Selenium (Se) is naturally occurring in most soils and is typically found in concentrations below 1 mg/kg soil. However, it can be found in high concentrations in alkaline soils with seleniferous (selenium-containing) parent rock. Like other elements, the availability of Se is determined by a number of environmental factors such as pH, redox potential, organic matter content, and others. Selenate, an oxidized form of selenium, is considered the primary bioavailable source of selenium for plants. Alkaline soils often have higher selenate content and Se availability, due to stronger oxidation. Soil selenium concentrations above 1 ppm are typically considered toxic and can cause chlorosis, reduced growth, oxidative stress, and poor crop yields.
Selenium Management
A helpful micronutrient.
Selenium in the Soil
A diagram of the selenium cycle. Image from Wikipedia.
Selenium in the Plant
Higher plants do not have specific transporter proteins for the uptake of Se. Instead, selenate travels through their sulfate transporters. There is evidence that selenite can be transported through phosphate transporter proteins as well. Therefore, the presence of sulfate and phosphate suppresses the uptake of selenium from the soil, and the absence of sulfate and phosphate allows selenium to enter the plant more easily.
Through a reduction process, selenate in the plant can be converted to selenide, which can then be processed into organic forms of selenium such as the amino acids selenocysteine, selenocystathionine, and selenomethionine. While consuming these organic forms of selenium is beneficial for human health, higher plants appear to have lost the ability to synthesize selenoproteins from these amino acids.
Selenium has not been shown to be essential for the growth of higher plants, but there is plenty of evidence to demonstrate that it is beneficial to the health of many plants in low concentrations. Crops grown in selenium-rich soils can have a greater yield, more biomass accumulation, and a better nutritional profile. Fertilization with low levels of selenium may not only promote plant growth, but also increase the concentrations of nutrients and metabolites such as proteins and sugars.
There is evidence that Se defends plants from damage by solar radiation and even promotes plant growth in high light conditions. In a greenhouse study comparing ryegrass and lettuce plants grown under normal light and high light, with and without Se fertilization, the authors found that high levels of UVB light inhibited plant growth and yield. However, low levels of Se counteracted these effects and even promoted plant growth in the presence of strong UVB by increasing the activity of the antioxidant enzyme glutathione peroxidase. Interestingly, Se did not increase any plant growth parameters under normal light conditions, indicating that bright UV light triggers the growth-promoting effects of selenium. There are many plants which accumulate high Se concentrations in their tissues. These Se hyperaccumulator species likely accumulate Se in their tissues to protect themselves from herbivory and increase their antioxidative capacity.
Applications of sodium selenate and sodium selenite have been demonstrated to increase Se content in plant tissues. Some selenium-rich ores and minerals have been used as fertilizers effectively. Both selenate and selenite are more bioavailable when applied foliarly compared to when applied to the soil.
Further Reading
El-Ramady, H., Abdalla, N., Taha, H. S., et al. (2015). Selenium and nano-selenium in plant nutrition. Environmental Chemistry Letters.
Hartikainen, H. and Xue, T. (1999). The promotive effect of selenium on plant-growth as triggered by ultraviolet irradiation. Journal of Environmental Quality.
Liu, Y., Huang, S., Jiang, Z., et al. (2021). Selenium biofortification modulates plant growth, microelements and heavy metal concentrations, selenium uptake,, and accumulation in black-grained wheat. Frontiers in Plant Science.
Sun, X., Wang, Y., Han, G., et al. (2020). Effects of different selenium forms on selenium accumulation, plant growth, and physiological parameters of wild peach. South African Journal of Botany.
Xia, Q., Yang, Z., Shui, Y., et al. (2020). Methods of selenium application differentially modulate plant growth, selenium accumulation and speciation, protein, anthocyanins and concentrations of mineral elements in purple-grained wheat. Frontiers in Plant Science.
