in doses greater than 40 \(\upmu\)M. Ascorbate peroxidase (APX) activity increased with IO\({}_{3}\)\({}^{-}\) application at any dose, while CAT activity also increased with IO\({}_{3}\)\({}^{-}\) as well as I\({}^{-}\) application, reaching its maximum value at IO\({}_{3}\)\({}^{-}\) doses \(\geq\)80 \(\upmu\)M [43]. Similarly, hydroponic lettuce grown under salt stress and biofortified with IO\({}_{3}\)\({}^{-}\) had increased activities of SOD, APX, Dehydroascorbate reductase (DHAR), and Glutathione reductase (GR) [12]. Soybeans grown under Cd\({}^{2+}\) stress and biofortified with IO\({}_{3}\)\({}^{-}\) at various concentrations (20, 40, and 80 \(\upmu\)M) demonstrated increased SOD, APX, and GR activity [62]. Tomato biofortified with I\({}^{-}\) or IO\({}_{3}\)\({}^{-}\), applied to soil or foliage, either daily (1 \(\upmu\)M) or biweekly (100 \(\upmu\)M), had reduced SOD activity following the biweekly I\({}^{-}\) treatments and the daily, foliar treatments of either iodine salt. However, the daily application of foliar I\({}^{-}\) led to increases in the concentrations of non-enzymatic antioxidants, namely ascorbate (22%) and glutathione (85%) [35].  
Plants have evolved several antioxidant systems to help them avoid damage from oxidative stress. One of those systems involves the enzymes SOD, CAT, and APX, acting in that order. SOD reduces O\({}_{2}\)\({}^{-}\) to H\({}_{2}\)O\({}_{2}\), which in turn is a substrate for CAT, APX, and other enzymes [60]. Iodide is considered an inorganic antioxidant that reacts with ROS (ozone, singlet oxygen, and superoxide) at rates 12 to 500 times those of ascorbate and glutathione [63]. In aquatic plants of the genus _Laminaria_, the enzyme haloperoxidase catalyzes the oxidation of iodide to hypoiodous acid and molecular iodine in the presence of H\({}_{2}\)O\({}_{2}\)[64].  
The results of the present work differed from those previously reported, although that could be attributed to a number of factors. For example, in this study, cultivation was substrate-based while previous reports have used hydroponic systems [43]. Additionally, the accumulation of iodine and other biomolecules can depend on factors such as the dose and species of iodine applied, the plant species, and the type of application, be it foliar or directly to the substrate, all of which can lead to the differential accumulation of iodine [15].