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Field Crops Research
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Increasing Se concentration in maize grain with soil- or foliar-applied selenite on the Loess Plateau in China*
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Jianwei Wanga b, Zhaohui Wanga b *, Hui Maoa, Hubing Zhaoa, Donglin Huanga
a State Key Laboratory of Crop Stress Biology in Arid Areas (Northwest AF&F University), Yangling 712100, Shaanxi, China
b Key Laboratory of Plant Nutrition and Agri-environment in Northwest China, Ministry of Agriculture (College of Natural Resources and Environment, Northwest AF&F University), Yangling 712100, Shaanxi, China
ARTICLE INFO
Article history:
Received 10 April 2013
Received in revised form 20 June 2013
Accepted 21 June 2013
Keywords:
Soil application Foliar application Grain Recovery
ABSTRACT
Selenium (Se) is an essential mineral nutrient for animal and human growth. Deficiency in this element is a worldwide nutrition problem. Thus, this study determined the potential of increasing Se content in maize grain by using various Se fertiliser application techniques to improve the nutritional status of local residents. Field experiments were conducted on the Loess Plateau for two growing seasons to investigate the effects of different Se fertiliser application methods and application rates on the Se content in maize grain as well as the Se recovery, yield and status of other nutrients in maize grain under rain-fed conditions. Results show that soil and foliar Se applications exhibited no significant effects on maize biomass and grain yield as well as N, P, K, Ca, Mg, Fe, Mn, Cu and Zn contents in maize grain. However, both foliar and soil Se applications significantly improved the Se content in maize grain. Selenium content in maize grain is found to be linearly correlated with Se application rates, increasing from 0.12 |g kg-1 to 0.33 |g kg-1 by soil application at 1 g of Se ha-1 and from 8.23 |g kg-1 to 8.67 |g kg-1 by foliar application at the same rate. Foliar application of Se showed higher Se recoveries in the grain compared with soil Se application: the former exhibited a maximum grain Se recovery rate of 52%o and 106%o in maize during the first and second growing seasons, respectively, whereas the latter was only 1.69% and 0.95%, respectively. On the Loess Plateau in China, both soil and foliar Se applications effectively improved the Se content in maize grain. Compared with soil Se application, foliar Se application can improve the grain Se content in maize at reduced costs.
© 2013 The Authors. Published by Elsevier B.V. All rights reserved.
1. Introduction
Selenium (Se) is an essential element for animals and human beings. This element reportedly prevents liver putrescence in mice (Schwarz and Foltz, 1957). Selenium is found to be an essential component of glutathione peroxidise (Rotruck et al., 1973), which can eliminate free radicals and peroxides in organisms to maintain cell membrane integrity. Selenium is also a component of numerous other enzymes (Berry et al., 1991; Pallud et al., 1997; Ramauge et al., 1996) and proteins (Behne et al., 1988, 1997; Gladyshev et al., 1998; Tamura and Stadtman, 1996; Whanger et al., 1997).
* This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited.
* Corresponding author at: Department of Plant Nutrition, College of Resources and Environment, Northwest Agricultural and Forestry University, Yangling 712100, Shaanxi, China. Tel.: +86 29 8708 2234; fax: +86 29 8708 2234.
E-mail address: w-zhaohui@263.net (Z. Wang).
An adequate dietary intake of Se is necessary to keep humans, livestock and poultry healthy.
Maize is the primary fodder source of livestock and poultry and one of the main food sources in some countries. In 2010, the maize-planted area was 325.18 million hectares, producing 17.75 billion tonnes maize grains in China, preceded only by rice production and second to that in the United States of America worldwide (FAO, 2013). However, low Se content in maize grain affects nutritional quality and cannot meet human, livestock and poultry health requirements. For example, the Se content in maize flour ranged from 16 |igkg-1 to 25 |igkg-1 in Jiangsu Province (Jia, 1999), whereas more than 66% of the maize grain samples were lower than 25 |gkg-1 in Chengdu, Sichuan (Jin et al., 2010). Wang (1982) analysed the Se content in maize grain in China and concluded that the average Se content in 543 maize grain samples was 19.4 |gkg-1, with most of the samples classified as Se-deficient. Therefore, Se deficiency in maize grain is related to human and animal Se deficiency in these areas. Relevant endemic diseases have drawn increasing attention, such as Keshan disease (Peng and Yang, 1991), Kashin-Beck disease (Chen et al., 1980; Ge et al., 1983),
0378-4290/$ - see front matter © 2013 The Authors. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fcr.2013.06.010
Behfet's disease (Delilba et al., 1991) and immune system diseases (McKenzie, 1998).
