Sodium accumulation has minimal effect on metabolite profile of onion bulbs

Onions (Allium cepa L.) are considered a salt-sensitive crop. However, to date, little evidence supports this claim and information about the physiological and metabolomic effects of Na⁺ accumulation in onion plants is lacking. The purpose of our research has been to assess changes in onion bulbs of three different cultivars after soil and foliar applications with moderate doses of chloride-free Na₂SO₄. The antioxidative defense mechanism in onion and the accumulation of Na⁺ within the plant has also been analyzed. Based on Na⁺ leaf and bulb concentrations, our findings demonstrate that Na⁺ is only transported from bulbs to leaves not vice versa, therefore foliar application does not lead to Na⁺ accumulation in the bulbs. Soil application with Na₂SO₄ results in an accumulation of Na⁺ in the leaves and bulbs, but with the exception of one onion variety this accumulation does not alter the metabolite profile of onions significantly. Even the K⁺ concentration and organic solute levels are unchanged after accumulation of Na⁺. Nevertheless, after Na₂SO₄ treatment, the antioxidative defense system moderately increases in onion bulbs. This study demonstrates that onion plants have the ability to exclude Na⁺ at moderate Na₂SO₄ treatment, and that the potential for quality onion production in soils with increased sodium concentration is much higher than previously assumed.