Edited by:
Reviewed by:
*Correspondence:
This article was submitted to Plant Nutrition, a section of the journal Frontiers in Plant Science
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
The low content of iodine (I) and selenium (Se) forms available to plants in soil is one of the main causes of their insufficient transfer in the soil-plant-consumer system. Their deficiency occurs in food in the majority of human and farm animal populations around the world. Both elements are classified as beneficial elements. However, plant response to simultaneous fertilization with I and Se has not been investigated in depth. The study (conducted in 2012–2014) included soil fertilization of carrot cv. “Kazan F1” in the following combinations: (1) Control; (2) KI; (3) KIO3; (4) Na2SeO4; (5) Na2SeO3; (6) KI+Na2SeO4; (7) KIO3+Na2SeO4; (8) KI+Na2SeO3; (9) KIO3+Na2SeO3. I and Se were applied twice: before sowing and as top-dressing in a total dose of 5 kg I⋅ha-1 and 1 kg Se⋅ha-1. No negative effects of I and Se fertilization were noted with respect to carrot yield. Higher accumulation and the uptake by leaves and storage roots of I and Se were obtained after the application of KI than KIO3, as well as of Na2SeO4 than Na2SeO3, respectively. Transfer factor values for leaves and roots were about a dozen times higher for Se than for I. Selenomethionine content in carrot was higher after fertilization with Na2SeO4 than with Na2SeO3. However, it was the application of Na2SeO3, KI+Na2SeO3 and KIO3+Na2SeO3 that resulted in greater evenness within the years and a higher share of Se from selenomethionine in total Se in carrot plants. Consumption of 100 g f.w. of carrots fertilized with KI+Na2SeO3 and KIO3+Na2SeO3 can supply approximately or slightly exceed 100% of the Recommended Daily Allowance for I and Se. Moreover, the molar ratio of I and Se content in carrot fertilized with KI+Na2SeO3 and KIO3+Na2SeO3 was the best among the research plots.
For the last few decades selenium (Se) has been considered as a beneficial element for plants – its indispensability for plants has not however been proved. Se can have a growth-promoting effect in many species of plants. Importantly, there is a strict relation between the uptake and metabolism of Se(IV) and P, as well as of Se(VI) and S (
The classification of iodine (I) into the group of beneficial elements for plants is not as unanimous as of selenium. There are known reports indicating its positive influence on higher plants (
What needs to be taken into account is the physiological role of I and Se for human and animals. The cause of insufficient transfer of I and Se in the soil-plant-consumer system is the low content of its available forms in the soil. It is also the reason for the limited supply of I and Se in food for several billion people and farm animals worldwide. It is estimated to affect nearly two-thirds of the global human population and is manifested by various diseases and health disorders (
The relatively cheapest, while still effective way of counteracting the problem could be applying methods of agrotechnical plant biofortification (enrichment) with I and Se, and also with Zn and Fe, whose deficiency is highly widespread round the world as well (
In many countries around the world, I prophylaxis is based on iodization of table salt. Due to the risk of developing various health problems associated with excessive salt consumption, the WHO has recently recommended searching for alternative ways of introducing I into food (
The implementation of agrotechnical methods of biofortification for combined fertilization with I and Se is difficult as neither element is a mineral nutrient for plants (
In general, irrespective of the cultivation type (field, soilless, hydroponic), the iodide form (I-) is more rapidly taken up by roots, and at the same time is more toxic to plants than the iodate form (IO3-;
In the case of Se, uptake preferences and plant toxicity of SeO42- and SeO32- also depend on the cultivation type – more precisely, on the environment in which the root system is developed. In the research of
The aspects of I and Se interaction due to their simultaneous application have not yet been sufficiently investigated. It is still unknown whether or to what extent increased concentrations of both these elements in the soil environment affect plant growth, development and, finally, yield. Only a few studies in this area have been conducted so far, including hydroponic cultivation of spinach (
The research hypothesis was that there is a possibility of increasing I and Se content in carrot plants by conducting simultaneous soil fertilization with mineral forms of these elements. Another one stated that simultaneous application of I and Se has a negative effect on the yield, as even small doses of both elements (not being plant nutrients) can be toxic to plants. It was also assumed that after plant fertilization with Se, a significant increase of organic forms of this element, including selenomethionine, would occur.