Numerous experiments showed that a Se fertiliser strategy can effectively improve the Se content in edible crop parts. Selenium fertiliser can be applied using four major techniques: seed dressing, seed soaking, soil application and foliar application. The two latter methods are widely used because of their simplicity and practicability. Addition of Se to growth medium increases the Se status in plants. For example, adding Na2SeO3 in a germinating solution significantly increased the Se content in chickpea sprouts (Zhang et al., 2011), and adding Se in a hydroponic solution significantly increased the Se content in Zea mays (Longchamp et al., 2013). Selenium application to soil, the major growth medium of plants, enhanced the Se content in plants. Selenium application to soil is used widely in Finland to increase the Se content in domestic food and successfully improve the low Se intake of the population (Eurola et al., 2006). In a greenhouse experiment, the Se contents in the upper leaves, roots, stolons and tubers of potatoes increased by Se application to soil (Turakainen, 2007). Similar results were obtained from field experiments in which 20gSeha-1 of soil applied at the jointing stage increased the Se content in wheat grain from 0.03 mg kg-1 in the control samples to 0.39 mgkg-1 (Curtin et al., 2006). The same fertiliser application at sowing increased the Se content in maize grain by 21 |igkg-1 for each gram of Se applied as Na2SeO4 (Chilimba et al., 2012a).
Foliar Se application is another commonly used technique for increasing the Se content in edible parts of crops. Foliar Se application of selenite or selenate solutions significantly promoted the Se content in carrot roots and leaves (Kapolna et al., 2009), garlic bulbs (Poldma et al., 2011), onion bulbs and leaves (Kapolna et al., 2012) as well as radish flowers and leaves (Hladun et al., 2013). In addition to vegetables, the nutritional status of cereal crops, particularly the edible parts was improved by foliar Se application. Selenium content in wheat grain increased from 0.03 mgkg-1 in the control samples to 0.45 mg kg-1 when Se was added at 20 g ha-1 by using foliar spray (Curtin et al., 2006).
Selenite and selenate are the main forms of Se fertilizer. Selenate is more effective than selenite for Se application to soil for the purpose of biofortification (Duma et al., 2011), although it is more easily leached to deep soil by the water from irrigation and rainfall due to its higher mobility (Wang et al., 2010). Furthermore, in edible parts of crops, Se in organic forms is more effective to human and animals than that in inorganic forms (Rider et al., 2010). Selenium in the leek was found with 79% in organic forms when selenite was applied to soil, while it was only 45% when selenate was applied (Lavu et al., 2012). Kapolna et al. (2012) also showed foliar application of selenite enhanced bio-synthesis of organic Se species compared to selenate. On the Loess Plateau in China, more than 65% of rainfall is received between July and September, when are right the maize growing season, and the leaching of soil applied Se
Soil Se rate (g ha"1)
Table 1
Main characteristics of soils from four experimental fields sampled before the experiment.
Soil property Soil field Foliar field
2009 2010 2009 2010
pH(H2O) 8.17 8.12 8.17 8.17
Organic matter (g kg-1) 13.0 14.1 12.9 13.1
Total N(g kg-1) 0.91 0.93 0.87 0.87
Nitrate-N (mgkg-1) 14.7 17.2 3.77 3.71
Ammonium-N (mg kg-1) 6.34 12.2 4.13 4.13
Available P(mgkg-1) 16.5 12.5 6.51 6.63
Available K (mg kg-1) 122 169 99.3 97.3
Total Zn (mgkg-1) 65.3 62.8 66.2 64.3
Available Zn (mg kg-1) 0.80 0.98 0.65 0.60
Total Fe (g kg-1) 2.04 2.04 2.04 2.04
Available Fe (mg kg-1) 5.08 6.39 7.22 5.08
Total Mn (gkg-1) 0.59 0.62 0.60 0.61
Available Mn (mg kg-1) 14.9 23.2 20.0 15.4
Total Cu (mgkg-1) 20.8 21.2 20.8 20.0
Available Cu (mg kg-1) 1.35 1.24 1.58 1.29
Total Se (|g kg-1) 142 100 170 93.0
Available Se (|g kg-1) 2.16 0.98 1.12 0.93
and its subsequently environmental risks should not be neglected. Hence, Se in the form of selenite should be a more rational choice for Se biofortification of maize in this area.
Studies on addressing human Se deficiency by Se fertilisation are rarely reported in the Loess Plateau areas. This study conducted field experiments on the Loess Plateau to investigate (1) the effect of both Se foliar and soil applications in the form of selenite on maize grain yield and Se content; (2) the efficiency of soil and foliar applications; (3) the relationship between Se content in maize grain and Se application rate; and (4) the effect of Se fertiliser on macronu-trients and other micronutrients in maize grain. This study was conducted to provide a more efficient method and determine the proper Se fertiliser application rate for improving the Se content in maize grain and alleviating Se deficiency in humans.
