Mélanie Longchamp

Maryse Castrec-Rouelle
email: maryse.rouelle@upmc.fr

Philippe Biron

Thierry Bariac

Variations in the accumulation, localization and rate of metabolization of selenium in mature Zea mays plants supplied with selenite or selenate

Keywords: Selenite, selenate, selenium bioavailability, enzymatic extraction, organoselenium compounds, biofortification

come     Variations in the accumulation, localization and rate of metabolization of selenium in mature Zea mays 2 plants supplied with selenite or selenate

INTRODUCTION

Selenium (Se) is an essential micronutrient in human and animal diets. More than 20 selenoproteins or selenoenzymes are involved in normal metabolism and selenium has also been proposed to lower the risk of cardiovascular diseases and cancer [START_REF] Rayman | Food-chain selenium and human health: emphasis on intake[END_REF][START_REF] Thomson | Assessment of requirements for selenium and adequacy of selenium status: a review[END_REF]. Food is the principal route of selenium intake. Meat and seafood contain the highest amounts of selenium, with 0.4-1.5 µg per gram [START_REF] Rayman | Food-chain selenium and human health: emphasis on intake[END_REF], but cereals, fruits and vegetables are also good food sources. Selenium enters the food chain through plants and especially crops, which are part of the diet of both primary and secondary consumers.

Selenium concentrations in food, including crops, depend not only on selenium concentrations in agricultural soils (which vary considerably between countries and regions) but also on selenium phytoaccessibility controlled by many abiotic and biotic factors such as soil pH, redox conditions, organic matter content, microbial activities, irrigation and compaction. In some countries or regions, low selenium levels in soil lead to low concentrations in feed or forage, which in turn can result in selenium deficiency in livestock and humans. For example, the average selenium intake is only 36 µg per day in France, 34 µg per day in the UK and 35 µg per day in Sweden [START_REF] Rayman | Food-chain selenium and human health: emphasis on intake[END_REF]; these levels are below the recommended dietary allowance of 40 to 70 µg per day (World Health Organization et al., 1996). To increase selenium levels in human and animal diets, several processes have been developed including mineral supplementation, genetic biofortification (plant breeding) and finally, the option chosen here, agronomic biofortification of food or forage.

In contrast to humans, the role of selenium for plants is more ambiguous, although studies on young plants have led to a better understanding of selenium pathways in higher plants [START_REF] Souza | Rate-Limiting Steps in Selenium Assimilation and Volatilization by Indian Mustard[END_REF][START_REF] Hopper | Plant avaibility of selenite and selenate as influenced by the competing ions phosphate and sulfate[END_REF][START_REF] Li | Selenium uptake, translocation and speciation in wheat supplied with selenate or selenite[END_REF][START_REF] Terry | Selenium in higher plants[END_REF][START_REF] Ximenez-Embun | Establishment of selenium uptake and species distribution in lupine, india mustard and sunflower plants[END_REF][START_REF] Zayed | Accumulation and volatilization of different chemical species of selenium by plants[END_REF][START_REF] Zhang | Uptake and transport of selenite and selenate by soybean seedlings of two genotypes[END_REF]. Plant development and selenium metabolism are strongly dependent on the form of supplied selenium. The greater mobility of selenate compared to selenite results in differences in the absorption, translocation and metabolism of selenium within the plant. Indeed, when plants are exposed to selenite, selenium accumulation is less than after selenate treatment [START_REF] Souza | Rate-Limiting Steps in Selenium Assimilation and Volatilization by Indian Mustard[END_REF][START_REF] Terry | Selenium in higher plants[END_REF][START_REF] Ximenez-Embun | Establishment of selenium uptake and species distribution in lupine, india mustard and sunflower plants[END_REF][START_REF] Zhang | Uptake and transport of selenite and selenate by soybean seedlings of two genotypes[END_REF], with a greater reduction in biomass production [START_REF] Hopper | Plant avaibility of selenite and selenate as influenced by the competing ions phosphate and sulfate[END_REF][START_REF] Ximenez-Embun | Establishment of selenium uptake and species distribution in lupine, india mustard and sunflower plants[END_REF]. After selenate treatment, selenium is almost entirely translocated to the leaves and weakly metabolized as selenoaminoacids, with a selenate concentration in shoots (i.e. stems and leaves) representing more than 90% of the total shoot selenium [START_REF] Souza | Rate-Limiting Steps in Selenium Assimilation and Volatilization by Indian Mustard[END_REF][START_REF] Hopper | Plant avaibility of selenite and selenate as influenced by the competing ions phosphate and sulfate[END_REF][START_REF] Li | Selenium uptake, translocation and speciation in wheat supplied with selenate or selenite[END_REF][START_REF] Mazej | Selenium species in leaves of chicory, dandelion, lamb's lettuce and parsley[END_REF][START_REF] Terry | Selenium in higher plants[END_REF][START_REF] Ximenez-Embun | Establishment of selenium uptake and species distribution in lupine, india mustard and sunflower plants[END_REF][START_REF] Zayed | Accumulation and volatilization of different chemical species of selenium by plants[END_REF][START_REF] Zhang | Uptake and transport of selenite and selenate by soybean seedlings of two genotypes[END_REF]. In contrast, when supplied as selenite, selenium accumulates principally in roots with little translocation, although selenoamino-acid production (principally selenomethionine, selenocysteine and selenomethylselenocysteine) is greater [START_REF] Souza | Rate-Limiting Steps in Selenium Assimilation and Volatilization by Indian Mustard[END_REF][START_REF] Hopper | Plant avaibility of selenite and selenate as influenced by the competing ions phosphate and sulfate[END_REF][START_REF] Li | Selenium uptake, translocation and speciation in wheat supplied with selenate or selenite[END_REF][START_REF] Liu | Selenium Accumulation in Different Brown Rice Cultivars and Its Distribution in Fractions[END_REF][START_REF] Terry | Selenium in higher plants[END_REF][START_REF] Ximenez-Embun | Establishment of selenium uptake and species distribution in lupine, india mustard and sunflower plants[END_REF][START_REF] Zayed | Accumulation and volatilization of different chemical species of selenium by plants[END_REF] and the selenium volatilization rate is about 2-fold higher from those plants [START_REF] Souza | Rate-Limiting Steps in Selenium Assimilation and Volatilization by Indian Mustard[END_REF].

