Difference of selenium uptake and distribution in the plant and selenium form in the grains of rice with foliar spray of selenite or selenate at different stages
Introduction
Selenium (Se) is an essential micronutrient in human body and a non-essential element for plants. Although the human demand for Se is not high, the abundance of Se is very low in the Earth’s crust. Thus, Se deficiency is still recognized as a global health problem that needs to be addressed (Valdiglesias et al., 2010). Increasing the production of Se-enriched dietary sources is necessary, because Se supplementation from natural food sources is considered safer than directly ingesting inorganic Se (Liu et al., 2012). Rice is a staple food in at least 33 countries and it provides ∼80% of the daily caloric intake to 3 billion people (Boldrin et al., 2013). The average Se concentration of rice is only 95 μg kg−1 in the major rice production areas of the world (Williams et al., 2009). Therefore, increasing Se concentrations in rice by application of Se fertilizer has great implications for improving Se nutrition in human beings (Giacosa et al., 2014, Hu et al., 2014).
Selenite and selenate are the two major types of exogenous Se fertilizer. When applied into the soil, selenite is less bioavailable due to adsorption onto ferric soil minerals or accumulation in plant tissue that is not part of human diet (Carey et al., 2012a, Keskinen et al., 2011, Li et al., 2015b). More than 80% of the Se applied as selenite exists in the forms of Fe-Mn oxide-bound Se, organic bound Se, and residual Se within a month, which are difficult for uptake and utilization by crops in the current season (Liu et al., 2015). Soil application of selenate is potentially a wasteful method of Se biofortification, as 80–95% of the Se added as selenate may be leached out by irrigation or rainfall (Keskinen et al., 2011). In addition, the antagonistic effect between sulfur and Se in soil can significantly reduce Se concentrations in crops (Liu et al., 2015, Liu et al., 2017). In particular, rice grows in flooded and anaerobic conditions for a long term, which could accelerate selenite fixation and selenate leaching. Therefore, soil application of Se fertilizer is considered disadvantageous for increasing Se concentrations in crops.
Numerous studies have shown that foliar spray of Se significantly improves Se concentrations in crops such as rice, wheat, maize, lentil, and table grape (Boldrin et al., 2013, Nawaz et al., 2015a, Rahman et al., 2015, Wang et al., 2013a, Zhu et al., 2017). However, the choice of the Se form supplied to plants strongly influences the amount of bioavailable Se in human and animal foodstuffs (Longchamp et al., 2015). For instance, there exists a huge difference in the uptake of selenite and selenate by crop roots. Selenate can migrate into the roots and immediately translocate to the shoots via high-affinity sulfate transporters, whereas selenite is mostly assimilated into organic Se in the roots and unlikely to translocate to the shoots (Sors et al., 2005). What are the differences in the translocation and distribution of Se in various parts of crops with foliar spray compared to soil application of Se? Do these differences vary with the spraying stage or not, and what are the differences in the results of foliar spray with different Se forms, such as selenite or selenate? A systematic report regarding these questions is still lacking.
The bioavailability of Se for humans and animals largely depends on the forms of Se in the edible parts rather than the total Se concentration in the plant (Premarathna et al., 2012), because the anti-cancer effects of selenium depend on its speciation (Carey et al., 2012b). Compared with inorganic Se, organic Se is more bioavailable for human health (Longchamp et al., 2015). Organic Se generally constitutes more than 80% of the total Se in crop grains (Eiche et al., 2015). Protein Se is the major form of organic Se in crops. It has been reported that organic Se accounts for more than 90% of the total Se in wheat grains, while protein Se accounts for more than 70% of the total Se (Cubadda et al., 2010). In potato tubers, 49–65% of the Se is present in protein components (Turakainen et al., 2006). The above results of Se analysis are mainly obtained from crops grown in Se-rich soils. However, the process of Se metabolism and assimilation by the roots differs from that by the leaves. Additionally, after Se uptake by the roots, there is sufficient time to assimilate inorganic Se into organic Se within the growth period of crops (Premarathna et al., 2012). Then, are there any differences in the assimilative capacity of plants for Se with foliar spray compared to soil application of Se? What are the differences in the assimilative capacity of plants for Se after foliar spray with different Se sources at different stages? Addressing these two questions is of great importance for safe production of Se-enriched rice.
Assuming foliar spray of Se is an effective approach to increase Se concentrations in crops and mitigate the environmental risks, then, elucidating the effect of foliar spray with different Se sources at different stages on the recovery efficiency of Se in rice is critical for commercial operations of Se-enriched rice in implementing efficient production. To this end, we sprayed selenite or selenate to the foliage of rice plants at the late tillering or full heading stage in the actual field conditions. The objectives were to determine the effects of time of foliar application and form of Se on 1) Se accumulation and distribution in rice plants; 2) organic and protein Se concentrations in brown rice; and 3) Se recovery efficiencies in the whole plant and brown rice.
Section snippets
Experimental site
A replicated field experiment was conducted for two seasons (2015 and 2016) in the Shekou village of Qianjiang, Hubei Province, China (30°33′17′′ N, 112°53′23′′ E). The annual precipitation was ∼1113 mm. The soil was fluvo-aquic soil with the following properties: pH, 8.19 (soil/water ratio = 1:2.5); organic matter, 28.39 g kg−1; available nitrogen, 91.07 mg kg−1; Olsen phosphorous, 6.14 mg kg−1; available potassium, 83.00 mg kg−1; and total Se, 0.35 mg kg−1. The seeds of rice (Oryza sativa L.) variety
Grain yield and total biomass
The average grain yield and total biomass of rice with no Se application were 6.8 and 16.6 t ha−1 in 2015, and 7.4 and 18.4 t ha−1 in 2016, respectively. Foliar spray of selenite or selenate at either growth stage exerted a significantly positive effect on grain yield and total biomass compared to CK. The resultant increases in average grain yield and total biomass were 5.1% and 4.4%, respectively. No significant difference was found between LT and FH treatments in terms of grain yield and total
Discussion
Whether the yield-increasing effect of Se on crops is significant or not is strongly influenced by the experimental conditions and management methods. Zhang et al. (2014) reported that soil application of 50 g Se ha−1 as selenite significantly improved grain yield in rice. Boldrin et al. (2013) observed no significant effects of soil application of selenite or selenate on grain yield in rice; rather, foliar spray of either Se source markedly improved grain yield in rice. In the present study,
Conclusions
Se source and spray stage significantly influenced the recovery efficiency of Se in the plant. The foliage of plants showed a significantly better absorption efficiency of selenate than selenite. An appropriate delay of spraying stage for Se application will facilitate the uptake and accumulation of Se in the edible portions of plants. It is worth noting that the spray of the two Se sources contributes to a relatively high conversion of Se to organic and protein forms in rice grains.
Acknowledgments
This research was supported by the Fundamental Research Funds for the Central Universities (2662016QD015), and the Rich Selenium Fertilization System of the Main Staple Crop in Jianghan Plain and Key Technology Research of Rich Selenium Product Processing (XKJ201501-21).
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