Dynamic Effects of Sodium Selenite on the Rhizospheric Microenvironment, Growth, and Antioxidative Responses of Wheat (Triticum aestivum L.)

Soil selenium (Se) speciation characteristics and their influence on the Se enrichment pattern and physiological characteristics of wheat are poorly understood. Based on the rhizobag experiment, we systematically investigated rhizosphere dynamics, as well as biomass and antioxidant responses, in wheat at five exogenous Se levels (0, 1.0, 2.5, 5.0, and 10.0 mg kg−1 Se in sodium selenite). The results showed that the rhizosphere pH and dissolved organic carbon (DOC) in the soil solution were higher than those in the non-rhizosphere soil solution and that the total and inorganic Se levels in the soil solution increased as the Se application concentration was increased. Meanwhile, in the rhizosphere soil, the concentrations of water-soluble Se (SOL-Se), exchangeable Se (EX-Se), and organically bound Se (OM-Se) significantly increased in response to increases in Se application rates. The ratio of the sum of the three forms of Se to total Se increased by 20.9–56.5%, 19.8–54.6%, and 17.9–53.0% at weeks 4, 6, and 8, respectively. The Se content in both the shoots and roots parts of wheat increased significantly as the Se application concentration was increased. The Se levels in the shoots and roots increased alongside wheat growth in low-level Se (≤2.5 mg kg−1). However, when using high-concentration Se treatments (≥5.0 mg Se kg−1), the trend in these plant parts was for the Se levels to initially increase and then decrease as the wheat grew, with the significant increases of 43-fold and 96-fold at week 6, reaching the highest levels. Under the 5 mg Se kg−1 treatment, the shoot bioaccumulation factor (BCFss) increased by 1.5-fold, 2.0-fold, and 1.6-fold at weeks 4, 6, and 8, respectively. The root bioaccumulation factor (BCFrs) increased with increasing Se concentration. The root-to-shoot translocation factor (TF) tends to increase and then decrease with application concentration increased; all factors had values of less than 1. The TF reached its maximum value at weeks 4 and 6 under 2.5 mg Se kg−1 treatment, while it was highest at week 8 under 5 mg Se kg−1 treatment. When using 5 mg Se kg−1 treatment, the shoot and root biomass of wheat increased by 17% and 22%, and 29% and 32%, respectively, at weeks 6 and 8, timepoints when the highest levels were reached. The application of 5.0 mg Se kg−1 treatment significantly increased the activity of superoxide dismutase (32%, 68%, and 17%) and glutathione peroxidase (34%, 70%, and 43%) in wheat leaves at weeks 4, 6, and 8, while reducing the malondialdehyde content (37%, 46%, and 26%). In summary, applying 5 mg kg−1 of Se to the soil is beneficial for wheat growth. The results of this study reveal the response of wheat to soil-applied Se in terms of wheat growth and physiological characteristics, rhizosphere and non-rhizosphere soil properties, and changes in the morphology of Se.