After ingestion by humans or animals, bioavailable selenium is the fraction that enters the systemic circulation [START_REF] Thiry | Current knowledge in species-related bioavailability of selenium in food[END_REF]. As with other micronutrients, selenium bioavailability strongly depends on the chemical form of the element: organic forms (such as Se-methionine and Se-cysteine), mainly from plant and animal sources, have more bioavailability than inorganic forms (selenate and selenite), which are principally found in dietary mineral supplements. Experimental designs used to measure selenium bioavailability vary widely in the literature, making it difficult to compare the results [START_REF] Knowles | Significance of Amount and Form of Dietary Selenium on Blood, Milk, and Casein Selenium Concentrations in Grazing Cows[END_REF][START_REF] Nicholson | Response of growing cattle to suplementation with organically bound or inorganic sources of selenium or yeast cultures[END_REF][START_REF] Podoll | Dietary selenate versus selenite for cattle, sheep, and horses[END_REF]. According to [START_REF] Thomson | Assessment of requirements for selenium and adequacy of selenium status: a review[END_REF], the apparent absorbed selenium (i.e. the difference between selenium ingested and selenium excreted in feces and urine) in humans was evaluated at about 90% for Se-met and Se-cys versus 50% for Canada, 2000). However, due to a lack of data on the bioavailable fraction across all food products, the recommended daily dietary allowances of selenium for humans are based only on the total selenium concentration, without taking into account the speciation. The two percentages (90% and 50%) estimated by [START_REF] Thomson | Assessment of requirements for selenium and adequacy of selenium status: a review[END_REF] are a "pseudo reference" value used in the present study to evaluate the selenium bioavailability in our so-called "organic" and "inorganic" fractions in Zea mays plants.

Due to the essential function of selenium in staple foods, a number of recent studies on grains and seeds have been carried out not only in wheat, but also in sesame, buckwheat pumpkin and Zea mays [START_REF] Broadley | Selenium biofortification of high-yielding winter wheat (triticum aestivum L.) by liquid or granular Se fetilisation[END_REF][START_REF] Cubadda | Changes in Selenium Speciation Associated with Increasing Tissue Concentrations of Selenium in Wheat Grain[END_REF][START_REF] Kapolna | Fate of selenium species in sesame seeds during simulated bakery process[END_REF][START_REF] Mbagwu | Selenium concentrations in crops grown on low-selenium soils as affected by fly-ash amendment[END_REF][START_REF] Moore | NanoSIMS analysis of arsenic and selenium in cereal grain[END_REF][START_REF] Smrkolj | Selenium uptake and species distribution in selenium-enroched bean (Phaseolus vulgaris L.) seeds obtained by two different cultivations[END_REF][START_REF] Smrkolj | Selenium species determination in selenium-enriched pumpkin (curcurbita pepo L.) seeds by HPLC-UV-HG-AFS[END_REF][START_REF] Stibilj | Enhanced selenium content in buckwheat (Fagopyrum esculentum Moench) and pumpkin (Cucurbita pepo L.) seeds by foliar fertilisation[END_REF]. In Brassica rapa [START_REF] Lyons | Selenium increases seed production in Brassica[END_REF], selenite fertilization increased seed number and weight produced by each plant. Regardless of the enrichment procedures employed in agricultural practice, the development and growth of plants and grains were not affected negatively by selenium supplementation [START_REF] Broadley | Selenium biofortification of high-yielding winter wheat (triticum aestivum L.) by liquid or granular Se fetilisation[END_REF][START_REF] Stibilj | Enhanced selenium content in buckwheat (Fagopyrum esculentum Moench) and pumpkin (Cucurbita pepo L.) seeds by foliar fertilisation[END_REF]. Independently of the selenium concentration added as amendment, grains seem to be an ideal storage tissue, with selenium concentrations higher than in shoots or fruits [START_REF] Cubadda | Changes in Selenium Speciation Associated with Increasing Tissue Concentrations of Selenium in Wheat Grain[END_REF][START_REF] Mbagwu | Selenium concentrations in crops grown on low-selenium soils as affected by fly-ash amendment[END_REF][START_REF] Stibilj | Enhanced selenium content in buckwheat (Fagopyrum esculentum Moench) and pumpkin (Cucurbita pepo L.) seeds by foliar fertilisation[END_REF]. It has previously been shown that the major selenium species in grains is selenomethionine accounting for 45% to 90% of total selenium [START_REF] Cubadda | Changes in Selenium Speciation Associated with Increasing Tissue Concentrations of Selenium in Wheat Grain[END_REF][START_REF] Kapolna | Fate of selenium species in sesame seeds during simulated bakery process[END_REF][START_REF] Smrkolj | Selenium uptake and species distribution in selenium-enroched bean (Phaseolus vulgaris L.) seeds obtained by two different cultivations[END_REF][START_REF] Smrkolj | Selenium species determination in selenium-enriched pumpkin (curcurbita pepo L.) seeds by HPLC-UV-HG-AFS[END_REF], with only very low levels of selenate detected [START_REF] Cubadda | Changes in Selenium Speciation Associated with Increasing Tissue Concentrations of Selenium in Wheat Grain[END_REF][START_REF] Lyons | Selenium distribution in wheat grain, and the effect of postharvest processing on wheat selenium content[END_REF].

In the present study, we investigated selenium enrichment in Zea mays grains grown in a hydroponic system. Cereal grains are rich in phytic acids, known for their antioxidant roles in humans and which strongly bind mineral and trace elements [START_REF] Hurrel | Influence of vegetable protein sources on trace element and mineral bioavailability[END_REF]. Zea mays grains contain more of this compound than wheat grains [START_REF] Egli | Phytic acid degradation in complementary foods using phytase naturally occurring in whole grain cereals[END_REF]. Moreover, Zea mays is the most widely cultivated cereal in the world, producing mainly forage and grains for animal feed but also grains as well as derived products for human consumption. In Malawi, for instance, 50% of the diet is derived from Zea mays [START_REF] Chilimba | Maize grain and soil surveys reveal suboptimal dietary selenium intake is widespread in Malawi[END_REF]. Consequently, the limited data available on selenium accumulation in Zea mays grains has been obtained in specific locations (selenium-deficient [START_REF] Chilimba | Maize grain and soil surveys reveal suboptimal dietary selenium intake is widespread in Malawi[END_REF] or seleniferous areas) or for Se-supplemention, fly-ash for example [START_REF] Mbagwu | Selenium concentrations in crops grown on low-selenium soils as affected by fly-ash amendment[END_REF].