2. Materials and methods
2.1. Experiment location
Two-year field experiments were conducted under rain-fed conditions in four plots in Yangmazhuang Village, Yongshou County (34°49' N 108°11' E, elevation= 1127.76m), Shaanxi Province, China. The annual on-site precipitation is approximately 600 mm, and more than 65% of rainfall is received between July and September.
The soil in the experiment fields is loessal sandy loam. The main characteristics of the soil in each field prior to any treatment application are shown in Table 1.
Foliar Se rate (g ha"1)
Fig. 1. Relationship between the Se content in maize grain and soil (A) and foliar (B) Se application rates.
2.2. Experimental design
The experimental design of soil Se application consists of a randomised complete block with four replications in 2009 and 2010. The plot sizes were 30 m2 and 16.25 m2 in 2009 and 2010, respectively. Treatments included 0, 150, 300, 450 and 600gSeha-1 applied to soil as a Na2SeO3 solution sprayed on the soil surface by a manual sprayer and then incorporated into soil with N and P fertilisers by a harrow. The maize (Z. mays L.) cultivar was identified as Zhengdan 958, sown 3.0 cm deep on April 21, 2009 and April 26, 2010 and thinned to 75,000 plants ha-1 after 15 d of seedling emergence. All plots received 100 kg N ha-1 as urea and 80 kg P ha-1 as triple superphosphate before sowing, and received another 100kgNha-1 as urea at jointing stage. Weeds were controlled in the experiment by using registered herbicides as required and no irrigation during growth.
The experimental design of foliar Se application was also a randomised complete block with four replications in 2009 and 2010. The plot size was 16.25 m2 in 2009 and 20 m2 in 2010. Selenium treatments included 0, 11, 57, 114, 171 and 228 g Se ha-1 in 2009 and 0, 57, 114, 171, 228 and 285gSeha-1 in 2010, applied as a Na2SeO3 solution sprayed twice on the plant surface by a manual sprayer at tasseling and one week after silking, respectively. Maize was sown on April 26, 2009 and April 26, 2010. Maize cultivar, density, N and P fertilisers as well as field management were the same as those in the soil Se application experiments.
2.3. Sampling
Maize was harvested at maturity on October 15, 2009 and October 1, 2010. Maize biomass and grain yield at harvest were measured by harvesting the entire plot through weighing the total straw and maize grain with the cob. After harvest, five maize plants with cob were sampled, maize grains were threshed from the cob, and subsamples of maize straw, grain and cob were washed twice with de-ionised water to remove the attached soil and other contaminants. Samples were then oven-dried at 60 °C to constant weight. The dried samples were ground into powder to pass through a sieve of 0.15 mm for nutrient analysis.
2.4. Chemical analysis
Nitrate-N and ammonium-N in soil samples were extracted with 0.1 M KCl; available P with 0.5 M NaHCO3; K with 1.0 M NH4OAc; Fe, Mn, Cu and Zn with DTPA and Se with 0.1 M KH2PO4. Soil pH was determined with a soil:water ratio of 1:2.5. Nitrate nitrogen, ammonium nitrogen and available P contents in the extract were measured by a continuous flowing analyser; K by a flame photometer; Fe, Mn, Cu and Zn contents by a flame atomic spectrophotometer; and Se by atomic fluorescence spectrometer. Total Fe, Mn, Cu, Zn and Se in soil were determined using the same method as their available forms after digestion by HCl-HNO3-HF (volume ratio 3:1:1). Reliability of the analysis was assessed by including soil reference materials in each soil extract. Measured contents matched the ranges in the standards.
The N content in maize grain was determined by the Kjeldahl method after digestion in a mixture system of highly concentrated sulphuric acid and hydrogen peroxide. Finely ground maize grain (0.5 g) was microwave-digested in pressurised perfluroalkoxy vessels with 5 mL of 70% superior-grade pure nitric acid and 2 mL of 30% hydrogen peroxide. Digested samples were diluted to 25 mL with ultrapure water (18.2 M^ cm). The Se content in the digestion solution was analysed by atomic fluorescence spectrometry, whereas other nutrient contents were analysed by inductively coupled plasma atomic emission spectrometry (Stavridou et al., 2012). Reliability of the analysis was assessed
by including standard reference materials [(durum wheat flour (GBW10011)] from the Institute of Geophysical and Geochemical Exploration (China) in each batch of the plant digests. Values for metal content matched the stated ranges in the standards.
2.5. Statistical analyses
All data were analysed using PROC GLM of the SAS software package (8.1). The homogeneity of the variances was verified, and the data were subjected to ANOVA. Least significant difference values were calculated and used to compare treatment means. Simple correlation coefficients were calculated based on treatment means. The level of significance was 0.05.