The research objective was to determine the effects of soil fertilization with different chemical forms of I (I- and IO3-) and Se (SeO32- and SeO42-) on crop yield, the efficiency of biofortification with these elements and selected chemical properties of carrot plants (
In the years 2012–2014, a field study with carrot (
Selected chemical properties of the 0–30 cm soil layer prior to the experiment in 2012–2014 (
Parameter | 2012 | 2013 | 2014 |
---|---|---|---|
pHH2O | 6.30 | 7.77 | 6.10 |
EC (dS m-1) | 0.13 | 0.12 | 0.04 |
Eh (mV) | +220.0 | +257.9 | +233.5 |
Iodine (mgkg-1) | 0.25 | 0.24 | 0.25 |
Selenium (mgkg-1) | 0.59 | 0.57 | 0.60 |
Organic matter (%) | 2.11 | 2.48 | 2.25 |
Particle size fraction (%): sand/silt/loam | 4/47/49 | 2/48/50 | 4/47/49 |
Soil texture class | Silty clay (heavy soil) | Silty clay (heavy soil) | Silty clay (heavy soil) |
Carrot was cultivated on a heavy soil with a silty clay texture (
The value of soil pH was higher in 2013, as in that year carrot was cultivated on another field located about 500 m from the field used in 2012 and 2014 (coordinates: 50.1868649 N, 20.0925089 E, 306 masl). Despite the pH differences, soils from the fields under study were characterized by similar I and Se content in each year.
The study included soil fertilization with I and Se in the following combinations: (1) Control; (2) KI; (3) KIO3; (4) Na2SeO4; (5) Na2SeO3; (6) KI+Na2SeO4; (7) KIO3+Na2SeO4; (8) KI+Na2SeO3; (9) KIO3+Na2SeO3. I and Se were applied twice: before sowing (before ridge formation) and as a top-dressing – at canopy closure, each in a dose of 2.5 kg I⋅ha-1+ 0.5 kg Se⋅ha-1. The total amount of introduced I and Se was 5 kg I⋅ha-1 and 1 kg Se⋅ha-1, respectively. Pre-sowing fertilization with I and Se was conducted on April 12, 2012, April 18, 2013, and April 04, 2014, and top-dressing application on June 29, 2012, July 3, 2013 and July 4, 2014. I was applied as KI and KIO3 (puriss. p.a., Avantor Performance Materials, Gliwice, Poland), and Se as Na2SeO4 and Na2SeO3 (puriss. p.a., Sigma–Aldrich Co. LLC, St. Louis, MO, USA). The experiment was arranged in a split-plot design. Each treatment was randomized in four repetitions on 4 m × 6 m (24 m2) plots. The total area of the experiment was 864 m2.
One day prior to bed formation, based on the results of soil chemical analysis, pre-sowing fertilization with N, P, and K was conducted (along with I and Se application), in order to supplement the soil to the level optimal for carrot: N-100, P-80, and K-200 (in mgdm-3 of soil). N fertilization was applied as urea (Zakłady Azotowe “Puławy,” Puławy, Poland), P and N as ammonium phosphate (Grupa Azoty SA, Zakłady Chemiczne “Police,” Police, Poland) and K as 60% potassium salt (Zakład Obrotu Towarami Sp. z o. o., Dwikozy, Poland). Doses of these fertilizers in individual years, respectively, for 2012, 2013, and 2014, were as follows: urea 0.25, 0.10, and 0.25 t⋅ha-1, ammonium phosphate 0.80, 1.25, and 1.00 t⋅ha-1 as well as potassium salt 0.90, 1.10, and 1.00 t⋅ha-1.
