Assessment of biofortification with iodine and selenium of lettuce cultivated in the NFT hydroponic system

• Higher efficiency of I and Se biofortification was noted after foliar application.
• Foliar spraying with IO3− and SeO42 did not affect root uptake of I and Se.
• Foliar application of I together with Se improved SeO42 absorption by leaves.
• Transport of I and Se in plants may occur from leaves to roots through phloem.

Iodine and selenium are not nutrients for plants but both play important roles in human and animal organisms. Diet of many populations around the world contains insufficient amount of these elements. Iodine and selenium biofortification of crop plants can increase its transfer into the food chain. A limiting factor for the development of agro-technical methods of I and Se application is poor recognition of its interaction with respect to plant growth and metabolism. The aim of the study was to determine the possibility of simultaneous biofortification of lettuce with iodine and selenium, applied foliarly or through the nutrient medium in the hydroponic system of nutrient film technique (NFT). Two-factor experiment with greenhouse cultivation of lettuce ‘Melodion’ cv. was conducted for two years. The following five sub-blocks with iodine and selenium introduction into the nutrient medium were distinguished: (1) control, (2) 0.5 mg Se dm−3, (3) 1 mg I dm−3, (4) 0.5 mg Se dm−3 + 1 mg I dm−3, (5) 1.5 mg Se dm−3 + 1 mg I dm−3 – the respective molar concentration were as follows: (2) 6.33 μM Se, (3) 7.88 μM I, (4) 6.33 μM Se + 7.88 μM I, (5) 19.00 μM Se + 7.88 μM I. Each sub-block included four combinations with five-time foliar treatment with: (A) distilled water, (B) 0.005% Se (0.633 mM Se), (C) 0.05% I (3.94 mM I) and (D) 0.005% Se + 0.05% (0.633 mM Se + 3.94 mM I). Iodine and selenium were applied in the form of KIO3 and Na2SeO4, respectively. There were three replicates for each treatment with eleven plants per one replicate. Tested factors did not negatively affect lettuce yield (average head weight) and nutritional status of leaves and roots. Only in lettuce from the sub-blocks with the nutrient medium containing 1.0 mg I dm−3, 0.5 mg Se + 1.0 mg I dm−3 and 1.5 mg Se + 1.0 mg I dm−3, foliar application of KIO3 and Na2SeO4 + KIO3 decreased the level of Ca, Mg and Fe in roots, when compared to respective plants from the control sub-block. Introduction of IO3− or SeO42− into the nutrient medium (in a dose of 0.5 mg Se and 1.0 mg I dm−3) had no negative impact on root uptake of SeO42− and IO3−, respectively and its further transport to leaves. Higher efficiency of iodine and selenium biofortification of lettuce plants was noted after foliar application of tested compounds rather than through its introduction into the nutrient medium. Foliar spraying with IO3− and SeO42 did not affect root uptake of iodine and selenium present in the nutrient medium. Foliar application of iodine together with selenium improved SeO42 absorption by leaves when compared to plants sprayed only with Se. Results obtained in the control sub-block may indirectly suggest that the transport of iodine and selenium in plants may occur from leaves to roots through phloem.

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