3. Results
3.1. Grain yield and biomass of maize
Both soil and foliar Se applications showed no significant effects on maize grain yield and biomass in either year (Table 2). The average maize grain yield and biomass with soil Se application were 5.41 and 9.13 tha-1 in 2009, and 7.93 and 12.25tha-1 in 2010. The grain yield and the biomass in 2010 were higher than those in 2009 because of the increased spikes per hectare, grains per spike and higher grain weight (data not shown).
The average maize grain yield and biomass of the foliar trail were 6.15 and 9.91 tha-1 in 2009, which were also lower than those in 2010 at 9.58 and 17.051 ha-1 for reasons similar to those in soil Se application experiments (Table 2).
3.2. Selenium content in maize grain
Soil Se application increased the Se content and its accumulation in maize grain (Table 3). When the Se rate was increased from 0 to 600, the grain Se content increased by 55-fold (from 3.7 |igkg-1 to 206 |gkg-1) and 118-fold (from 0.6 |gkg-1 to 71.5 |gkg-1) in 2009 and 2010, respectively. The Se accumulation in grain increased by 17-fold (from 59 mg ha-1 to 1073 mg ha-1) and 123fold (from 4.55 mgha-1 to 565 mgha-1) correspondingly, whereas the Se recovery in grain showed no significant change across the Se rates with an average of 1.58% in 2009 and 0.81 % in 2010. Soil Se application can be used to increase the maize grain Se level, with the effects changing significantly from year to year. Correlation analysis showed that 1 g of Se ha-1 increased the grain by 0.33 |g kg-1 in 2009 but only 0.12 |g kg-1 in 2010 (Fig. 1). The grain Se recovery was very low, with an average of 1.19% over the two-year period.
Foliar application of Se also remarkably increased the Se content in maize grain (Table 3). When the foliar-applied Se was increased from 0 to 228, the Se content in grains increased by 168-fold (from 11 |gkg-1 to 1863 |gkg-1) and 329-fold (from 7 |gkg-1 to 2312 |gkg-1) successively during the two-year period. Selenium accumulation in grains correspondingly increased by 168-fold (from 65 mg ha-1 to 10973 mg ha-1) and 373-fold (from 53 mg ha-1 to 19800mgha-1). Selenium recovery in grains showed no significant change across Se rates in 2009, and no significant change was detected between most Se rates in 2010 with averages of 42% and 90%. Foliar Se application showed a significantly greater potential for increasing the Se level in maize grains, with 1 g of Se ha-1 increasing the grains by 8.67 and 8.23 |gkg-1 in 2009 and 2010, respectively. Selenium recovery in maize grains significantly increased but remained very low with an average of 66.10% over the two-year period.
Table 2
Effects of different levels of both soil and foliar Se applications on maize grain yield and biomass.
Method Se rates (g Se ha-1) 2009 Se rates (g Se ha-1 ) 2010
Grain Yield (t ha-1) Biomass (t ha-1) Grain Yield (t ha-1) Biomass (t ha-1 )
Soil application 0 5.82a 9.62a 0 8.41a 14.6a
150 5.31a 8.71a 150 8.07a 12.8a
300 5.43a 9.51a 300 8.57a 13.0a
450 5.67a 9.51a 450 7.18a 11.1a
600 5.24a 8.79a 600 7.91a 12.1a
Foliar application 0 6.53a 10.2a 0 9.98a 18.0a
11 5.45a 8.83a 57 9.96a 17.1a
57 6.37a 10.5a 114 10.0a 17.4a
114 6.36a 10.4a 171 9.75a 17.8a
171 6.26a 9.90a 228 9.23a 16.9a
228 5.91a 9.64a 285 8.55a 15.1a
Values in the same column followed by the same small letter are not significantly different at P = 0.05.
Table 3
Effects of different soil and foliar Se application rates on Se content in maize grain and accumulation and recovery by the grain.
Fertilisation Se rates (g Se ha-1 ) 2009 Se rates (g Se ha-1 ) 2010
Se content (^g kg-1) Grain Se accumulation (mg ha-1) Grain Se recovery (%o) Se content (^gkg-1) Grain Se accumulation (mg ha-1) Grain Se recovery (%o)
Soil application 0 3.7e 59e 0 0.6d 4.55d
150 51d 272d 1.42a 150 12.8cd 104cd 0.66a
300 100c 536c 1.59a 300 40.2bc 290bc 0.95a
450 138b 783b 1.61a 450 52.0b 319b 0.70a
600 206a 1073a 1.69a 600 71.5a 565a 0.93a
Foliar application 0 11e 65c 0 7e 53c
11 96e 513c 27a 57 600d 5952b 103a
57 457d 2889c 40a 114 961c 9606b 84ab
114 754c 4754b 52a 171 1866b 18165a 106a
171 1487b 9361a 43a 228 2224a 20428a 89ab
228 1863a 10973a 48a 285 2312a 19800a 69b
Grain Se accumulation (mg ha-1 ) = grain Se concentration (|xgkg-1) x grain yield (gha-1). Grain Se recovery (%>>) = (grain Se accumulation in the Se treatment (mgha-1) -grain Se accumulation in the control (mgha-1))/rate of applied Se (gha-1) x 1000. Values in the same column followed by the same small letterare not significantly different at P=0.05.
