The interaction between mycorrhizal inoculation, humic acids supply and elevated atmospheric CO2 increases energetic and antioxidant properties and sweetness of yellow onion
The combined application of humic substances and mycorrhizal inocula under increased CO2 in the air is a promising horticultural technique for improving the quality of greenhouse-grown onion seedlings. The objective of this study was to evaluate if the development and qualitative characteristics of bulbs from the yellow onion Allium cepa L. Alfa São Francisco improved with the application of the abovementioned agronomic, biotic, and environmental factors. The lowest dry matter was achieved in bulbs of onions not inoculated with mycorrhizal fungi, non-amended with humic acids (HA), and grown at ambient CO2. Mycorrhizal inoculation significantly increased bulb biomass in plants cultivated at ambient CO2, and not fertilized with HA, which may be a consequence of the enhanced acid phosphatase activity in roots. The application of HA always increased bulb dry matter, which were more evident at elevated rather than ambient CO2 conditions and correlated with improved acid phosphatase activity in the plants at elevated CO2. Mycorrhizal inoculation, HA supplementation, and elevated CO2 in the air had an additive effect of increasing the content of soluble sugars, proteins, and phenolics in bulbs. While high levels of sugars and proteins enhanced the energetic value of onions, the increase in phenolics improved their antioxidant properties. The triple interaction between mycorrhization, HA fertilization, and elevated CO2 also increased the ratio between soluble solids and total titratable acidity, which may enhance the perception of sweetness and make onions more pleasant for consumption.