Influence of iodine form and application method on the effectiveness of iodine biofortification, nitrogen metabolism as well as the content of mineral nutrients and heavy metals in spinach plants (Spinacia oleracea L.)

Biofortification of vegetables with iodine can become an alternative (in reference to salt iodization) method of introducing this element to human diet. Side-effects of iodine application on mineral nutrition of plants have not yet been thoroughly documented. The aim of the study was to evaluate the effect of iodine form and the method of I fertilization on iodine biofortification, nitrogen metabolism and the level on mineral nutrients and selected heavy metals in spinach plants. In the present research, pot cultivation of spinach ‘Olbrzym zimowy’ cv. was carried out on heavy soil including the following treatments: 1 – control (without iodine application), 2 – pre-sowing KI fertilization, 3 – pre-sowing KIO3 fertilization, 4 – fertigation with KI, 5 – fertigation with KIO3. In treatments no. 2 and 3 iodine was applied in a dose of 1 mg I per 1 dm3 of soil. In treatments no. 4 and 5, water solution of iodine in the concentration of 0.0004% was used – approximately 1.1 mg I dm−3 of soil was introduced during the cultivation. Introduction of iodine by fertigation proved to be much more effective for iodine biofortification of spinach than pre-sowing fertilization with this element – by 339% and 498% respectively for KI and KIO3. Pre-sowing soil fertilization with KI and KIO3 improved accumulation of nitrates(V) and chlorides in spinach. Soil application of iodine (in the form of KI and KIO3 – combination no. 2 and 3) increased N-total content in spinach plants. Fertigation of plants with KI and KIO3 significantly reduced the level of ammonium ions in spinach. Lowered content of Ca as well as higher accumulation of Fe were found in plants from treatments no. 2, 4 and 5. Application of iodine (irrespective of its chemical form and method) in spinach cultivation (treatments 2–5) decreased Na and Zn content in plants. Pre-sowing fertilization with KI and KIO3 reduced Pb accumulation in spinach. No influence of iodine fertilization and fertigation was found in reference to yield as well as the content of P, K, Mg, S, B, Cu, Mn, Mo and Cd in spinach plants.

► Fertigation was more effective (339%–498%) for I biofortification than fertilization.
► Fertigation can contribute to reduce costs of iodine biofortification programs.
► Fertilization with KI and KIO3 improved accumulation of NO3− and Cl− in spinach.
► Soil application of KI and KIO3 increased N-total content in spinach plants.
► Pre-sowing fertilization with KI and KIO3 reduced Pb accumulation in spinach.