Iodine biofortification of field-grown strawberries - Approaches and their limitations

The potential of iodine biofortification in strawberry fruits by means of soil and foliar fertilization was investigated in three field experiments and a preliminary phytotoxicity test in the greenhouse. In the main experiment iodine was applied by one-time potassium iodate soil drenches two weeks after planting or, alternatively, by using potassium iodide foliar sprays from the beginning of flowering. Beside the iodine accumulation in fruits, effects on crop yield and quality were determined. The soil fertilization resulted in a relatively low iodine accumulation in strawberry fruits, probably because the concentration of phytoavailable iodine in the soil rapidly decreased after its application. A markedly higher iodine content in fruits was achieved when it was aerially applied, either by a single treatment shortly before harvest or by repeated sprays during the flowering period. Yield, firmness and total acidity concentration of strawberry fruits were not significantly affected by any of the tested iodine applications. However, as a result of repeated foliar sprays the concentration of soluble solids in fruits was slightly diminished. Attemps to substantially increase the iodine content in fruits of strawberry plants cultivated in the second and third year failed, even following frequent sprays. In conclusion the results of this study suggest that only a relatively small proportion of exogenously applied iodine enters the fruits of field-grown strawberries due to its strong retention in soil and low phloem mobility in plants.