Biofortification of parsley (Petroselinum crispum Mill.) with iodine (potassium iodide) in soil culture

Iodine is an essential micronutrient for human nutrition that has a unique role in regulating metabolic processes such as the synthesis of thyroid hormones, which are involved in the synthesis of sugars, nucleic acids, and proteins. The recommended dietary allowance for iodine is 90 mg for young children (1-8 years), 120 mg for older children (9-13 years), 150 mg for adults, and 220-270 mg for pregnant and lactating women. The low amount of iodine available in the soil is one of the main reasons for their insufficient transfer to the plant and as a result, the lack of accumulation of this element in plants. Their deficiency in food occurs in most human and farm animal populations worldwide. Biofortification of products with iodine is proposed as a new strategy to further enrich the human diet with a potentially affordable and readily available source of iodine. Understanding how iodine affects and functions in plants is critical to developing appropriate biofortification approaches. Considering the importance and high consumption of parsley, this study aimed to investigate the effects of applying different levels of iodine in the soil on the enrichment, growth, and physiological characteristics of parsley.

The research was carried out in the form of a completely randomized design with three replications in the conditions of the research greenhouse of the Department of Horticultural Sciences, Agricultural Sciences and Natural Resources University of Khuzestan, in the cropping season of 2017-2018. The experimental treatments included five levels of potassium iodate (concentrations of zero, 12, 25, 50, 100, and 150 mgkg-1 soil), which were mixed with soil before filling the pots, and then each pot was filled with enriched soil corresponding to each treatment. At the end of the experiment, the fresh and dry weight of the shoot, amount of iodine, Ascorbic acid, soluble proteins, photosynthetic pigments, and content of soluble carbohydrates and total phenol in parsley were examined.

According to the obtained results, the highest amount of fresh and dry weight of parsley shoot was obtained in the treatments of 25 and 50 mg of iodine per kg of soil, and it showed a decreasing trend with the increase of iodine concentration of more than 50 mgkg-1 of soil. Iodine consumption up to 50 mgkg-1 of soil not only did not have a negative effect on the growth and development of parsley but also showed increasing effects. However, only the treatment of 150 mgkg-1 of soil had a statistically significant difference from the other treatments. Similarly, the levels of chlorophyll a, chlorophyll b, total chlorophyll, chlorophyll b ratio to total chlorophyll, chlorophyll b ratio to chlorophyll a, antioxidant capacity, ascorbic acid, and soluble proteins increased up to the treatment of 50 mg iodine per kilogram of soil. However, with further increases in iodine concentration in treatments of 100 and 150 mg iodine per kilogram of soil, these indicators showed a significant decrease. By increasing the amount of iodine consumed in the treatments, the content of soluble carbohydrates, the amount of total phenol, and iodine in shoot of parsley showed an increase. No significant difference was observed in any of the measured traits except the amount of iodine in shoot of parsley between the treatment of 50 mg and 25 mg of iodine per kg of soil. Overall, the results showed that parsley biofortification with amounts of 25 to 50 mg of iodine per kg of soil can improve the growth and biochemical indicators of the parsley plant and be used as a solution to meet human iodine needs.

In general, according to the results obtained in this research, the plants treated with low to medium concentrations of iodine had more soluble carbohydrates, soluble protein, ascorbic acid, antioxidant capacity, and photosynthetic pigments, which indicates a positive response to low to medium amounts of iodine in the root environment. Quantitative data obtained showed that a large amount of iodine is stored in the vegetative tissue. Based on the results of this research, it can be concluded that parsley will be a promising target plant for biofortification with iodine. That is by consuming 50 mg of iodine per kilogram of soil, the concentration of iodine in the aerial parts of parsley can be increased by one order, and by consuming 50 to 100 grams of fresh parsley, the daily iodine requirement for an adult person will be provided.