3.3. Micromineral contents in maize grain
Both soil and foliar Se applications showed no significant effects on Fe, Mn, Cu and Zn contents in maize grain in both years (Table 4). The average Fe, Mn, Cu and Zn contents in maize grain across the soil-applied Se rates were 28.5,4.51, 2.10 and 17.1 mg kg-1 in 2009, respectively, and 37.7, 4.23, 2.30 and 22.8 mg kg-1 in 2010, respectively. The average grain Fe, Mn, Cu and Zn contents across the foliar-applied Se rates were 24.1, 3.13, 1.66 and 15.5 mgkg-1 in 2009, respectively, and 34.2, 3.89, 2.25 and 18.2mgkg-1 in 2010, respectively.
3.4. Macromineral contents in maize grain
Similar to micronutrients, macronutrients in maize grain were not significantly affected by soil- or foliar-applied Se in most cases. However, grain K was significantly decreased by 150 g ha-1 of soil-applied Se, whereas N increased by 300gha-1 of soil-applied Se in 2010 (Table 5). For soil Se application, the average N contents across the Se-applied treatments were 16.2 and 14.8 g kg-1 in 2009 and 2010, respectively. The corresponding averages were 2.13 and 2.68gkg-1 for P, 2.86 and 3.51 gkg-1 for K, 0.15 and 0.17gkg-1 for Ca and 0.97 and 1.01 gkg-1 for Mg. For foliar Se application,
Table 4
Effects of soil and foliar Se applications on Fe, Mn, Cu and Zn contents in maize grain.
Fertilisation Se rates (gSe ha-1) Nutrient contents (mg kg-1) in 2009 Se rates (gSe ha-1) Nutrient contents (mgkg-1) in 2010
Fe Mn Cu Zn Fe Mn Cu Zn
Soil application 0 33.2a 4.61a 2.17a 16.7a 0 40.0a 4.57a 2.64a 24.5a
150 30.0a 4.46a 2.01a 16.0a 150 40.8a 4.39a 2.34a 23.1a
300 27.9a 4.28a 2.04a 18.5a 300 39.2a 4.02a 2.17a 21.4a
450 25.5a 4.62a 2.14a 16.9a 450 33.4a 4.19a 2.15a 24.2a
600 26.0a 4.60a 2.16a 17.2a 600 35.2a 3.98a 2.19a 20.6a
Foliar application 0 24.1a 3.07a 1.65a 14.7a 0 44.7a 3.88a 2.14a 17.4a
11 21.8a 3.18a 1.81a 16.5a 57 23.9a 5.10a 2.17a 17.5a
57 21.7a 2.99a 1.62a 14.4a 114 26.1a 3.37a 2.13a 17.4a
114 27.3a 3.14a 1.58a 14.5a 171 39.1a 3.41a 2.20a 18.2a
171 25.4a 3.20a 1.68a 16.8a 228 30.2a 3.75a 2.41a 18.9a
228 24.5a 3.19a 1.61a 16.2a 285 41.2a 3.82a 2.45a 19.8a
Values in the same column followed by the same small letter are not significantly different at P= 0.05.
Table 5
Effects of soil and foliar Se applications on N, P, K, Ca and Mg contents in maize grain.
