The role of exogenous humic and fulvic acids in iodine biofortification in spinach (Spinacia oleracea L.)

The strong sorption of iodine in soils is a major factor that limits its uptake by plants. The aim of this study was to evaluate the influence of Humistar (a concentrate of exogenous humic acids 12 % w/w and fulvic acids 3 % w/w; abbreviated: HU) applied with KI or KIO3 on the efficiency of iodine biofortification, yield and the accumulation of undesirable compounds in spinach. Methods of iodine application with or without HU were tested in two pot experiments and a hydroponic study with spinach cultivation. The spinach biomass and the content of iodine, soluble oxalates, Cd and Pb in leaves were determined. The physicochemical properties and chemical composition of the soil and media were analysed. No negative influence of iodine and HU on spinach yield was observed. Fertigation with iodine resulted in a higher efficiency of iodine biofortification than its application pre-sowing (particularly for KIO3), with no increase in the Cd and Pb content in spinach plants compared to the control. The improvement of iodine uptake and root-to-leaf transfer (represented by higher transfer factor values) in spinach can be obtained by the application of HU via plant fertigation with IO3 (-). The level of soluble oxalates in spinach grown in the pot experiments was not affected.