Sodium Chloride in Nutrient Solutions Can Affect Onion Growth and Flavor Development

Onion is classified as a salt-sensitive crop, though it is found in production on saline soils around the world. While onion flavor intensity has been studied in response to various growing conditions, little is known about its response to salt stress. To understand if NaCI affects growth, flavor development, and mineral content in onion, 'Granex 33' plants were grown to maturity with six different concentrations of NaCl ranging from 0 (control) to 125 mM in nutrient solutions. NaCI affected onion fresh weight and altered onion flavor intensity and quality. Plants did not survive the 125 mM NaCI treatments and are not included in the results. As bulb Na + and Cl - content increased in response to increasing NaCI concentrations, leaf and bulb fresh weight of mature plants decreased. Total bulb S content also decreased with increasing NaCI solution concentrations, while bulb SO 4 2- content increasing linearly, indicating that less S was entering the S metabolic stream. Though bulb soluble solids content was not influenced by NaCI concentrations, pungency increased, but only at the highest NaCI concentration. Total flavor precursors and methyl cysteine sulfoxide content increased in response to NaCI, but only at the 100 mM treatment. 1-Propenyl cysteine sulfoxide was generally unresponsive to the salt treatment. Propyl cysteine sulfoxide content decreased then increased in responses to increasing NaCI levels, but was found as a minor flavor precursor. Peptide intermediates measured in the pathway leading to 1-propenyl cysteine sulfoxide and propyl cysteine sulfoxide decreased linearly with increasing NaCI exposure. While NaCI affected onion flavor in this study, severe reductions in growth would prevent onion production under similar saline conditions. For practical purposes, the effects of NaCl on flavor are, therefore, minimal.