Furthermore, the influence of the chemical form of selenium in Zea mays plants, on accumulation including location (i.e. roots, stems, shoots and grains), has not been widely studied. The first objective of the present study was, therefore, to quantify the effects of those two inorganic chemical forms (selenate and selenite) on Zea mays growth and seed production. The second aim was to investigate the uptake, translocation and speciation of selenium in different Zea mays tissues: roots, stems, leaves and grains.

MATERIALS AND METHODS

Seed germination and culture conditions

Three weeks after germination, Zea mays sups.mays (L.) corn seedlings were cultivated in hydroponic conditions in 20 L plastic tanks filled with a modified Hoagland nutrient solution consisting of KNO 3 (3 mmol.l -1 ), Ca(NO 3 ) 2 .4H 2 0 (2.72 mmol.l -1 ), NH 4 NO 3 (2 mmol.l -1 ), NaCl (0.2 mmol.l -1 ), KH 2 PO 4 (0.98 mmol.l -1 ), MgSO 4 .7H 2 0 (0.70 mmol.l -1 ), (NH 4 ) 6 Mo 7 O 24 .4H 2 O (0.04 µmol.l -1 ), H 3 BO 3 (24 µmol.l -1 ), MnSO 4 (13 µmol.l -1 M), ZnSO 4 (6 µmol.l -1 ), CuSO 4 (1.5 µ mol.l -1 ) and FeEDDHA (6%) (4 µmol.l -1 ). Two nutrient solutions were supplemented with 12 µmol.l -1 selenium as either Na 2 SeO 4 or Na 2 SeO 3 (solutions Se VI -T and Se IV -T), respectively. Under control conditions (C-T), no selenium was added. Five corn seedlings were transplanted in each tank and placed into a RUBIC5 plant growth chamber (Reactor Used for Continental Isotopic Biogeochemistry), a 9 m 3 sealed chamber (Servathin, France) the atmospheric compositions of which are controlled. Lighting was provided by 15x400 watt Philips Son-T Agro bulbs over an 8-hour photoperiod set at 600 µM.m -2 .s -1 photosynthetically active radiation at plant height. Air temperature was set at 25°C during the day and 18°C at night. Air humidity was controlled by a dew point condenser in order to maintain a set-point of 70% relative humidity. Beyond this set point, excess water vapor was condensed and collected using an Isco 3700 water sampler (so called "condensates"). The CO 2 concentration was measured using a LI-COR (Lincoln, Nebraska USA) Li620 infrared gas analyzer set at 400 ppmv. The chamber had a slight positive pressure of +20 Pa to avoid entry of outside air.

Data were logged by a computer and averaged at 10 min intervals.

The change in aerial biomass production was followed by recording the leaf area five times during the experiments.

At maturity, plants (five for each treatment) were harvested and roots briefly rinsed in deionized water to remove traces of nutrient solution. The selenium concentration in this rinse water fell below the detection threshold of CRC-ICP-MS. The leaves, stems, roots and grains were then separated. Plant samples were freeze-dried, ground with an automatic agate mortar, and dry weights (DW) were measured.

Total selenium analysis

A suitable amount of powdered plant tissue (about 100 mg DW) was digested in 2 ml of HNO 3 (70%) at 100°C for 24 hours in a closed digestion vessel. After cooling, 1 ml of H 2 O 2 (30%) was added and the sample was heated again at 100°C for 24 hours. The selenium concentration in the digested tissues samples was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, JY 2000, LOD: 50 µg.l -1 ). A blank and a reference material (White clover, BCR402-IRMM) were included in each batch of samples. In the C-T plants, the selenium concentrations fell below the detection threshold of ICP-AES.

Condensate selenium concentrations were determined by graphite furnace atomic absorption spectrometry (GFAAS, UNICAM 989 QZ, LOD: 1.5 µg.l -1 ). A certified reference material (TMDA-64-Environment Canada) and a blank were included in each batch of samples.

Separation and quantification analysis of selenate and selenite

For enzyme hydrolysis, a DW sample of about 100 mg was digested with 20 mg of Streptomyces protease (Protease Type XIV ≥ 3.5 units/mg solid from Streptomyces griseus, Sigma Aldrich, Saint-Quentin Fallavier, France), dissolved in 5 ml of 30 mmol/l -1 Tris-HCl buffer (pH=7, Rockland) and heated at 37°C for 24 hours under regular agitation (DigiPREP Jr). Samples were then centrifuged at 4000 rpm for 30 min (Eppendorf 5810 centrifuge). The supernatants were filtered and stored at 4°C in 0.1% mercaptoethanol (β-mercaptoethanol molecular biology grade 99.8%, Calbiochem) to avoid oxidation. To determine the efficiency of enzymatic extraction, total selenium concentrations for each sample were determined by ICP-AES (LOD: 10µg.l -1 ). To identify selenate and selenite fractions, the two inorganic forms were separated by HPLC (Dionex, ICS 3000) using a high pressure pump and an anion exchange column (AS15, 4x250 mm) under the following conditions:

-mobile phase: 30 mmol.l -1 KOH -flow rate: isocratic at 1 ml/min -injected sample volume: 200 µl -column temperature: 30°C Standard solutions of the selenium species -Se(IV) (Sodium selenite 99%, Sigma Aldrich, Saint-Quentin Fallavier, France) and Se(VI) (Sodium selenate anhydrous , Sigma Aldrich)were prepared at suitable concentrations. The chromatography system was off-line to GFAAS (LOD: 1.5 µg.l -1 ) used for detection and quantification. The detection limits for each inorganic species were 5 µg.g -1 (i.e. 5% of total selenium) in plant tissues.