Carrots were cultivated in one row on 40 cm wide and 30 cm high raised beds at a seeding rate of 37 seeds⋅m-1 (approximately 600,000 seeds per hectare). The seeds were sown on April 19, 2012, April 25, 2013, and April 05, 2014. The carrot roots were harvested on September 26, 2012, September 11, 2013, and September 9, 2014. During harvest, yield of carrot leaves and storage roots as well as plant density per hectare were determined. Marketable yield consisted of storage roots of cylindrical or close-to-cylindrical shape with a head diameter of ≥3 cm, undamaged by pests, not infected by fungi or bacteria, with no fractures and heads greened to a maximum of 0.5 cm. The length of a storage root was 15 cm minimum. At harvest, approximately 10 kg samples of carrot storage roots were chosen from each of the four plots (replications) for laboratory analysis. Only roots qualifying as marketable yield were taken for further chemical analysis.
During carrot harvest, soil samples from the layers 0–30 cm, 30–60 cm, and 60–90 cm were also collected separately for each of the research plots. Soil samples from each layer from each of the four plots (replications) were collected by soil drill (diameter 3 cm) for laboratory analysis. Additionally, before cultivation, eight individual samples from the whole area of the experimental fields were randomly collected (
Samples of carrot leaves and storage roots were dried at 70°C in a laboratory dryer with forced air circulation and ground in a Pulverisette 14 Fritsch (Idar-Oberstein, Germany) variable speed rotor mill, using a 0.5 mm sieve. Samples thus prepared were subsequently analyzed with respect to the content of I and Se with an ICP-OES spectrometer (Prodigy, Leeman Labs, New Hampshire, MA, USA) and selenomethionine (SeMet) using a capillary electrophoresis (CE) analyzer PA 800 Plus CE system with DAD detection (Beckman Coulter, Indianapolis, IN, USA)
Iodine content in leaf and root samples was determined with the application of the cold vapor I2 generation (CVG) technique (
Selenium content was analyzed after sample digestion in nitric acid (
The SeMet content in leaf and root samples was determined using the following procedure. In 10 cm3 Falcon tubes, 5 cm3 of solution containing 40 mg protease and 20 mg lipase in demineralized water were added to 0.25 g of air-dried plant samples. The samples were incubated for 16 h at 20°C and centrifuged for 15 min at 4500 rpm (
Before carrot cultivation, soil samples were taken from the 0–30 cm layer, in order to characterize the physical and chemical properties of the soil site (
Iodine and Se content in the soil before and after carrot cultivation was analyzed using the following procedure. Soil samples were dried at 70°C in a laboratory dryer with forced air circulation, ground in a mortar and sieved through a 1 mm sieve. Soil samples (2.5 g) were put into 30 cm3 Falcon tubes; 10 cm3 of double-distilled water and 1 cm3 of 25% TMAH (tetramethylammonium hydroxide - Sigma–Aldrich Co. LLC, St. Louis, MO, USA) were added. After mixing, the samples were incubated for 3 h at 90°C. After incubation, the samples were cooled to a temperature of approximately 20°C and filled to 30 cm3 with double-distilled water. After mixing, the samples were centrifuged for 15 min at 4500 rpm. The measurements, using an ICP-OES spectrometer (Prodigy, Leeman Labs, New Hampshire, MA, USA), were conducted in the supernatant (without decanting it). The method described above is our own modified procedure (
Analysis of the I and Se content in leaves and storage roots of carrot and soil was conducted using a ICP-OES Prodigy spectrometer (Prodigy, Leeman Labs, New Hampshire, MA, USA). Calibration of the instrument was performed by maintaining the same matrix as for the analyzed samples. For I determination in plant samples using the CVG technique, the most sensitive line of I: I-178.276 nm (with the detection limit of 0.5 μg I⋅dm-3) was chosen, as for this method no interference from P affects it (
Each year, carrot was grown from April to September. Meteorological data from dekadal (10 days) periods are presented including the mean daily air temperature, mean daily PAR value and total precipitation (
For comparison, the mean air temperature for the period 1971–2000 from April to September was 14.3°C, and total precipitation was 435 mm (
The most unfavorable weather conditions during carrot cultivation occurred in 2012 and mainly concerned the amount and distribution of rainfall (
With respect to PAR, its highest total was noted in 2014, and at the same time it was the most evenly distributed throughout the entire period of carrot cultivation within all 3 years of the study (
The I and Se transfer factor (TF) in the soil-plant (carrot leaves or storage roots) system was calculated using the following formula:
with C standing for I and Se content in plant/soil dry matter. Based on the results of crop yield measurement, I and Se determination (and dry weight) in carrot leaves and storage roots, values of I and Se uptake by leaves, storage roots and whole plants of carrot were calculated. Using the results of Se and SeMet content, the percentage ratio of Se present in SeMet in relation to the total Se content in the leaves and storage roots of carrot was calculated.
