Trichoderma asperellum promotes the development and antioxidant activity of white onion (Allium cepa L.) plants

Allium cepa L. (onion) is a crop with high nutritional value and antioxidant activity. The growth and antioxidant activity of onion plants may depend on their interaction with fungi from the Trichoderma genera, which was this study’s subject. White onion ‘Cirrus’ plants inoculated with Trichoderma asperellum had improved photochemical performance compared with that of uninoculated plants; the chlorophyll and carotenoid contents were 130% and 40% greater, respectively. In all plant organs, protein and macronutrient contents were 0.8–3.0 times greater, and the peroxidase activity was 14.7 and 2.0 times greater in bulbs and roots, respectively, in inoculated plants compared with in uninoculated plants. Inoculated plant bulbs were more spherical than uninoluted plant bulbs, and their fresh and dry biomass increased by 1.2 and 1.1 times, respectively. Nuclear magnetic resonance (NMR) metabolomic profiles of onion plant bulbs were similar for inoculated and uninoculated plants. Thirty-nine metabolites were identified in the NMR spectra corresponding to six sugars, sixteen amino acids, nine organic acids, five nucleosides, choline, ethanol, and trigonelline. However, the onion bulbs of inoculated plants had lower concentrations of fructose and acetic, malic, and 2-hydroxyisobutyric acids than those of uninoculated plant bulbs. The content of phenolic compounds was two-fold higher and the antioxidant activity was 15–70% higher in inoculated plants than in uninoculated plants, while there was no difference in flavonoid content. In conclusion, T. asperellum promotes the growth of onion plants via an increase in photosynthetic activity and quantities of macronutrients and protein. The fungus also increases the antioxidant activity due to the onion bulbs’ peroxidase activity and content of phenolic compounds.