Calculation of leaf area

Measuring leaf area is a non-destructive method of monitoring plant growth during experimental studies. Leaf area was calculated using the following formula [START_REF] Fakorede | A comparative study of methods used for estimating leaf area of mays (Zea mays L.) from non destructive measurements[END_REF][START_REF] Ruget | Estimation simple de la surface foliaire de plantes de maïs en croissance[END_REF]:

(L in m: length of leaf; l in m: width of leaf; n: leaf number per plant)

Statistical analysis

In our study, the number of samples was less than 30 (i.e. five plants). Non-parametric tests were used for the statistical analysis. The significance of the effect of treatment conditions was determined with a bilateral Mann-Whitney test (to compare 2 groups) or Kruskal-Wallis test (to compare more than 2 groups), with an alpha risk equal to 0.05. These tests calculated the probability P of the difference between groups being random. P values less than 5% were considered statistically different. In the figures, the results of statistical tests are represented by the letters a, b and c. In this study, values are presented with the median (Q1; Q3).

RESULTS

Biomass production

Plant growth was monitored throughout the experiment by measuring leaf area, internode size and dry weights after harvest and drying.

Before day 20 and after day 75, there was no significant difference in the leaf areas between the three treatments (Figure 1.A). However, between days 30 and 70, the leaf area of Se IV -T plants was on average 42% smaller than for control plants. At the end of experiment, the internode size of each plant was measured (Figure 1.B). For Se IV -T, the internodes No. 3 to No. 9 were on average 2.3 (1.7; 2.5) times shorter than those of control plants. The largest difference was measured for the seventh internode, which was 2.9 times smaller in Se IV -T plants versus controls.

Moreover, again for Se IV -T, the biomass production of plants (Figure 2.A) as well as the biomass production of shoots (data not shown) was about 70% less than C-T plants.

For grains, dry weights decreased by 60% and 80% in Se VI -T and Se IV -T plants, respectively, compared to the control plants (Figure 2.B). The grain number produced by each plant (Figure 2.C) decreased significantly (70%) with selenite. Biomass allocation was affected by selenium; the ratio of grain dry weights to shoot dry weights was less with both selenium treatments compared with C-T. In fact, grain biomass for the C-T plants represented 33% of aerial biomass, but fell to 21% when selenium was present in the nutrient solution.

Uptake and accumulation of total selenium in Zea mays

The total selenium concentration in the plant is the sum of the selenium concentration in each tissue (Figure 3). Supplementation with selenite versus selenate resulted in significant differences in the distribution of selenium in whole plants and between tissues: in whole plants, selenium concentration was 68% higher in Se IV -T versus Se VI -T plants, with selenium concentrations of 210 µg.g -1 (156; 225) and 125 µg.g -1 (103; 126), respectively (Figure 3A).

Similarly but to a greater extent, selenium concentrations in roots were much higher (675%) in Se IV -T versus Se VI -T plants (Figure 3 Regardless of the inorganic form of selenium, the selenium concentrations in stems were similar (Figure 3

.C).

Based on the data for each tissue of Zea mays plants (Figure 3), we calculated the selenium concentration in shoots (stems + leaves) and in tops (stems + leaves + grains). In shoots and tops, selenium concentrations were 72 µg.g -1 (66; 80) and 86 µg.g -1 (86; 88) after selenite treatment, and 151 µg.g -1 (114; 154) and 126 µg.g -1 (105; 130) after selenate treatment, respectively.

Following root uptake, selenium can be redistributed to various degrees in the different plant tissues. The translocation factor 'root-tops' (ratio of tops to roots concentrations) reflects the capacity of selenium to be transferred to roots from aerial tissue. This ratio was lower with selenite (0.13 (0.12; 0.13)) than with selenate (1.45 (1.16; 1.49)). The translocation factor 'shoot-grains' (ratio of grains to shoots concentrations) reflects the capacity of selenium to be transferred to aerial vegetative tissue from grains. This ratio was higher with selenite (2.03 (1.66; 2.09)) than with selenate (0.51 (0.49; 0.53)).

Selenium amount (i.e. quantities in µg per plant or tissues) in tissues not only depends on selenium concentrations but also on the biomass, which can vary considerably from one tissue to another. Therefore, selenium amounts in each tissue provide important information about selenium uptake by the plant. Selenium levels in whole plants were similar for selenite (4278 µg (4031; 5452)) and selenate (4813 µg (3833; 5418)). For selenate-treated plants, tops accounted for more than 90% of the total selenium amount, with around 50% of total selenium found in the leaves (Figure 4). After selenite treatment, selenium amounts in the three tissues differed dramatically: selenium amount in tops was low (about 40% of total plant selenium) whereas around 60% of total plant selenium was found in the roots. Regardless of the form of selenium supplied in the nutrient solution, selenium amount in grains represented 15% of total plant selenium.

Rate of selenium metabolization in Zea mays

Concentrations of inorganic selenium species were determined after protease hydrolysis and are presented in Table 1. The organic selenium fraction (i.e. pool of various organic selenium species) was estimated as the difference between total selenium extracted by protease hydrolysis and the sum of the inorganic species. This estimation is satisfactory for stems, leaves and grains because the efficiency of enzyme hydrolyis is high, approximately 90%. In roots, where the efficiency of enzyme hydrolyis is only about 35%, the fraction of nonextractable selenium corresponds to chemically or physically sequestered organic selenium; the percentage of inorganic selenium species is slightly over-estimated in this case.

After selenite treatment, neither selenate nor selenite was detected in any of the plant tissues.

Conversely, after selenate treatment, no trace of selenite was detected, but selenate was identified in roots, stems and leaves, with a higher percentage in stems and leaves (54 ± 16% and 39 ± 9%, respectively) than in roots (20 ± 5%). Finally, whatever the form of selenium supplied, selenium was converted completely to organo-selenium compounds in grains.

DISCUSSION

Crop growth of Zea mays

Leaf area is used to monitor aerial biomass production throughout plant development.

