Iodine uptake and translocation in apple trees grown under protected cultivation

Abstract

Background and Aims: Agronomic biofortification of food crops with iodine may improve the dietary intake of this trace element, which is essential for human development and health. So far, little is known about the suitability of this technique in pome fruits. The objectives of this study were (1) to investigate uptake and translocation of exogenously applied iodine in apple trees, (2) to identify possible strategies of iodine biofortification for this type of fruit, and (3) to evaluate interactions between foliar applied iodine and selenium.Methods: Apple trees were cultivated in a plastic tunnel for two growing seasons. Iodine was applied via leaves or substrate. During the 2nd year, simultaneous foliar application of iodine and selenium were tested as well. At harvest time, iodine and selenium content in leaves and fruits were determined. The phytoavailable iodine concentration in the growing medium was analyzed following an extraction with calcium chloride. In addition, the dynamics of iodine applied as potassium iodide and iodate in a peat‐based substrate was investigated in an incubation experiment without plants.Results: The iodine concentration in washed apples increased more than 100‐fold, valuing around 50 µg (100 g FM)−1 by foliar application of iodine as compared to the control treatment. However, this level was only achieved in fruits which were directly wetted by the spray solution. The translocation of leaf‐absorbed iodine to fruits was negligible. Following a substrate fertilization, the fruit iodine content remained rather low due to a strong retention of iodine in the growing medium. When using foliar sprays, the addition of selenium did not affect the iodine enrichment of the apple fruits.Conclusions: Foliar fertilization of iodine seems to be a promising method to biofortify apples with iodine. The level of I achieved in apple fruits by means of foliar fertilization can significantly contribute to the daily I intake requirement of humans.

Simultaneous Biofortification of Rice With Zinc, Iodine, Iron and Selenium Through Foliar Treatment of a Micronutrient Cocktail in Five Countries

“…Similarly, also I and Se seem to be not antagonistic ( Table 6 ). Also in previous studies, no clear interaction was found between I and Se during their leaf absorption and transportation to wheat grains ( Zou et al, 2019 ), lettuce roots ( Smoleń et al, 2014 ), kohlrabi tubers ( Golob et al, 2020 ) and apple fruits ( Budke et al, 2020 ).…”

“…Similar findings related to phloem transportation of I have been also reported by Smoleń et al (2016) and Landini et al (2011) . By contrast, there are studies showing poor phloem transport of I in the plants such as in young spinach plants ( Humphrey et al, 2019 ) and apple trees ( Budke et al, 2020 ). In case of the studies with apple trees, the increases in I concentration of the fruits, which were kept in plastic bags during the foliar spray, were around 4- to 5-fold compared to the trees without foliar I spray (i.e., from 0.4 μg up to 2.0 μg per 100 g FW) ( Budke et al, 2020 ).…”

“…By contrast, there are studies showing poor phloem transport of I in the plants such as in young spinach plants ( Humphrey et al, 2019 ) and apple trees ( Budke et al, 2020 ). In case of the studies with apple trees, the increases in I concentration of the fruits, which were kept in plastic bags during the foliar spray, were around 4- to 5-fold compared to the trees without foliar I spray (i.e., from 0.4 μg up to 2.0 μg per 100 g FW) ( Budke et al, 2020 ). These increases were, indeed, very significant; but the dietary relevance seems to be minimal.…”

“…The age of the leaves (i.e., sink or source status of the leaves) of the experimental plants used in the short-term studies might be also another factor. Considering published data and reported large gradient in I concentrations between the sink and source organs (Tsukada et al, 2008;Cakmak et al, 2017;Budke et al, 2020), it can be suggested that I is not highly phloem mobile in the plants, like nitrogen, phosphorus or potassium (Marschner, 2012); but it can be considered as moderately phloem mobile as also suggested by Hurtevent et al (2013). Further studies are required for better clarification and understanding of the phloem mobility of I in different plant species considering source and sink status of the treated leaves and other factors highlighted above.…”

Iodine Biofortification of Apples and Pears in an Orchard Using Foliar Sprays of Different Composition

“…This is important because a previous study on apples showed that more than half of the foliar-sprayed I is localized in the fruit peel. Nevertheless, I in the peel is hardly affected by washing of the fruit under running water – this reduced the fruit I content by only 8% ( Budke et al, 2020a ). Thus, when fresh pome fruits are consumed, most of the biofortified I usually becomes nutritionally effective.…”