Method Se rates (g Se ha-1) Nutrient contents (g kg-1) in 2009 Se rates (g Se ha-1 ) Nutrient contents (gkg-1) in 2010
N P K Ca Mg N P K Ca Mg
Soil application 0 15.8a 2.19a 2.77ab 0.14a 0.99a 0 14.2b 2.68a 3.44a 0.19a 1.12a
150 16.8a 1.80b 2.60b 0.13a 0.96a 150 15.1ab 2.59a 3.53a 0.15a 1.03a
300 16.0a 2.06b 2.81ab 0.16a 0.91a 300 15.3a 2.74a 3.55a 0.20a 0.96a
450 16.2a 2.24a 3.01a 0.16a 0.99a 450 14.8ab 2.74a 3.56a 0.12a 0.99a
600 16.2a 2.35a 3.10a 0.17a 0.99a 600 14.8ab 2.63a 3.49a 0.17a 0.96a
Foliar application 0 16.6a 2.66a 3.20a 0.11a 0.81a 0 14.1a 2.05a 3.19a 0.12a 0.90a
11 16.3a 3.08a 3.26a 0.12a 0.97a 57 13.4a 2.06a 3.79a 0.18a 1.01a
57 15.7a 2.69a 3.05a 0.10a 0.79a 114 13.8a 1.92a 3.10a 0.11a 0.94a
114 16.5a 2.53a 3.05a 0.09a 0.87a 171 13.7a 2.01a 3.49a 0.18a 0.97a
171 16.0a 3.39a 3.79a 0.14a 0.90a 228 13.6a 2.05a 3.47a 0.16a 0.98a
228 16.9a 3.35a 3.81a 0.13a 0.87a 285 13.7a 2.17a 3.87a 0.13a 1.04a
Values in the same column followed by the same small letter are not significantly different at P = 0.05.
the average contents across the Se-applied treatments were as follows: 16.3 and 13.7gkg-1 for N; 2.95 and 2.04gkg-1 for P; 3.36 and 3.49 g kg-1 for K; 0.12 and 0.15 g kg-1 for Ca; and 0.87 and 0.97 g kg-1 for Mg in 2009 and 2010, respectively. Compared with the corresponding control, neither soil nor foliar Se applications affected the micronutrient (N, P, K, Ca and Mg) contents.
4. Discussion
4.1. No significant effect of Se fertiliser on maize grain yield
No significant effect of Se fertiliser application on maize grain yield was observed in our experiments over the two-year period. This result is consistent with that in hydroponic experiments, which showed that Se addition to the nutrient solution did not affect the biomass production of maize seedlings (Longchamp et al., 2012) and lettuce (Duma et al., 2011). Similar results in a greenhouse study showed that selenite-pelleted seeds at rates up to 60gSeha-1 exerted no influence on the yield of shoots and roots of three ryegrass cultivars (Cartes et al., 2011). Several field experiments confirmed this result, and no significant effects of Se application were observed on the yield and harvest index in winter wheat (Broadley et al., 2010), garlic bulb yield (Poldma et al., 2011) and maize grain or stover yield (Chilimba et al., 2012a).
However, several studies showed that Se application positively affected the plant. In a pot experiment, the Se-treated potato plants produced higher tuber yields than did the control plants, which was related to its antioxidative effect in delaying senescence (Turakainen, 2007). Similarly, in a hydroponic experiment, Se treatment was associated with a 43% increase in Brassica seed production (Lyons et al., 2009), which was attributed to higher total respiratory activity in leaves and flowers (Lyons et al., 2009). In a chicory experiment, Se also increased the respiratory potential in young chicory (Germ et al., 2007). Addition of Se significantly enhanced the antioxidant activity, antioxidant level and corn grain yield when drought stress level was increased (Sajedi et al., 2011). Thus, Se exerts positive effects on plants by increasing the antioxi-dant activity and respiratory potential in plants.
4.1.1. Effect ofSe on plant growth is related to its application rates and crop variety
The effect of Se on plant growth depends on rate and crop variety. For example, the total dry matter of plants was higher in Se-sprayed plants than that in controls for cultivar Monivip. However, no significant differences in plant growth were observed between the sprayed and the control plants for cultivar Monivip (Germ et al., 2007). Similarly, selenate applied at a low rate (1 mg Se kg-1) stimulated plant growth in the Prunus rootstock variety GF 677, whereas
no significant change was observed in the biomass of Prunus root-stock variety Mr.S.2/5 (Pezzarossa et al., 2009).
4.1.2. Excessive application inhibits plant growth
With dual effects on plants, the proper rate of Se application stimulated plant growth, whereas excessive Se application inhabited plant growth. In a germination experiment, a significant decrease in the weight of chickpea sprout was observed when Na2SeO3 content exceeded 50 mgL-1 in the germinating solution, due to decreased mean frequency of germination, extended germination time, shortened root and shoot length of the seedling and increased rotten seeds, compared with the control sample germinated in tap water (Zhang et al., 2011). Similarly, in a pot experiment, Se added at a rate of 2.5 and 5.0mgSekg-1 were found highly toxic for the two young peach rootstocks, resulting in reduced plant growth and a high mortality rate (Pezzarossa et al., 2009). Similar results were observed in crop plants. For example, total biomass of all wheat genotypes decreased on the seventh day because of Se supply (Inostroza-Blancheteau et al., 2012). Thus, managing the Se application rate is important to avoid toxic effects on plants.