Changes in leaf area usually follow three successive stages also observed in the three treatments of our experiment: 1) a growing stage where aerial biomass production is exponential, 2) a reproductive stage where foliar development becomes weak or null, and 3) finally a shoots (i.e. stems and leaves) senescence stage when grains are mature [START_REF] Gitelson | Remote estimation of leaf area index and green leaf biomass in maize canopies[END_REF].

Based on the dry weights of plants or tissues and the leaf areas, supplementation with inorganic selenium at high concentration (12 µM) was harmful to Zea mays growth. In selenite-treated plants, the development of all tissues was affected, with a decrease in dry forage biomass as well as quantity (number) and quality (dry weight) of grains. A decrease in the leaf area of plants treated with selenite was observed only during the reproductive stage, i.e. between days 30 and 70. The internodes that were affected by selenite treatment corresponded to those developed during the vegetative stages. Selenite toxicity has already been observed in white lupine and in sunflower, for example, with a biomass reduction of 20% and 40%, respectively (with 12 µmol.l -1 selenite) [START_REF] Ximenez-Embun | Establishment of selenium uptake and species distribution in lupine, india mustard and sunflower plants[END_REF]. These results contrast with data on Brassica rapa [START_REF] Lyons | Selenium increases seed production in Brassica[END_REF], which showed that at very low selenium concentrations (0.05 µ mol.l -1 selenite) in hydroponic solution, plant biomass and dry weight of each grain were not affected, and moreover grain number increased by 43% for each plant. In selenate-treated plants, vegetative tissues of Zea mays were not affected, according to literature data on different varieties of crops (such as Zea mays and wheat) or other plants (such as pumpkin, buckwheat, dry beans) fertilized with different techniques (foliar application of selenate, selenate liquid or solid addition in soils, or fly-ash amendment) [START_REF] Broadley | Selenium biofortification of high-yielding winter wheat (triticum aestivum L.) by liquid or granular Se fetilisation[END_REF][START_REF] Cubadda | Changes in Selenium Speciation Associated with Increasing Tissue Concentrations of Selenium in Wheat Grain[END_REF][START_REF] Mbagwu | Selenium concentrations in crops grown on low-selenium soils as affected by fly-ash amendment[END_REF][START_REF] Smrkolj | Selenium species determination in selenium-enriched pumpkin (curcurbita pepo L.) seeds by HPLC-UV-HG-AFS[END_REF][START_REF] Stibilj | Enhanced selenium content in buckwheat (Fagopyrum esculentum Moench) and pumpkin (Cucurbita pepo L.) seeds by foliar fertilisation[END_REF]. However, contrary to [START_REF] Mbagwu | Selenium concentrations in crops grown on low-selenium soils as affected by fly-ash amendment[END_REF] and [START_REF] Broadley | Selenium biofortification of high-yielding winter wheat (triticum aestivum L.) by liquid or granular Se fetilisation[END_REF], grain biomass decreased in our study when plants were supplied with high selenate concentrations, although there was only a small decrease in the number of grains. Selenate does not appear to influence the quantity of grains but seems to inhibit their normal filling.

Uptake, accumulation and speciation of selenium in Zea mays

To control for possible volatilization of selenium from the plant tissues, which could also result in a decrease in selenium content, condensate samples were collected throughout the experiments. Selenium concentrations measured in those condensate samples (data not shown) indicated that Zea mays does not significantly volatize selenium, which is why this factor is not taken into account in the remainder of the discussion.

Although selenate is the most mobile form of selenium, the total selenium concentration in Zea mays was higher in the presence of selenite (selenium concentration 12µ mol.l -1 ). However, as significant toxicity was manifested as a reduction in biomass production, the accumulated selenium in whole plants was similar with both selenate and selenite treatments.

Ours results differ from a majority of studies concluding that accumulation is higher after supplementation with selenate compared to selenite [START_REF] Souza | Rate-Limiting Steps in Selenium Assimilation and Volatilization by Indian Mustard[END_REF][START_REF] Terry | Selenium in higher plants[END_REF][START_REF] Ximenez-Embun | Establishment of selenium uptake and species distribution in lupine, india mustard and sunflower plants[END_REF][START_REF] Zayed | Accumulation and volatilization of different chemical species of selenium by plants[END_REF]. However, some studies on rice, wheat or soybean show that selenite can accumulate as much as [START_REF] Li | Selenium uptake, translocation and speciation in wheat supplied with selenate or selenite[END_REF][START_REF] Zayed | Accumulation and volatilization of different chemical species of selenium by plants[END_REF] or even more [START_REF] Zhang | Uptake and transport of selenite and selenate by soybean seedlings of two genotypes[END_REF] than selenate. It should be noted that our results cannot be compared directly to any previous data because our experiments were carried out on the mature plant, unlike previous experiments conducted in young plants. Among those studies, [START_REF] Lyons | Selenium increases seed production in Brassica[END_REF] measured total selenium concentrations in roots, shoots and also seeds of Brassica rapa grown in hydroponic conditions, but the sodium selenite concentration in this study was very low (0.05 µmol.l -1 selenium), i.e. 240-fold lower than in our experiments.

With selenite treatment, most of the selenium accumulated in roots. In [START_REF] Li | Selenium uptake, translocation and speciation in wheat supplied with selenate or selenite[END_REF], [START_REF] Terry | Selenium in higher plants[END_REF], [START_REF] Souza | Rate-Limiting Steps in Selenium Assimilation and Volatilization by Indian Mustard[END_REF] and [START_REF] Ximenez-Embun | Establishment of selenium uptake and species distribution in lupine, india mustard and sunflower plants[END_REF], the 'roots-tops' translocation factor (0.5) was greater than that suggested by our result (0.13), but similar to that of Lyons et al. (2009) (0.08). These two very similar results are the only data obtained from experiments carried out up to the reproductive stage in a hydroponic system. The developmental stage of the plant seems to influence the root storage capacity of selenium: root uptake and accumulation appear to increase as the plants mature. Moreover, with selenite treatment, organo-selenium compounds were produced to a greater extent than with selenate treatment: in whole plants, no traces of inorganic selenium were detected. In several papers, traces of selenite were detected in roots or shoots, but always less than 7%, indicating that selenium in plants is overwhelmingly organoselenium compounds [START_REF] Ximenez-Embun | Establishment of selenium uptake and species distribution in lupine, india mustard and sunflower plants[END_REF]. With selenate treatment, most of the selenium taken up by Zea mays was translocated and accumulated in the tops of plants, especially in the leaves; much less accumulated in roots [START_REF] Pickering | Quantitative, chemically specific imaging of selenium transformation in plants[END_REF]. In our study, the 'roots-tops' translocation factor (1.45) was the same order of magnitude as published data (1.5-17) [START_REF] Souza | Rate-Limiting Steps in Selenium Assimilation and Volatilization by Indian Mustard[END_REF][START_REF] Li | Selenium uptake, translocation and speciation in wheat supplied with selenate or selenite[END_REF][START_REF] Terry | Selenium in higher plants[END_REF]. Selenate was metabolized less than selenite in whole plants.