The percentage of Recommended Daily Allowance for I (RDA-I) and Se (RDA-Se) supplied from one serving of a 100 g portion of fresh carrot storage roots was calculated. In these calculations, results of I and Se determination in carrot as well as the recommended daily intake of these two elements: 150 μg I and 55 μg Se per day for adults were used (
All data were subjected to analysis of variance using the ANOVA module of Statistica 10.0 PL. For determining the significance between the means, the Tukey test was used. The significance was declared at
In the following years of the research, no statistically significant effect of I and Se fertilization was observed with respect to all measured yield parameters (
In 2012 and 2014, a lower planting density was observed than in 2013 (
Fertilization with I and Se had a significant effect on the accumulation of I in carrot leaves and storage roots (
Each year, the highest I content in carrot leaves and storage roots along with the highest I uptake was characterized by plants fertilized solely with KI (
Fertilization only with Se in the form of Na2SeO4 and Na2SeO3, as compared to the control, caused a substantial increase of I content in leaves and storage roots (
The highest accumulation, TF and I uptake by roots (
In comparison to the control and plots with the application of only I, in each year of the research, a significant increase of Se accumulation, TF values for Se, Se uptake and SeMet content in carrot leaves and storage roots were noted in plants from the plots with Se and Se+I (
In relation to the control, fertilization with KI or KIO3 alone caused a considerable increase of Se content in storage roots – with the exception of KI application in 2014 (
For each year of the research, diverse effects of relations between the application of Na2SeO4 alone or together with KI and KIO3 were noted with respect to Se content, Se TF values, Se uptake and SeMet content in carrot leaves and storage roots (
The percentage of total Se found in SeMet calculated for carrot leaves and storage roots (further expressed as “Se-SeMet in total Se”) was the parameter with the highest variability within the years (
Soil fertilization with I and Se compounds substantially increased the percentage of the RDA-I and RDA-Se supplied by the intake of 100 g of fresh carrot roots (
In the case of I, a single portion of 100 g f.w. of carrot fertilized with KI in each research year (as well as KI+Na2SeO4 and KI+Na2SeO4 in 2013) would supply more I than recommended for adults (
In the case of Se, each year a single portion of 100 g f.w. of carrot fertilized with Na2SeO4, KI+Na2SeO4 and KIO3+Na2SeO4 would supply Se in a supraoptimal quantity - from 500 to 650% RDA-Se for adults (
It should be underlined that soil fertilization solely with KI or KIO3 also caused a significant increase of about 12% of daily consumer allowance of Se supplied with a single portion of fresh carrots when compared to the control – the relation was noted in 2012 and 2013 (
In the years 2012–2014, a highly varied effect of I and Se fertilization on the content of these two elements in soil after carrot cultivation was noted (
In each research year, I content in soil from plots fertilized with this element exceeded that in the control and combinations treated with Se alone (
In the study conducted by
The lack of negative response of carrot plants (in terms of yield) to I fertilization may be the consequence of its tolerance to iodine. In a pot experiment carried out in a greenhouse,
In the present study, iodine uptake by carrot plants was substantially improved when this element was applied as I- than IO3-. Similar observations were noted for lettuce cultivated in perlite (
Generally, the values of I and Se TF in carrot roots (and leaves) can vary to a large extent depending on the cultivation site (climatic zone) and physical and chemical properties of soils (
The dose of Se used for fertilization (1 kg Se⋅ha-1) was five times lower than of I (5 kg I⋅ha-1). Still, irrespective of the research plot, the values of Se TF values for leaves and storage roots were higher than for I. This indicates considerably higher Se mobility, and therefore availability, to plants in the soil environment, since I undergoes very strong sorption in soil (
The transport of SeO32- ions in plants occurs using the same protein transporters as of phosphate ions, while for SeO42-, sulfate carriers are engaged (
From our study we may assume that reduced I uptake in 2012 was probably a consequence of lower precipitation volume than in the years 2013–2014. In this context, however, the results of Se determination in soil after carrot cultivation are surprising. Its highest content was noted in 2014, while the lowest was in 2012, which was accompanied by the smallest Se uptake by carrot plants. These results indicate that in that latter year Se underwent the highest soil sorption, which reduced its uptake by plants.
