## 2 Materials and Methods  
### Plant Material and Cultivation  
In the spring season of 2022, early potatoes _Solanum tuberosum_ L. _'Denar' c.v._ were grown in a pot experiment. The experiments were located in a foil tunnel with a one-side vent at the campus of the Faculty of Biotechnology and Horticulture of the University of Agriculture in Krakow (50\({}^{\circ}\)05\({}^{\prime}\)03\({}^{\prime\prime}\) N 19\({}^{\circ}\)57\({}^{\prime}\)01\({}^{\prime\prime}\) E). Seed potato tubers were planted on 8 March 2022 in 7.5 L pots filled with heavy mineral soil. To characterise the chemical properties prior to the experiment (Table 1), the following chemical analyses were carried out on soil samples taken before potato planting: soil texture, acidification (pH(H\({}_{2}\)O)), salinity (EC) (mS-cm\({}^{-1}\)), soil oxyreduction potential (Eh mV), organic matter content and mineral composition. The content of macroelements, including N-NH\({}_{4}\), N-NO\({}_{3}\), N-NH\({}_{4}\)\({}^{+}\)N-NO\({}_{3}\), P, K, Mg, Ca and S, was analysed using the methods and technologies presented by [PERSON] et al. [28]. The soil content of hydroxides, Al, Fe, and Mn was analysed according to methods proposed by [PERSON] and [PERSON] [29], and [PERSON] et al. [30]. Three-tenths of a gram of dry soil were weighed into 30 mL. falcon tubes. Then, 25 mL of extraction solution was added. The extraction solution was composed of 0.22 M trisodium citrate, 0.11 M sodium bicarbonate and 0.1 M sodium dithiolite. The samples were shaken at 45 \({}^{\circ}\)C for 22 h. Then, they were centrifuged for 20 min at 3000 rpm and filtered using medium qualitative filters. Al, Fe, and Mn were analysed using ICP-OES (Prodigy Spectrometer, Leeman Labs, New Hampshire, MA, USA). Before planting the tubers, Polifoska 10, 5, 15, 5, and 14 (N, P, K, Mg, S), produced by the Polish granular fertiliser manufacturer Grupa Azoty Police, were applied to the soil in the amount of 0.55 g/kg of soil. Polifoska 5 is a complex multi-nutrient fertiliser that is particularly recommended for use before sowing, but its late application in early spring is also possible. This represents the only instance of the fertilisation of the plants via the soil for the entire cultivation period. Plants were grown in four replicates for each treatment in a randomised design. There were 12 plants per one replication (in each treatment), which equalled 48 plants per each treatment; there was a total of 192 plants per experiment. The subject of the study was plant fertilisation with iodine in the form of organic compounds; this included iodoquinolines, and an inorganic form of iodine that was presented as KIO\({}_{3}\), which was used as a reference treatment and as a positive control against the iodoquinolines. The following treatments were tested in the studies: (1) Control (without iodine application); (2) KIO\({}_{3}\), (3) 8-OH-7-I-5 QSA and (4) 5-Cl-7-1-8-Q. All the iodine compounds were applied to soil once a week through manual fertigation (manual watering) with 50 \(\upmu\)M (molar mass equivalents) of the solutions of the studied compounds, at a dose of 500 mL/pot-1 (one plant-1). During the experiment, nine applications of these compounds were made every seven days; the first application was made after the emergence of plants (26 April 2022) and the last one was made on 7 June 2022, one week before the harvest. Finally, 1.77 \(\upmu\)mol of I per pot was applied. The working solutions of the tested iodine compounds used for the fertigation were prepared in time intervals of every two weeks. On the days between the application of iodine compounds, the plants were watered via a drip irrigation system with tap water in the amount of approximately 200-500 mL/pot-1 (depending on the growth phase and weather conditions). Plants were harvested on 15 June 2022. During the plant harvesting, the number of tubers from one plant and the total weight yield of tubers from one plant were measured separately for each replication.