Growth and metabolism of onion seedlings as affected by the application of humic substances, mycorrhizal inoculation and elevated CO2

• Effects of mycorrhizae, humic substances and high CO2 on onion seedlings were tested.
• Each of the three abovementioned factors improved shoot biomass.
• Interaction among factors highly increased sugars, proteins and proline in leaves.
• Interaction among factors induced the accumulation of phenolics in leaves.
• The three abovementioned factors favoured acid phosphatase activity in roots.

Onion (Allium cepa L.) is a crop with great economic importance over the world. The vigor of seedlings plays a crucial role in the posterior growth and quality of bulbs. Application of humic substances (HS), inoculation of arbuscular mycorrhizal fungi (AMF) in the soil and enhancement of atmospheric CO2 are three factors that can influence plant growth and development. Therefore, our main objective was to assess the effect of each of the abovementioned factors, separately or interacting, on the metabolism and growth of onion seedlings before bulb formation and under greenhouse conditions. Results showed that these three factors appear as valid horticultural techniques for improving growth and quality of onion seedlings cultivated in greenhouse even when mycorrhizal colonization of roots does not achieve high rates. Beneficial effects of HS were additive to those of mycorrhizal inoculation or elevated CO2 (ECO2) on shoot and root biomass production. The triple interaction between exogenous HS application, mycorrhizal inoculation and ECO2 induced the highest accumulation of soluble sugars, proteins and proline in leaves, suggesting that such interaction was the most effective for increasing the quality of onion shoots as source organs for posterior growth and quality of bulbs and also for enhancing the tolerance of onion seedlings to environmental stresses.