Changes in the Chemical Composition of Six Lettuce Cultivars (<i>Lactuca sativa</i> L.) in Response to Biofortification with Iodine and Selenium Combined with Salicylic Acid Application

A two-year greenhouse study was conducted to assess the effects of the application of I (as KIO₃), Se (as Na₂SeO₃), and salicylic acid (SA) in nutrient solutions on the chemical composition of six lettuce cultivars, i.e., two butterhead lettuces (BUTL), &#8220;Cud Voorburgu&#8221; and &#8220;Zimująca&#8221;; two iceberg lettuces (ICEL), &#8220;Maugli&#8221; and &#8220;Kr&#243;lowa lata&#8221;; and two <i>Lactuca sativa</i> L. var. <i>crispa</i> L. (REDL) cultivars, &#8220;Lollorossa&#8221; and &#8220;Redin&#8221;, grown in the NFT (nutrient film technique) system. The treatments were as follows: control, I+Se, I+Se+0.1 mg SA dm^&#8722;3, I+Se+1.0 mg SA dm^&#8722;3, and I+Se+10.0 mg SA dm^&#8722;3. KIO₃ was used at a dose of 5 mg I dm^&#8722;3, while Na₂SeO₃ was used at 0.5 mg Se dm^&#8722;3. The application of I+Se was a mild abiotic stress factor for the plants of the ICEL and REDL cultivars. In contrast, I+Se did not have a negative impact on the BUTLcultivars. The application of 1.0 mg SA dm^&#8722;3 improved the biomass productivity in all cultivars compared with I+Se. In the majority of the cultivars, the applied combinations of I+Se and I+Se+SA resulted in a reduction in the nitrate(V) content that was beneficial to the consumer and increased levels of sugars, phenols, phenylpropanoids, flavonols, and anthocyanins. In addition, an increase in ascorbic acid content was observed, but only in the BUTL cultivars and REDL &#8220;Redin&#8221;. The application of I, Se, and SA had upward or downward effects on the concentrations of N, K, P, Ca, Mg, S, Na, B, Cu, Fe, Mn, Mo, and Zn in the leaves.