This finding is coherent with the fact that reduction of selenate into selenite is the ratelimiting step in selenate metabolism in plants [START_REF] Souza | Rate-Limiting Steps in Selenium Assimilation and Volatilization by Indian Mustard[END_REF][START_REF] Li | Selenium uptake, translocation and speciation in wheat supplied with selenate or selenite[END_REF][START_REF] Terry | Selenium in higher plants[END_REF]. Selenate absorbed by roots is metabolized to organic selenium compounds (that represent only 20% of total selenium in roots) and/or is quickly translocated to the tops of plants. The percentage of selenate in leaves (39%) is less than in stems (54%), which seems to indicate that selenate is also metabolized in leaves. [START_REF] Mazej | Selenium species in leaves of chicory, dandelion, lamb's lettuce and parsley[END_REF] and [START_REF] Li | Selenium uptake, translocation and speciation in wheat supplied with selenate or selenite[END_REF] showed that on average, 60 to 100% of selenium in leaves and roots is selenate. In the results presented by [START_REF] Ximenez-Embun | Establishment of selenium uptake and species distribution in lupine, india mustard and sunflower plants[END_REF] in India mustard, the selenate form represents 30% in roots and 90% in shoots; moreover in sunflower, selenate in leaves (35%) is similar to that in Zea mays, and is also less than in the stems (97%). Thus, in our study, the metabolization rate, which is higher than in the literature, can probably be attributed to the difference in the developmental stage, which was more advanced in our case. This increased metabolization of selenate in fully developed mature plants could be explained by 1) increased enzymatic generation (increase in the amount synthesized or in the activity rate of enzyme) and/or 2) a decrease in selenate absorption at the reproductive stage involving a larger proportion of selenate metabolized.

In the literature, selenium accumulation in grains has been studied mainly in wheat or rice [START_REF] Broadley | Selenium biofortification of high-yielding winter wheat (triticum aestivum L.) by liquid or granular Se fetilisation[END_REF][START_REF] Cubadda | Changes in Selenium Speciation Associated with Increasing Tissue Concentrations of Selenium in Wheat Grain[END_REF][START_REF] Eurola | Selenium in Finnish foods after beginning the use of selenate supplemented fertilizers[END_REF][START_REF] Lyons | Selenium distribution in wheat grain, and the effect of postharvest processing on wheat selenium content[END_REF], but to date, few data exist for selenium accumulation in Zea mays grains, except in [START_REF] Chilimba | Maize grain and soil surveys reveal suboptimal dietary selenium intake is widespread in Malawi[END_REF] and [START_REF] Mbagwu | Selenium concentrations in crops grown on low-selenium soils as affected by fly-ash amendment[END_REF], for example. Moreover, studies on grains or seeds are usually carried out in soil (pot or yield), naturally or manually enriched with Se-supplementation , but with little information on water-soluble selenium bioavailable for plants [START_REF] Broadley | Selenium biofortification of high-yielding winter wheat (triticum aestivum L.) by liquid or granular Se fetilisation[END_REF][START_REF] Chilimba | Maize grain and soil surveys reveal suboptimal dietary selenium intake is widespread in Malawi[END_REF][START_REF] Cubadda | Changes in Selenium Speciation Associated with Increasing Tissue Concentrations of Selenium in Wheat Grain[END_REF][START_REF] Eurola | Selenium in Finnish foods after beginning the use of selenate supplemented fertilizers[END_REF][START_REF] Kapolna | Fate of selenium species in sesame seeds during simulated bakery process[END_REF][START_REF] Lyons | Selenium distribution in wheat grain, and the effect of postharvest processing on wheat selenium content[END_REF][START_REF] Mbagwu | Selenium concentrations in crops grown on low-selenium soils as affected by fly-ash amendment[END_REF][START_REF] Smrkolj | Selenium uptake and species distribution in selenium-enroched bean (Phaseolus vulgaris L.) seeds obtained by two different cultivations[END_REF][START_REF] Smrkolj | Selenium species determination in selenium-enriched pumpkin (curcurbita pepo L.) seeds by HPLC-UV-HG-AFS[END_REF][START_REF] Stibilj | Enhanced selenium content in buckwheat (Fagopyrum esculentum Moench) and pumpkin (Cucurbita pepo L.) seeds by foliar fertilisation[END_REF]. Thus, our results can only be compared with Zea mays grown in soils or other plant species. In our hydroponic system, selenium concentrations obtained in Zea mays grains (93 -226 µg.g -1 ) were much higher than in the majority of studies [START_REF] Eurola | Selenium in Finnish foods after beginning the use of selenate supplemented fertilizers[END_REF][START_REF] Lyons | Selenium increases seed production in Brassica[END_REF][START_REF] Lyons | Selenium distribution in wheat grain, and the effect of postharvest processing on wheat selenium content[END_REF][START_REF] Mbagwu | Selenium concentrations in crops grown on low-selenium soils as affected by fly-ash amendment[END_REF][START_REF] Smrkolj | Selenium species determination in selenium-enriched pumpkin (curcurbita pepo L.) seeds by HPLC-UV-HG-AFS[END_REF][START_REF] Stibilj | Enhanced selenium content in buckwheat (Fagopyrum esculentum Moench) and pumpkin (Cucurbita pepo L.) seeds by foliar fertilisation[END_REF]. For example, in Zea mays grains in Malawi, selenium concentrations were only 45 to 500 ng.g -1 [START_REF] Chilimba | Maize grain and soil surveys reveal suboptimal dietary selenium intake is widespread in Malawi[END_REF], while winter wheat grains [START_REF] Broadley | Selenium biofortification of high-yielding winter wheat (triticum aestivum L.) by liquid or granular Se fetilisation[END_REF] can accumulate up to 2.6 µg.g -1 . Only grains of wheat harvested in the Nawanshahr-Hosshiarpur region of India had selenium concentrations similar to ours with 29 and 185 µg.g -1 [START_REF] Cubadda | Changes in Selenium Speciation Associated with Increasing Tissue Concentrations of Selenium in Wheat Grain[END_REF]. These differences are probably due to the growing conditions, specifically the soils or hydroponic solution. In our study, the hydroponic experiments allowed us to study the process of uptake in roots and translocation to shoots, which cannot be clearly identified in soil due to its complex composition. Another explanation may be that phytic acid (a chelating compound for trace elements) concentrations are higher in Zea mays grains than, for example, in wheat grains [START_REF] Egli | Phytic acid degradation in complementary foods using phytase naturally occurring in whole grain cereals[END_REF], causing Zea mays to accumulate more selenium. .

