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

Abstract

Many people across the world suffer from iodine (I) deficiency and related diseases. The I content in plant-based foods is particularly low, but can be enhanced by agronomic biofortification. Therefore, in this study two field experiments were conducted under orchard conditions to assess the potential of I biofortification of apples and pears by foliar fertilization. Fruit trees were sprayed at various times during the growing season with solutions containing I in different concentrations and forms. In addition, tests were carried out to establish whether the effect of I sprays can be improved by co-application of potassium nitrate (KNO₃) and sodium selenate (Na₂SeO₄). Iodine accumulation in apple and pear fruits was dose-dependent, with a stronger response to potassium iodide (KI) than potassium iodate (KIO₃). In freshly harvested apple and pear fruits, 51% and 75% of the biofortified iodine was localized in the fruit peel, respectively. The remaining I was translocated into the fruit flesh, with a maximum of 3% reaching the core. Washing apples and pears with running deionized water reduced their I content by 14%. To achieve the targeted accumulation level of 50-100 μg I per 100 g fresh mass in washed and unpeeled fruits, foliar fertilization of 1.5 kg I per hectare and meter canopy height was required when KIO₃ was applied. The addition of KNO₃ and Na₂SeO₄ to I-containing spray solutions did not affect the I content in fruits. However, the application of KNO₃ increased the total soluble solids content of the fruits by up to 1.0 °Brix compared to the control, and Na₂SeO₄ in the spray solution increased the fruit selenium (Se) content. Iodine sprays caused leaf necrosis, but without affecting the development and marketing quality of the fruits. Even after three months of cold storage, no adverse effects of I fertilization on general fruit characteristics were observed, however, I content of apples decreased by 20%.

Keywords: agronomic biofortification; foliar fertilization; iodate; iodide; pome fruit; potassium nitrate; selenium; total soluble solids.

FIGURE 1

Examples of fruit trees included in the second field experiment and fruit appearance shortly after the harvest: Apple tree cv. ‘Fuji’ (A), pear tree cv. ‘Williams Christ’ (B). Selection of 10 harvested apple (C) and pear fruits (D) from treatment no. 5 consisting of a combined foliar spray with KNO₃, KIO₃ and Na₂SeO₄ which did not negatively affect external fruit characteristics. Partitioning of fruits for further preparation and analysis steps (E).

FIGURE 2

Iodine content in washed fruit segments of apples cv. ‘Jonagold’ (A) and pears cv. ‘Alexander Lucas’ (B) at harvest time as affected by the dose and form of iodine foliar sprays in the first field experiment. Means ± standard deviation (n = 4).

FIGURE 3

Iodine distribution in washed apples cv. ‘Jonagold’ (A) and pears cv. ‘Alexander Lucas’ (B) at harvest time as affected by the dose and form of iodine foliar sprays in the first field experiment.

FIGURE 4

Iodine and selenium content in washed fruit segments of apples cv. ‘Fuji’ (A,B) and pears cv. ‘Williams Christ’ (C,D) in the second field experiment as affected by different foliar spray treatments and fruit storage at 2°C for a period of three months. Means ± standard deviation (apple n = 6, pear n = 4). Means not sharing a letter in one chart or indicated by an asterisk are significantly different according to Tukey-HSD test at α = 0.05.

FIGURE 5

Cumulative decrease of the iodine and selenium content in fruit segments by washing and peeling of apples cv. ‘Fuji’ (A,B) and pears cv. ‘Williams Christ’ (C,D) in the second field experiment at harvest time and after fruit storage at 2°C for a period of three months. Means ± standard deviation (apple n = 6, pear n = 4).

FIGURE 6

Development of leaf damage during the growing season until fruit harvest in the second field experiment. Images of scanned leaves of apple trees cv. ‘Fuji’ (A) and pear trees cv. ‘Williams Christ’ (B) from treatment no. 5 consisting of a combined foliar spray with KNO₃, KIO₃ and Na₂SeO₄. Score values indicate the degree of the damage (Score value 1 = no damage, 3 = slight damage 5 = moderate damage, 7 = severe damage, 9 = very severe damage). Detail view of ‘Fuji’ apple trees (C,D) and ‘Williams Christ’ pear trees (E,F) in the second field experiment at harvest time. Picture C and E: treatment no. 1 (control). Picture D and F: treatment no. 5 (spray solution composition as described above).

FIGURE 7

Appearance of pear trees cv. ‘Alexander Lucas’ in the first field experiment 19 days after harvest (Oct. 13) as affected by the dose and form of iodine foliar sprays applied two weeks before fruit harvest.

FIGURE 8

Total soluble solid content in fruit segments of apples cv. ‘Fuji’ (A,B) and pears cv. ‘Williams Christ’ (C,D) in the second field experiment as affected by different foliar spray treatments and fruit storage at 2°C for a period of three months. Means ± standard deviation (apple n = 6, pear n = 4). Means with same letters for one fruit group and one time of measurement do not differ according to Tukey-HSD test at α = 0.05.