Selenium (Se) is an essential micronutrient for animals to humans, and it is an important component of selenoproteins, glutathione peroxidase (GPx), and thioredoxin reductase (TrxR), which regulate the cellular oxidative stress response through the redox response system. A total of 25 selenoproteins are identified in human cells, which exhibit various biological activities, such as redox signaling, antioxidant defense, and immune response, and they have important functions in antioxidant, immune regulation, and disease prevention. In addition, Se is involved in cell growth, apoptosis and modification of cell signaling systems and transcription factors ^[1][2]. Se deficiency in the human body can lead to the occurrence of diseases such as Keshan disease, Kaschin-beck disease, and infertility. Given the uneven distribution of selenium resources in the global soil and the Given of selenium-deficient zones, about one billion people are currently facing health risks because of insufficient intake of selenium in their diets, resulting in selenium deficiency. Therefore, scientific and reasonable selenium supplementation through daily diet is essential to maintain human health ^[3][4].

As one of the top three non-alcoholic beverages in the world, tea has received considerable attention from the public for its unique taste and nutritional and health benefits. Studies have shown that long-term tea consumption can reduce serum cholesterol levels and the risk of cardiovascular disease in humans ^[5]. The tea tree (Camellia sinensis (L.) O. Kuntze) has a high-selenium enrichment capacity, converting absorbed inorganic Se into organic Se, only about 8% of Se in tea leaves is inorganic, such as selenite, and the rest of Se is organic such as Se-containing proteins and amino acids. Organic Se is safer and more bioavailable than the inorganic one ^[6]. The exogenous application of Se can increase the biomass of tea trees while promoting the uptake of mineral elements and inhibiting the uptake of harmful elements.

2. Selenium Uptake and Metabolism in Tea Plant

Inorganic Se uptake by plants primarily consists of selenate and selenite, with selenate primarily existing in alkaline-oxidizing environments and selenite primarily existing in acidic and neutral environments. Tea plants can grow and develop in weakly acidic soils, thereby indicating their stronger uptake of selenite ^[7]. The organic matter in the soil can fix Se, and when the pH of the soil changes from acidic to alkalescence, the elemental Se and selenides in the soil will slowly oxidize to selenate and selenite. Moreover, the divalent Se produced by the decomposition of organically bound Se will also oxidize to selenate and selenite, thereby increasing the effective Se content in the soil ^[8]. The roots of tea plants are more efficient at selenite uptake than selenate ^[9]. When selenite is absorbed by the roots of tea plants, it can be accumulated in the roots through phosphate transport proteins and rapidly converted into organoselenium compounds such as selenocysteine (SeCys), selenocystine (SeCys₂), selenomethionine (SeMet), and methylselenocysteine (MeSeCys) before being transported aboveground (Figure 1). The roots of tea trees take up selenate via sulfate transport proteins and transport it rapidly from the xylem to the aboveground. However, selenate is toxic to plants because it is incompletely transformed in the roots ^[10]. Previous studies have hypothesized that higher selenate concentrations in plant cells can reduce plant water potential, resulting in the rapid translocation of selenate to the aboveground. Selenate is converted into organoselenides, such as SeCys, SeMet, and MeSeCys, in the leaves through the sulfate metabolism pathway. These organoselenides have oxidative, anti-inflammatory, and other biological activities, and they are important components of the nutritional quality of tea leaves ^[11]. Tea plant leaves have not evolved a specific structure to absorb Se. Studies have shown that under the conditions of open leaf stomata, the absorption rate of external solutes by plant leaves can be significantly increased ^[12]. However, plants have evolved protective structures such as waxy layers and cuticular layers to protect the fragile leaf tissue; for example, the upper and lower epidermal layers of tea tree leaves are covered with a cuticular layer composed of keratin and wax, which hinders the absorption of Se in the leaves ^[13]. Research shows that plant leaves absorb Se as selenate or selenite. They can also absorb organic Se such as SeCys and SeMet but not insoluble elemental forms of Se (Se0) or Se metal compounds ^[14]. Consequently, most of the inorganic Se absorbed by the leaves is converted to organic Se, primarily in the form of soluble proteins ^[15]. The uptake and metabolism of selenite and selenate in rice (Oryza sativa) and tobacco (Nicotiana tabacum) is similar to that of tea trees. The selenite is absorbed and metabolized in the underground part of the plant and then transferred to the aboveground part. However, selenate is absorbed by roots and then transported to the aboveground part, which is metabolized and converted into organic selenium in leaves. Studies on wheat also showed similar results ^[16][17]. At present, the research on the absorption, metabolism, and regulation mechanism of selenium in tea leaves is still insufficient, which needs to be further explored in future work. However, studies on the uptake and metabolism of Se by tea leaves remain insufficient and need to be further explored at a later stage.

