Relatively little is known on the interaction between iodine and selenium in plants. It may become a drawback in developing agrotechnical rules of plant biofortification with these elements. The aim of the study was to determine the influence of soil fertilization with various forms of iodine (I^- and IO₃^-) and selenium (SeO₃^2- and SeO₄^2-) on yield, biofortification efficiency and selected chemical properties of lettuce plants. The study (conducted in 2012–2014) included soil fertilization of lettuce cv. ‘Valeska’ in the following combinations: control (without iodine and selenium fertilization), KI, KIO₃, Na₂SeO₄, Na₂SeO₃, KI + Na₂SeO₄, KIO₃ + Na₂SeO₄, KI + Na₂SeO₃, KIO₃ + Na₂SeO₃. Iodine and selenium were applied twice: before sowing and as a top-dressing in a total dose of 5 kg I·ha^-1 and 1 kg Se·ha^-1. Only the application of Na₂SeO₄ (individually or together with iodine) exhibited strong toxic effect on plants which was accompanied by the highest accumulation of Se, selenomethionine (SeMet) and selenocysteine (SeCys) in lettuce. The accumulation of I and Se in lettuce was respectively higher after fertilization KI than KIO₃ and Na₂SeO₄ than Na₂SeO₃. Simultaneous application of iodine and selenium decreased the level of Se, SeMet and SeCys in lettuce particularly in the combination with KIO₃ + Na₂SeO₃. Simultaneous application of KI with both forms of selenium decreased iodine content in lettuce as related to the treatment with KI alone. In the case of lettuce from the combinations with KIO₃, KIO₃ + Na₂SeO₄ and KIO₃ + Na₂SeO₃, comparable results of iodine concentration were obtained.

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