Selenium plants are botanical species distinguished by their ability to absorb and concentrate the trace element selenium from the soil. While not universally essential for plant growth, selenium is necessary in small quantities for many organisms, yet it can become toxic in higher concentrations. These plants influence selenium’s distribution and availability in various environments.
Understanding Selenium and Its Role
Selenium (Se) is a naturally occurring trace element found in soils, water, and some foods. For humans and animals, it is an essential micronutrient, required for proper physiological function. Selenium is a component of selenoproteins, which are crucial for antioxidant defense, immune function, thyroid hormone metabolism, and male reproduction.
While beneficial at lower levels, selenium can cause toxicity in excessive amounts. In humans, chronic exposure to high selenium levels can lead to selenosis, characterized by symptoms such as hair loss, nail changes, skin discoloration, and neurological issues. Animals can also suffer from selenosis, manifesting as hair loss, lameness, and reproductive problems. For plants, selenium is not universally required for growth, but low concentrations can offer benefits like improved growth and increased resistance to environmental stresses. Plants primarily take up selenium from the soil as selenate and selenite.
Categories of Selenium Accumulating Plants
Plants are categorized by their capacity to accumulate selenium. Hyperaccumulators concentrate exceptionally high levels, often exceeding 1,000 milligrams per kilogram of dry matter. These plants, including Astragalus species, Stanleya pinnata, and Neptunia amplexicaulis, typically thrive in selenium-rich soils. Their ability to tolerate and store such high concentrations is linked to unique physiological mechanisms for selenium metabolism and sequestration.
Accumulators can accumulate moderate, yet significant, levels of selenium, generally above 100 mg Se kg–1 dry matter but below the hyperaccumulator threshold. This group includes Brassica species like broccoli and kale, onions, and garlic. These plants do not necessarily require high selenium for their growth but can take it up if available. In contrast, non-accumulators do not concentrate significant amounts of selenium, even in selenium-rich soils, typically maintaining tissue concentrations below 100 mg Se kg–1 dry matter. Most plant species fall into this non-accumulator category and often cannot tolerate elevated selenium levels.
Applications and Considerations of Selenium Plants
Selenium-accumulating plants offer various practical applications in environmental management and nutritional enhancement. Phytoremediation uses hyperaccumulating plants to clean up selenium-contaminated soils and water. These plants absorb and stabilize selenium, which can then be harvested for permanent removal. This method provides an environmentally friendly and often more cost-effective approach compared to traditional cleanup techniques.
Biofortification involves cultivating accumulator plants to enrich food sources with selenium. This strategy addresses dietary selenium deficiencies in human and livestock populations, especially where soil selenium levels are low. By increasing selenium content in staple crops, biofortification improves nutritional quality.
Despite their benefits, selenium plants also pose significant toxicity considerations. Livestock grazing on hyperaccumulating plants, such as Astragalus species (known as locoweeds), can suffer from selenium poisoning. These plants, while often unpalatable, can be consumed when other forage is scarce, leading to conditions like “blind staggers” or “alkali disease” in animals. For humans, consuming plants with extremely high selenium content, particularly from wild sources or contaminated areas, carries health risks. Understanding the origin and type of plant is important when considering consumption, especially for those growing in naturally seleniferous regions.