3. Effect of Selenium on Tea Plant Growth

3.1. Effect of Selenium on Tea Plant Biomass

The health functions and pharmacological mechanisms of tea are constantly being explored and utilized by modern science. Tea has become the world’s largest cultivated and most widely consumed beverage crop to improve tea production and promote the early sprouting and expansion of tea buds; thus, the spring tea early market can effectively improve the economic benefits of the market. Investigation shows that exogenous application of Se can significantly increase the yield of tea leaves and promote early germination of tea plants ^[18]. Huang et al. showed that foliar application of Se could effectively increase the 100-bud weight (weight of one 100 growing buds) of tea trees, and the 100-bud weight gradually increased with the spraying dosage when the concentration of exogenous Se was controlled at 30 and 50 mg/kg, and gradually decreased with the spraying dosage at 100 mg/kg. This result indicates that the tea yield can be significantly increased under the appropriate concentration of Se treatment, but the concentration of Se is excessively high to reduce the 100-bud weight of tea plants ^[19]. Wu et al. used the tea variety “Baiye No. 1” as the test material for foliar spraying of nano-Se and found that the yield of fresh tea leaves was significantly improved (Table 1). In particular, when the foliar spraying of nano-Se was applied at a concentration of 13.5 g/hm², the photosynthetic performance and yield of tea trees could be significantly improved. This result could be attributed to the fact that the net photosynthetic rate, transpiration rate, and stomatal conductance of tea trees were enhanced, and the intercellular CO₂ con-centration was reduced at this Se concentration, resulting in the greatest increase in photosynthetic performance ^[20]. Xu et al. also found that Se-containing biologics at a concentration of 100 mg/L and at a rate of 50 g/ha significantly promoted earlier germination of tea plants in early spring and increased the yield of high-grade (Table 1), high-quality tea leaves by a factor of two in early spring ^[21]. Se treatment not only increases the net photosynthetic rate of tea leaves under low-temperature stress to stabilize plant photosynthesis and membrane systems but also improves the cold tolerance of tea plants. Compared to the CK treatment, the Fv/Fm value of tea leaves increased by 10.63%, and the photochemical quenching value increased by 39.45% under 2 mg/mL exogenous selenium treatment ^[22]. Therefore, the appropriate Se concentration not only enables tea trees to germinate earlier but also significantly increases the biomass of tea trees and improves tea production.

3.2. Effect of Selenium on the Uptake of Mineral Elements by Tea Plant

Tea leaves are rich in a variety of mineral elements, and they have not only an important role in the growth and development of tea trees but are also an important expression of the nutritional value of tea. Based on relevant studies, the exogenous application of Se can have different effects on the uptake of mineral elements in tea plants. Foliar spraying of low concentrations (5.0~50.0 mg/L) of exogenous Se (sodium selenite and sodium selenate) can increase the Zn, K, Ca, and Mg content of tea leaves to some extent, and 50.0 mg/L Se treatment has a significant effect on the Fe content of leaves (Table 1) ^[27]. In the case of Se deficiency, the fluorine (F) content in tea leaves and roots increased significantly with the increasing exogenous Se concentration. In the case of Se sufficiency, the Fe, Ca, and Al content in tea leaves increased, the Se and Mg content in leaves and roots increased significantly, and the total F, water-soluble F, and malondialdehyde (MDA) content decreased significantly ^[23][28][29]. There are few reports on the effect of selenium on the absorption of mineral elements in tea plants. The above studies showed that exogenous application of selenium could increase the accumulation of mineral elements such as Zn, Mg, and Fe but inhibit the absorption of F by roots. However, the absorption of other mineral elements has not been reported, so it needs to be verified by subsequent experiments.