Effects of various forms of selenium biofortification on photosynthesis, secondary metabolites, quality, and lignin deposition in alfalfa (Medicago sativa L.)

This study investigated the effects of various selenium(Se) sources on the lignin deposition and quality attributes of alfalfa and their application methods. A field experiment conducted in 2020 and 2021 was divided into eight treatments. At the seedling stage, four root irrigation treatments using 2.5, 50, and 5 µMol/L of sodium selenite (S-RI), selenomethionine (SS-RI), Se nanoparticles (SSS-RI), and deionized water (CK-RI) was performed. At the branching stage, four treatments of foliar spraying with the above three Se fertilizers at a concentration of 0.5 mMol/L (S-FS, SS-FS, and SSS-FS) and deionized water (CK-FS) was applied. The net photosynthetic rate (Pn), photochemical quenching (qP), and nonphotochemical quenching (NPQ) of the upper-layer leaves of alfalfa treated with Se root irrigation and foliar spray increased by 1.81–23.65%, 2.50–7.69%, 16.95–71.88%, 7.15–22.08%, 13.40–88.89%, and 29.63–81.48%, respectively, compared with the control during the 2 years. The lower-layer leaves increased by 1.28–47.51%, 2.86–20.00%, 10.00–23.33%; 4.77–34.38%, 5.00–172.73%, and 3.45–110.00%, respectively, compared with the control. In alfalfa, Se treatment increased the total flavonoid, anthocyanin, soluble sugar, and reducing sugar contents and phenylalanine ammonia-lyase (PAL) activity; however, it decreased caffeic acid 3-O-methyltransferase (COMT) and caffeioyl-CoA-3-O-methyltransferase (CCoAOMT) activities and cellulose and hemicellulose contents. S-RI and SSS-FS treatments in 2020 and SS-FS (RI) treatments in 2021 significantly reduced lignin content in alfalfa leaves (P < 0.05). SSS-RI and SS-FS treatments in 2020 and 2021, respectively, significantly reduced the above ground lignin content in alfalfa compared with the control, and SS-RI and SSS-FS treatments over the 2 years (P < 0.05). Se application increased the total Se, Fe, Mn, and N contents and forage yield of alfalfa, but significantly decreased both the total C content as well as the C:N ratio, especially the SSS-FS treatment, in the 2 years (P < 0.05). Therefore, SSS-FS treatment was the most effective Se application for reducing the lignin content and improving quality of alfalfa production.