Iodine is an essential nutrient for humans, which is often not ingested through food in adequate quantities. Currently, Germany is once again one of the countries in which there is an iodine deficiency in the population. Women between the ages of 20 and 40 are particularly affected, a critical situation since pregnant and lactating women have an increased iodine requirement. Iodization of table salt is a widely used prophylactic measure. However, this method is not sufficient and may become less important in the future if further dietary salt reduction occurs, as nutritionists are demanding. Alternative approaches are therefore needed to improve the supply.
One of these approaches is the agronomic biofortification of food crops. In this process, iodine is applied via fertilization measures during the cultivation of the plants. This gives the plants the ability to take up the mineral, which is only available in the soil to a very limited extent. In recent years, many studies have been published on the biofortification of vegetables and cereals. Foliar fertilization measures have proven to be significantly more efficient than soil fertilization measures. Nevertheless, up to now few results are available on the biofortification of fruit crops. However, fruit is also important for a healthy diet and the iodine supply of humans can only be improved if as many iodine-rich foods as possible are available. Therefore, the aim of this work was to investigate iodine biofortification of berry and tree fruit species in more detail.
In order to be able to achieve this objective, trials were performed over several years with strawberries, apple and pear trees. In addition to suitable application methods, the aim was to determine the iodine form (iodide and iodate) and the necessary iodine quantity. On the one hand, the measured iodine contents in the fruit and leaf tissue allowed conclusions to be drawn about the translocation of iodine in the plant. On the other hand, this made it possible to evaluate the basic suitability for iodine biofortification of the fruit crops investigated. Since iodine has a phytotoxic effect above a certain amount, the plant compatibility should also be tested. In addition, common household processing methods, such as washing or peeling the fruit, as well as fruit storage over several months, should provide information on the extent to which such measures could reduce the iodine content. Another study parameter was the soluble solids content, as there is evidence that iodine can affect the sugar content of fruit. Furthermore, a combined application of potassium nitrate and selenium was carried out and their influence on iodine and sugar content was investigated. Selenium is also an essential trace element, which is usually inadequately absorbed through the diet.
The results of the investigations showed that it was possible, in principle, to raise the iodine content of strawberries, apples and pears to a level of 50 to 100 µg iodine per 100 g fresh mass. In the case of strawberries, however, this was only feasible if the plants were in their first year of cultivation and the iodine was applied by foliar fertilization shortly before harvest. In the 2nd and 3rd year of cultivation, the plants had a very dense canopy, which prevented direct wetting of the fruit. However, direct wetting of the fruit surface with the iodine solution is imperative, as this was the only way to achieve a reliably high iodine content in the fruit mass. Soil fertilization proved to be completely unsuitable in trials with strawberries and apple trees. The translocation of iodine after soil fertilization occurred mainly via the xylem transport into the strongly transpiring leaves and not into the fruits. In addition, compared to a foliar application, a significantly higher iodine application rate was required. Furthermore, experiments with apple trees cultivated in a plastic tunnel, protected from precipitation, showed that the iodine transfer via the phloem into the fruits was only marginal.
With regard to the phytotoxic effect of iodine application, no consistent difference was observed between potassium iodide and potassium iodate. Both forms of iodine did not affect yield or average individual fruit weight. Damage to fruit was not observed in any variant. However, with increasing iodine levels, significant damage to leaves was noticeable. Apple and pear trees also showed early leaf fall. Iodide generally led to significantly higher iodine contents in the plant mass after foliar application, but this was also associated with high fluctuations. With iodate, it was possible to reliably achieve the targeted iodine content in the fruit mass of apple and pear trees with an application rate of 1.5 kg iodine per hectare and meter canopy height.
Washing the fruit reduced the iodine content of strawberries by up to 30%. For apples and pears, this value was about 14% at harvest and about 12% after 3 months of storage. Peeled apples and pears showed a significantly reduced iodine content. 51% of the iodine in apples was bound in the fruit peel or the cuticular waxes. A reduction of 73% was determined for pears. Cold storage for 3 months resulted in a significant loss of iodine in parts of the apple peel. At this point, the release of volatile iodine compounds is probably the cause of the reduction. However, this would still have to be confirmed by further investigations.
Iodine application had a negative effect on the soluble solids content of strawberries above a certain level. It was not possible to observe significant changes for pome fruit in the trials conducted. However, the application of potassium nitrate (alone and in combination with iodine) resulted in an increase. Iodine uptake remained unaffected by the combined application of potassium nitrate and selenium. However, it was shown that selenium has a comparable uptake and translocation pattern to iodine and that a combined biofortification with both minerals is, in principle, possible.
Accordingly, apple and pear trees are well suited for biofortification with iodine by foliar fertilization. However, further trials in commercial orchards are necessary to implement this process. In the future, appropriately fortified fruit could make an important contribution to the alimentary iodine supply for humans.
| Identifer | oai:union.ndltd.org:uni-osnabrueck.de/oai:repositorium.ub.uni-osnabrueck.de:urn:nbn:de:gbv:700-202110265518 |
| Date | 26 October 2021 |
| Creators | Budke, Christoph |
| Contributors | Prof. Dr. Gabriele Broll, Prof. Dr. Diemo Daum |
| Source Sets | Universität Osnabrück |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis |
| Format | application/pdf, application/zip |
| Rights | http://rightsstatements.org/vocab/InC/1.0/ |
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