Sodium chloride timing and length of exposure affect onion growth and flavor

This study investigated how sequentially exposing plants to sodium chloride (NaCl) would affect growth and the flavor quality of onion (Allium cepa L.) bulbs at harvest. In a greenhouse experiment beginning 74 d before harvest, 100 mM concentrations of NaCl were applied at biweekly intervals to onions growing in nutrient solutions. At harvest, fresh weights (FW) were measured and the bulbs were analyzed for soluble solids content (SSC), bulb pungency as measured by total pyruvate (TPY), bulb sulfur (S) and sulfate (SO4-2) accumulation, flavor precursors, and their biosynthetic intermediates. Bulb and leaf FW decreased linearly the earlier NaCl was added during plant growth and development. While total bulb S was significantly affected by sequential addition of NaCl, bulb SO4-2 was unaffected. Bulb pungency was significantly reduced by NaCl, especially when NaCl was introduced during early bulb development. NaCl generally reduced flavor precursor accumulation in a quadratic response, with the greatest depression occurring when plants were exposed to NaCl beginning in the early stages of bulbing. Significant reductions in plant growth and changes in the S-compounds associated with flavor suggested that the duration and timing of NaCl exposure are important in onion.