Ammonium nitrate fertility levels influence flavour development in hydroponically grown ‘Granex 33’ onion

High levels of nitrogen (N) fertility have been shown to influence bulb flavour characteristics in onion ( Allium cepa L). To test the effects of lower levels of N fertility on onion bulb flavour, ‘Granex 33’ onions were grown hydroponically in a greenhouse with varying solution N levels. Eleven levels were tested by increasing the concentration of NH 4 NO 3 in solutions from 20 to 140 mg l −1 N. Mature plants were harvested and evaluated for plant leaf and bulb fresh weights (FWs), bulb soluble solids content (SSC), bulb total pyruvic acid, bulb total sulphur (S), and bulb sulphate (SO 4 2− ). To determine the effect of N on the flavour biosynthetic pathway of onion, total and individual S ‐alk(en)yl cysteine sulphoxides and related peptide intermediates were also tested. Leaf and bulb FWs responded quadratically to N concentration, as did total bulb S. Bulb SO 4 2− and SSC, though significantly influenced by N concentration, did not respond with a meaningful trend. Bulb pyruvic acid increased linearly with N level increases, as did (+)‐ S ‐propyl‐ L ‐cysteine sulphoxide. Total precursors, (+)‐ S ‐methyl‐ L ‐cysteine sulphoxide, and trans ‐(+)‐ S ‐1‐propenyl‐ L ‐cysteine sulphoxide responded quadratically to N levels. At lower N levels, trans ‐(+)‐ S ‐1‐propenyl‐ L ‐cysteine sulphoxide content was highest relative to the other precursors. However, at elevated N levels, (+)‐ S ‐methyl‐ L ‐cysteine sulphoxide accumulated in the highest concentrations. Peptide intermediates 2‐carboxypropyl glutathione and γ‐glutamyl propenyl cysteine sulphoxide responded linearly and quadratically respectively to increasing N fertility levels. Nitrogen fertility levels can influence flavour intensity and quality and should be considered when growing onions for flavour attributes.