Selenium enrichment of Zea mays to improve the quality of human and livestock food

Unlike many studies, we were able to compare both selenate and selenite uptake in Zea mays under the same experimental conditions: variations in the inorganic chemical form of supplied selenium greatly influenced the ability of grains to accumulate selenium.

With selenate treatment, Zea mays grains with the lowest selenium concentrations accumulated less selenium than other Zea mays tissues. However, with granular fertilization of selenate, [START_REF] Gissel-Nielsen | Comparison of selenium treatments of crops in the field[END_REF] found that selenium concentrations in barley grains were equal to or slightly higher than in barley straw. On the other hand, with selenite treatment and according to the literature [START_REF] Cubadda | Changes in Selenium Speciation Associated with Increasing Tissue Concentrations of Selenium in Wheat Grain[END_REF][START_REF] Mbagwu | Selenium concentrations in crops grown on low-selenium soils as affected by fly-ash amendment[END_REF], selenium concentrations are higher in grains than in shoots. In our study, grains contained twice as much selenium as shoots, which is slightly higher than the levels reported by [START_REF] Cubadda | Changes in Selenium Speciation Associated with Increasing Tissue Concentrations of Selenium in Wheat Grain[END_REF] and [START_REF] Mbagwu | Selenium concentrations in crops grown on low-selenium soils as affected by fly-ash amendment[END_REF] (around 1.15 times). Similarly, in field experiments and with different methods of selenium application (granular fertilization and foliar application), selenium concentrations in barley grains treated with selenite were on average 1.6 times higher than in straw [START_REF] Gissel-Nielsen | Comparison of selenium treatments of crops in the field[END_REF]). Thus, with selenite supplementation, grains appear to be a secondary tissue for selenium storage after the roots.

These findings suggest that the use of selenite fertilizer could be attractive because (i) after roots, selenium accumulates principally in grains; (ii) selenite is less mobile than selenate, thereby enriching the soil in selenium at each fertilization, meaning that in the long term, plants grown on this soil will be enriched in selenium without the use of Se-fertilizers; and

(iii) the low mobility of selenite also limits selenium dispersion in the surrounding environment. Moreover, another technique involving foliar application of selenite was found to be more effective than granular fertilization for soybeans [START_REF] Yang | Effect of the application of selenium on selenium content of soybean and its products[END_REF].

Although not confirmed for rice [START_REF] Hu | Determination of selenium concentration in rice and the effect of foliar application of Se-enriched fertiliser or sodium selenite on the selenium content of rice[END_REF], it would be interesting to test this technology with Zea mays.

To improve dietary intake of selenium, the amount of selenium ingested is important but the quality and quantity of bioavailable selenium are also key factors. According to [START_REF] Thomson | Assessment of requirements for selenium and adequacy of selenium status: a review[END_REF], the bioavailability of organic selenium is 90% compared with 50% for selenite or selenate. Based on these data, we calculated selenium bioavailability for humans and animals in Zea mays in our experiment (Table 2).

Despite growth in highly variable conditions (species, concentrations and techniques of selenium supplementation), all previous published results [START_REF] Cubadda | Changes in Selenium Speciation Associated with Increasing Tissue Concentrations of Selenium in Wheat Grain[END_REF][START_REF] Kapolna | Fate of selenium species in sesame seeds during simulated bakery process[END_REF][START_REF] Smrkolj | Selenium uptake and species distribution in selenium-enroched bean (Phaseolus vulgaris L.) seeds obtained by two different cultivations[END_REF][START_REF] Smrkolj | Selenium species determination in selenium-enriched pumpkin (curcurbita pepo L.) seeds by HPLC-UV-HG-AFS[END_REF], as well as the present data, show that selenium in grains is, over-whelmingly, present as organo-selenium compounds. Furthermore, Kalpona et al. (2007), [START_REF] Cubadda | Changes in Selenium Speciation Associated with Increasing Tissue Concentrations of Selenium in Wheat Grain[END_REF] and [START_REF] Smrkolj | Selenium species determination in selenium-enriched pumpkin (curcurbita pepo L.) seeds by HPLC-UV-HG-AFS[END_REF] showed that selenomethionine represents around 80% of the total selenium in grains of sesame, wheat and pumpkin, respectively. Consequently, the evaluation of bioavailable selenium for humans is straightforward because bioavailable organic selenium accounts for 90% of total selenium in grains. In our study, the bioavailable selenium per plant did not differ according to the form of inorganic selenium supplied (Table 2). However, despite a decrease in grain biomass for the selenite treatment at 12 µM, the bioavailable selenium concentration in grains was higher than with selenate. In fields with granular selenium fertilization, selenium concentrations in wheat or barley grains were higher with selenate than with selenite fertilizer [START_REF] Gupta | Effect of selenate vs. selenite forms of selenium increasing the selenium concentration in forages and cereals[END_REF][START_REF] Singh | Selenium content of wheat as affectd by selenate and selenite contained in a Cl-or SO 4 -based MPK fertilizer[END_REF]. This difference is probably due to the fact that, in soil, selenite has lower mobility and so is less bioavailable for plants compared to selenate. However, selenite can enrich soil over the long term and avoid environmental pollution. Our results show that, at equal ratios (i.e equal grain mass), and despite an observed decrease in grain biomass production, the grains treated with selenite supply 73% more bioavailable selenium than those treated with selenate (Table 2). Selenite is, therefore, the best treatment to enrich grains with bioavailable selenium for animals and humans.

