Factors influencing the efficacy of iodine foliar sprays used for biofortifying butterhead lettuce (Lactuca sativa)

Abstract

Iodine biofortification of butterhead lettuce (Lactuca sativa) via foliar sprays was investigated in field trials, focusing on assessing the influence of the time and application method. The iodine (I) concentrations in the edible plant parts increased when potassium iodide (KI) and potassium iodate (KIO3) solutions were sprayed at doses up to 0.25 kg I ha−1 on different dates close to harvest. Crop yield and marketable quality were not significantly affected by I treatments. A greater efficacy of KI was frequently observed and probably related to its lower point of deliquescence and smaller anion size in comparison with KIO3. KI sprays on butterhead lettuce at different times of the day resulted in a higher I enrichment when applied at 11:00 and 15:00 h. The diurnal variation in I uptake may reflect the impact of fluctuating climatic conditions at the time of application. Iodine treatments at different application dates near harvest led to an increasing I concentration in the vegetable produce that could be related to the rising shoot fresh mass and leaf area. When KI and KIO3 were sprayed simultaneously with commercial calcium fertilizers, fungicides or insecticides, I accumulation in butterhead lettuce was not negatively affected or in some cases even significantly enhanced. The results show that foliar sprays of KI and KIO3 are an effective method to biofortify butterhead lettuce with I and this approach may easily be implemented as a routine method in commercial cultivation.

Agronomic biofortification as a means of enriching plant foodstuffs with iodine

“…Willd.] from 61 to 73 μg 100 g −1 FW [25]. Biofortification by means of fertigation becomes even more effective when iodine salts are used together with humic substances or organic acids, as confirmed in the cultivation of spinach, where satisfactory biofortification effects were achieved when KIO 3 and humic acids were applied simultaneously [30].…”

“…Similarly, adding KNO 3 to KI increased iodine absorption, but there was no increase in iodine content in the case of Ca(NO 3 ) 2 [26]. The leaves of butter lettuce absorbed iodate at a higher rate when CaCl 2 was added to the solution [25]. The positive effect of CaCl 2 on iodine absorption by the leaves was explained as being a result of the humectant effect.…”

“…Spraying leaves with a solution containing iodine salts is an effective biofortification method, which has been confirmed in numerous experiments. In lettuce cultivation, foliar spraying contributed more effectively to accumulation of iodine than did soil application, and furthermore it had no negative effect on crop quality or biomass [25]. However, foliar application of iodine was found to be less effective than root treatment in tomato cultivation [28].…”

“…Under field conditions, iodide from the soil is more susceptible to losses caused by leaching, due to its greater solubility as compared to the iodate form [25]. In soil, IO 3 − is more efficiently absorbed by plants than is I − , and in soil-less cultures it is less toxic than iodide.…”

“…They show considerable homology to methyltransferases involved in responses to salinity or glucosinolate metabolism, suggesting that methyl iodide production may be a defence mechanism in plants [28]. Literature research indicates that there is no single pattern according to which chemical forms of iodine affect plants [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. Each species will react in a different way, depending on the method of cultivation and application of iodine.…”