Even more surprising were the extremely different concentrations of I and Se in 2012 and 2014, as in these years carrot was cultivated on various parts of the same field. Most probably, diversity with respect to pH, EC and the content of I, Se and organic matter along with diverse weather conditions in 2012–2014 affected these changes in I and Se uptake by plants. Another affecting factor could include speciation processes of I and Se that occur in soil, sometimes leading to its volatilization. We did not, however, study this problem. Additionally, overall soil diversity as well a varying history of cultivation of two separate parts of the field in 2012 and 2014 may have contributed to obtained differences in I and Se content in soil.
There is also no certainty which form of I and Se is analyzed in soil after sample incubation in TMAH. According to
Results of our study indicate that climatic conditions may have a substantial influence on obtaining diverse plant reaction to simultaneous application of I and Se. It cannot be clearly stated which of the monitored weather parameters (with its values varying in the years 2012–2014) contributed mostly to observed changes. Such effect can be determined only in studied conducted in strictly controlled conditions, e.g., in growth chambers
Both tested elements are expelled via leaves through methylation (
After uptake, SeO32- ions in roots are converted into organoselenium compounds (mainly Se-amino acids) and transported in that form to leaves. SeO42- ions, however, after root uptake, are firstly transported through the xylem to stems and then to leaves, in which they are reduced to SeO32-. The reduction process is followed by the synthesis of Se-amino acids, e.g., selenocysteine (SeCys) and SeMet. Biosynthesis of organoselenium compounds from SeO42- ions (after previous SeO32- reduction) does not occur in roots, or has a marginal effect, in comparison to Se-amino acid synthesis directly from SeO32- (
A basic parameter crucial for assessing the efficiency of simultaneous biofortification of plants with I and Se is RDA, which for these elements is 150 μg I and 55 μg Se for adults, and 200–300 μg I and 60–70 μg Se for pregnant and lactating women, respectively (
In the 3-year research period, the molar ratio of I:Se in a single portion of 100 g f.w. of carrot from the respective research plots was within the following values: (1) Control (2.2–3.9:1), (2) KI (29.4–59.3:1), (3) KIO3 (8.4–42.1:1), (4) Na2SeO4 (0.06–0.009:1), (5) Na2SeO3 (0.32–0.65:1), (6) KI+Na2SeO4 (0.45–1.4:1), (7) KIO3+Na2SeO4 (0.31–0.61:1), (8) KI+Na2SeO3 (2.9–4.1:1), (9) KIO3+Na2SeO3 (1.6–4.4:1). According to this data, even in control plants the molar ratio of I:Se differs from the optimal for consumer need. It should be noted that for that combination the obtained RDA-I values ranged from 6.14% in 2014 to 7.6% in 2012, and of RDA-Se from 8.4% in 2012 to 41.2% in 2014. These results indicate a greater capacity of the control carrot to satisfy consumer demand for Se than I – increasing with the natural content of I and Se in soil.