All aerial parts of plants (stems + leaves + grains = tops) are used as forage for livestock. To evaluate the bioavailable selenium for animals in our Zea mays plants, it was necessary to take into account not only the selenium amount or concentration, but also the selenium speciation. With selenite treatment, the biomass production of tops decreased, but 95% of the selenium was organo-selenium whereas, with selenate, the biomass production of tops was greater but the selenium was present as both organo-selenium compounds or selenate (less bioavailable). So, based on selenium amount and speciation in tops, we conclude that plants treated with selenate supply 148% more bioavailable selenium per plant compared to those treated with selenite (Table 2). However, at equal ratios (i.e equal tops mass), tops treated with selenite or selenate supply the same amount of bioavailable selenium (Table 2). Thus, since selenate treatment does not affect shoot biomass, it is the best supplement to enrich forage in bioavailable selenium for animals.

CONCLUSIONS

Our data suggest ways to improve agronomic biofortification of Zea mays with selenium.

The absorption, accumulation, distribution and metabolization of selenium in mature Zea mays plants depend on the form of selenium supplied. Despite a decrease in grain biomass in the presence of selenite or selenate in the nutrient solution, selenium is present mainly as organo-selenium compounds in grains. The choice of the form of selenium supplied strongly influences the amount of bioavailable selenium in human and animal foodstuffs: to obtain the highest selenium content for consumers (human or animal), selenate should be used for animal feed and selenite for human food. Because health benefits associated with selenium as well as its toxicity, the creation of dietary recommendations is a key challenge for human and animal health. Nonetheless, data on selenium bioavailability in food are scarce in the literature. Despite our specific experimental conditions (i.e. hydroponic), and although cereals are considered non-accumulators, they do accumulate and metabolize selenium to organoselenium compounds: this study estimated, for the first time, selenium bioavailability in edible parts for human and animals of an important cereal in the diet, Zea mays. Table 1. Selenium species in Zea mays after enzyme hydrolysis.

LIST OF FIGURES

Table 2. Estimated concentration (µg/g) and amount (µg/plant) of selenium bioavailable for animals (tops of plant) or humans (grains of plant) with the two selenium treatments: Se VI -T and Se IV -T. Values are medians (Q1; Q3)

  selenite or selenate supplements (Panel on Dietary Antioxidants and Related Compounds, Subcommittee on Upper Reference Levels of Nutrient, Subcommittee on Interpretation and Uses of DRIs, Standing Committee in the Evaluation of Dietary Reference Intakes of the Food and Nutrition Board, Institute of Medecine & the National Academies and Health

  .B); and selenium concentrations in grains were 1.7 times greater, i.e. 73%, in Se IV -T versus Se VI -T plants (Figure 3.E). Conversely, selenium concentration in leaves was 73% lower in Se IV -T compared to Se VI -T plants (Figure 3.D).

Figure 1 .

 1 Figure 1. Change in leaf area (cm²) (A) and internode length (cm) (B) of Zea mays plants

Figure 2 .

 2 Figure 2. Dry biomass production (%) in Zea mays plants (A) or in grains (B) and number of grains per plant (C) with the three different treatments: C-T (dots), Se VI -T (light gray) and Se IV -T (dark gray).

Figure 3 .

 3 Figure 3. Selenium concentrations (µg/g DW) in whole Zea mays plants (A) or in different

Figure 4 .

 4 Figure 4. Schematic representation of a Zea mays plant showing the selenium amount (%) in roots, stems, leaves, and grains treated with Se VI -T (left) or Se IV -T (right).

Figure 1 .BFigure 3 .Figure 4 .

 134 Figure 1. Change in leaf area (cm²) (A) and internode length (cm) (B) of Zea mays plants

Table 1 .

 1 Selenium species in Zea mays after enzyme hydrolysis. % of Se species after protease hydrolysis b difference between total Se extracted by protease hydrolysis and sum of inorganic species Values are means ± SD

				Se VI -T			Se IV -T
		Efficiency of enzyme hydrolyis		Se fraction (%) a			Se fraction (%) a
		(%)	Selenate Selenite Se-organic b Selenate Selenite Se-organic b
	Roots	36 ± 8	20 ± 5	ND	80	ND	ND	≥ 95
	Stems	89 ± 13	54 ± 16	ND	46	ND	ND	≥ 95
	Leaves	93 ± 6	39 ± 9	ND	61	ND	ND	≥ 95
	Grains	104 ± 7	ND	ND	≥ 95	ND	ND	≥ 95

a

Table 2 .

 2 Estimated concentration (µg/g) and amount (µg/plant) of selenium bioavailable for animals (tops of plant) or humans (grains of plant) with the two selenium treatments: Se VI -T and Se IV -T.

			Se VI -T	Se IV -T
	Grains	µg/g µg/plant	71 a (61; 78) 669 a' (541; 760)	126 b (119; 133) 520 a' (482; 723)
	Tops	µg/g µg/plant	97 a# (84; 100) 3023 a* (2593; 3903)	72 a# (71;82) 1362 b* (1104; 2288)
	Values are medians (Q1; Q3)
	a, b: results of Mann-Whitney test

ABBREVIATIONS USED

CRC-ICP-MS: Collision

/reaction cell -Inductively coupled plasma mass spectrometry; ICP-AES: Inductively coupled plasma atomic emission spectrometry; GFAAS: graphite furnace atomic absorption spectrometry; DW: Dry weight; LOD: Limit of detection; SD: Standard deviation; FeEDDHA: Iron-Ethylenediaminedi-Q-hydroxyphenylacetic acid; IRMM : Institute for Reference Materials and Mesurements