4.2. Enhanced Se nutritional status in maize grain by Se application
Regardless of the applied method, Se application increased the Se content in maize grain. Other studies obtained similar results. For example, the Se contents in seeds, tops and roots of Brassica were higher in the Se-supplemented plants (Lyons et al., 2009), whereas the Se content in the chicory head was about twice [43 ngSeg-1 dry matter (DM) to 46 ng Se g-1 DM] as much as that in the control (21 ngSeg-1 DM to 24ngSeg-1 DM) (Germ et al., 2007). Selenium application increased the Se status of plants. However, many factors affect the increase in Se efficiency in plants. These factors include the time, rate, method of Se application, Se form, crop spices and variety as well as soil and climatic conditions (Rengel et al., 1999).
4.2.1. Increase in grain Se depends on time of application and rate
The time of application showed a highly significant effect on the increase in grain Se. Soil Se application at sowing was less effective than at joining stage in increasing grain Se, particularly for wheat (Curtin et al., 2006). Grain Se contents in spring wheat were higher following Se application after tillering than after sowing (Govasmark et al., 2008). In addition, Se contents in maize grains at all six sites were higher following late Se application compared with early Se applications, with an overall difference of 33% (Chilimba et al., 2012a). Thus, Se applied at an appropriate time increases the efficiency of enhancing Se status in plants. The recommended time of Se application is the late stage of plant growth.
These experiments indicated that the Se content in maize grain was linearly correlated with the Se application rate. This finding is consistent with the results of several studies, including hydroponic, soil pot and field experiments. Two pot experiments conducted in greenhouse found the Se content in the green parts of wheat and oilseed rape was dependent on the selenium dose applied (Kaklewski et al., 2008). The Se content in roots, twigs and leaves of Prunus (Pezzarossa et al., 2009) increased with increasing amounts of Se added to the soil. In the hydroponic experiment, the Se content in lettuce was correlated with the Se dose given to plants (Duma et al., 2011). A greenhouse study also showed that Se content in ryegrass shoots and roots steadily improved because of increased Se supply (Cartes et al., 2011). In the field experiment, the increase in the Se content in maize grain was approximately linear for all Se application rates (R2>0.90 for 27 of 30 experimental units) (Chilimba et al., 2012a), as well as that in wheat grain (Broadley et al., 2010).
4.2.2. Increase in grain Se depends on crop species and variety
The amount of increase in Se content in crop caused by Se application depends on the crop variety. In a pot experiment, Kaklewski et al. (2008) found that at reduced H2SeO3 doses applied to soil (0.05 mM kg-1), the Se content in wheat plant was higher than that in rape, whereas at increased H2SeO3 doses applied to soil (0.15 and 0.45 mMkg-1), a different pattern showed that Se content in rape was higher than that in wheat. In a hydroponic experiment, similar results showed that Se accumulation depends on the plant and its variety. The lettuce variety 'Riga' accumulated higher Se amounts than did the lettuce variety 'Tarzan', whereas the average Se content in leaf lettuce exceeded iceberg lettuce by 2.2 times (Duma et al., 2011). A wheat hydroponic experiment similarly showed that wheat varieties Tinto and Kumpa accumulated lower Se amounts than did Puelche (Inostroza-Blancheteau et al., 2012). By contrast, field experiments revealed that the cultivar effects on wheat grain Se and maize grain or stover Se were either small or not significant (Chilimba et al., 2012a; Curtin et al., 2006). Thus, Se accumulation in plants differed among varieties and species.
4.2.3. Increase in grain Se depends on soil and climatic conditions
Climatic conditions affected the Se status of plants, with water
condition being one of the major factors. The grain Se levels of two wheat varieties following Se application at sowing were higher at the irrigated site than their corresponding treatments at the dryland site (Curtin et al., 2006). By contrast, Zhao et al. (2007) found that irrigation significantly decreased grain Se contents by 30% to 75%. Soil properties, especially soil type, also affected the increase in plant Se. In the experiment conducted by Stephen et al. (1989), increases in plant Se contents caused by Se application were 32% greater in silt loam than in clay loam.
4.2.4. Increase in grain Se depends on the application method and the form ofSe
Both soil and foliar Se applications effectively enhance the Se content in maize grain. However, the effects differed; foliar Se application caused a higher increase in the Se content in maize grain. The results of our study indicates that to obtain the recommended level of 100 |igkg-1, 301 gSeha-1 in 2009 and 835gSeha-1 in 2010 were needed for soil application, whereas only 1 and 14 g Se ha-1 were needed for foliar application in 2009 and 2010, respectively. Similar results were obtained in the experiment conducted by Curtin et al. (2006), in which significantly higher Se contents in wheat grain were detected with foliar Se application. Reduced efficiency to increase the Se status in plants by soil Se application is attributed to the lower plant availability of the applied Se in soil. A field study verified this result, indicating
that the plant availability of applied Se can decrease rapidly in soil (Curtin et al., 2006).