The most promising results in terms of biofortification purposes were obtained for simultaneous fertilization of Na2SeO3 with KI or KIO3. Not only did the values of RDA-I and RDA-Se oscillate, or slightly exceeded 100%, but the molar ratio of I:Se content in carrot was the closest to optimal – as compared to other combinations. Also, in the above-mentioned research by
Future research in this area needs to be expanded to establish the possibility of balancing daily human diets of I and Se by biofortified crop plants. Additionally, it is necessary to investigate the assimilability of these elements by consumers.
A common health problem is I and Se deficiency in the human and animal population. The increase of I and Se content in plants tissues through various strategies of plant biofortification can be included as one of the ways of innovative crop production. Simultaneous plant enrichment with I and Se stays within the scope of functional food production. Both tested elements are not essential for plants. The results of conducted studies widened the knowledge on its interaction in plants, at the same time proposing the implementation of simultaneous plant fertilization with I and Se into the agricultural practice, mostly in the areas with endemic deficiency of both beneficial elements.
Soil fertilization with KI, rather than KIO3, contributed to greater uptake and accumulation of I. For Se, it was the introduction of Na2SeO4 into the soil that improved Se uptake and increased its content, also in the organic form of SeMet, in carrot leaves and roots. Fertilization with Na2SeO3, however, stimulated the processes of Se conversion into an organic form, which was reflected by the increased percentage of total Se present in SeMet.
An interaction between I and Se with respect to the rate of accumulation of both beneficial elements was revealed. The clearest relation concerned the negative effect of Se (mainly Na2SeO3) on I content and uptake. The reverse interaction describing the limiting influence of I on Se uptake and accumulation was most distinctive when KI and Na2SeO3 were applied together. Interaction of Na2SeO4 with I with respect to Se uptake, TF values and the content of SeMet was far more diverse within the years of the study.
Our research has demonstrated the possibility of conducting simultaneous soil fertilization with both beneficial elements (I and Se) without the risk of reducing the carrot crop yield. The applied doses of I and Se increased I and Se enrichment of carrot to a level exceeding the possibility of balancing the diet with respect to RDA-I and RDA-Se. In further studies conducted on soils with low content of I and Se as well as in agricultural production, lower doses of both elements should be applied in order to avoid the excessive intake of I and Se with biofortified carrots by both humans and animals.
Combined soil fertilization with I+Se needs to be applied in areas with insufficient concentration of both beneficial elements in soils, and thus in plants/food. Because of higher TF values for Se than I, Se doses for fertilization should be significantly lower than those of I. Despite the lower uptake of Se from Na2SeO3, application of this compound (and not Na2SeO4) gave better results in terms of the percentage ratio of Se from SeMet in total Se content in carrot plants.
When cultivating plants on soils intrinsically rich in I and/or Se, additional introduction of I or Se can be conducted but in much lower doses, in order to ensure proper balancing of both beneficial elements content in plants and food. Particular attention should be paid to maintaining the optimal molar ratio of I:Se in plants within the values of: 4.4–8.8:1.
SS: Leader of the project, author of the method of carrot biofortification with iodine and selenium, coordinator of field experiments and laboratory analyses, conducted the analysis of results and prepared the manuscript; ŁS: conducted field experiments with carrot cultivation; conducted iodine and selenium analysis using ICP-OES; IL-S: co-founder of the project; involved in the preparation of the research project, author of the method of iodine biofortification of carrot, conducted statistical analysis of results; helped prepare the manuscript; RR: involved in conducting field experiment with carrot cultivation; conducted laboratory analyses using capillary electrophoresis; AKop: helped conducting carrot cultivation; prepared soil and plant samples. EP: helped conducting carrot cultivation; prepared soil and plant samples. RB-K: helped conducting carrot cultivation; prepared soil and plant samples. AKor: helped conducting carrot cultivation; prepared soil and plant samples. JK-D: helped conducting carrot cultivation; prepared soil and plant samples.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We would like to thank Mr. Mariusz Doniec for allowing us to conduct studies with carrot cultivation on his farm.