Iodine and selenium together fulfill important functional roles in organisms of humans and animals. Conducting simultaneous biofortification (enrichment) of plants with these elements is justified as combined endemic deficiency of I and Se (hidden hunger) is often encountered. Relatively little is known about the interaction between I and Se in plants, not only with respect to their accumulative efficiency, but also its influence on mineral nutrition or biological quality of crop. The study (conducted in 2012–2014) included soil fertilization of lettuce cv. ‘Valeska’ with I and Se in the following combinations: control, 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 (each of 2.5 kg I·ha^-1 + 0.5 kg Se·ha^-1) – a total dose of 5 kg I·ha^-1 and 1 kg Se·ha^-1 was used. Diversified weather conditions significantly modified the impact fertilization with I and Se had on tested aspects of nitrogen metabolism and biological quality of lettuce – except for the lettuce heads mass and the sucrose content. Based on the results concerning average lettuce weight as well as the level of sugars, phenolic compounds, phenylpropanoids, flavonols and antioxidizing activity it was concluded that the application of Na₂SeO₄ alone or together with iodine acted as a stress factor for cultivated plants.

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