Although soil Se application increased the Se content in maize grain, a higher Se application rate is needed. The recovery of grain Se in maize was at most 1.69%o only. This result indicates that most soil-applied Se was not assimilated by maize plants and instead remained in soil. The residual Se with small residual effect on the next crop (Chilimba et al., 2012b; Gupta et al., 1993) leached into the deeper soil with the rainwater or flowed with the ground runoff, causing environmental pollution. Compared with soil Se application, foliar Se application requires lower Se application rates. The grain Se recovery was higher, achieving the highest value of 106%. This result was consistent with those of the wheat and maize experiments, which showed that grain Se recovery in wheat at foliar application rates of 10 and 20 g Se ha-1 as selenate were only 80.5% and 96.5% in experiment 1 but reached 124.2% and 122.1% in experiment 3 (Stroud et al., 2010). By contrast, the Se recovery in maize grain applied in exogenous forms averaged 84.4% and 85.0% for the 2008 to 2009 and 2009 to 2010 periods, respectively (Chilimba et al., 2012a). This result indicated that increased foliar-applied Se was assimilated by maize plants. Thus, foliar Se application can more effectively and more economically enhance Se content in maize grain. This finding is consistent with the results of a study regarding Se application on maize (Curtin et al., 2006) and Zn application on maize and wheat (Wang et al., 2012; Zhang et al., 2012).
Two forms ofSe are widely used, viz., selenite and selenate. Most research of plant Se enrichment with Se fertilizers concluded that selenate was more available to plant than selenite when applied to soil (Cartes et al., 2005). In our previous study at the same site as the present study, soil application of selenate elevated the Se concentration in potato tubers from 20 |gkg-1 to 1510-2150 |gkg-1, and in cabbage leaf from 90 |gkg-1 to 13030-19440 |gkg-1, showing 70-100-folds and 130-200-folds increment of Se concentration (Wang et al., 2012). This indicated that selenate application to soil caused much higher concentration and more increment of Se in plants than selenite. To the contrary, the result from a hydroponic experiment showed that Se accumulation in maize following selenite treatment was always higher than that with selenate (Longchamp et al., 2013). This inconsistence showed that the properties of the growth medium has significant influence on Se absorption of plant. The Se form not only affect the Se concentration in plant, but also affect the Se distribution in plant. For example, Longchamp et al. (2013) showed that the maize applied with selenate accumulated more Se in shoot, while that applied with selenite accumulated more Se in root. The plants (onion and leek) with selenite application accumulated more portion of organic Se than that with selenate application (Kapolna et al., 2012; Lavuet al., 2012).
4.3. Effects ofSe application on other nutrients in maize
In the present study, the status of nutrients other than Se in maize grain was not affected by either soil or foliar Se applications. By contrast, an experiment conducted over a two-year period by Poldma et al. (2011) indicated that foliar application at 50gha-1 significantly decreased the S, K and Ca contents in garlic, whereas low Se application rate (not more than 25 g Se ha-1) exerted no significant decreases in garlic. This finding suggested that the effect of foliar Se application on S, K and Ca contents in garlic is related to the Se rate.
4.4. Importance of monitoring maize grain after Se application
The increase in Se contents in maize grain caused by soil Se application varied remarkably in different years. For example, 1 g Se ha-1
soil application increased the Se content by 0.33 |igkg-1 in 2009, which is 2.7-fold of that in 2010. Thus, the increase in Se content in maize grain changed dramatically after soil Se application in different years. Monitoring the Se content in maize grain before use is necessary to ensure safety when Se fertiliser is applied to soil.
Compared with soil Se application, the increase in Se contents in maize grain caused by foliar Se application varied slightly in different years. As observed in this experiment, 1 g Se ha-1 increased the grain Se content by 8.23 and 8.67 |gkg-1 in 2009 and 2010, respectively, during foliar application. Despite the narrow range of Se content in maize after foliar Se application, Se content in maize grain must be detected prior to human and animal consumption to guarantee the safety of food made from biofortified maize grain.
5. Conclusion
Both soil and foliar Se applications can reliably and effectively increase the Se content in maize grain without negatively affecting the N, P, K, Ca, Mg, Fe, Mn, Cu and Zn contents. However, foliar application is more cost-efficient and effective than soil application. Monitoring of Se content in maize grain prior to human and animal consumption is necessary.
Acknowledgements
This study was supported by the National Key Basic Research Special Funds (2009CB118604), the 111 Project of Chinese Education Ministry (B12007), the National Natural Science Foundation of China (30871596) and the Innovative Research Team Program of Northwest A&F University.
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