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To determine the extent to which sulfur (S) and nitrogen (N) fertility interact to influence the flavor biosyn- thetic pathway in onion (Allium cepa L.), 'Granex 33ʼ onions were grown in hydroponic solution culture with varying levels of S and N availability. Plants were grown at 5, 45, or 125 mg·L-1 sulfate (SO4 2-), and 10, 50, 90, or 130 mg·L-1 N, in a factorial combination. Total bulb S, total and individual fl avor precursors and their peptide intermediates in intact onion tissue were measured. To measure the effect of S and N on alliinase activity, fl avor precursors were also measured in onion macerates. Sulfur and N availability in the hydroponics solution interacted to infl uence all fl avor compounds except S-methyl-L-cysteine sulfoxide. Levels of S-methyl-L-cysteine sulfoxide were infl uenced by N and S levels in the solutions; however, no interaction was present. At the lowest SO4 2- or N levels, most precursors and pep- tides measured were present in very low concentrations. When SO4 2- or N availability was adequate, differences among fl avor compounds were small. Results indicated that S fertility had a greater infl uence on trans-S-1-propenyl-L-cysteine sulfoxide (1-PRENCSO) accumulation, while N availability had a greater infl uence on S-methyl-L-cysteine sulfoxide levels. Flavor precursors remaining in the onion macerates revealed that the percentage of intact precursors hydrolyzed by alliinase were not signifi cantly infl uenced by either SO4 2- or N levels in the solutions, except for 1-PRENCSO, which was affected by N levels. Nitrogen and S fertility interacted to infl uence the fl avor biosynthetic pathway and may need to be considered together when manipulating onion fl avor compounds. Onions (Allium cepa L.), have been cultivated since antiquity primarily for their fl avoring properties. Onion fl avor is dominated by the presence of organosulfur compounds, in particular the S- alk(en)yl-L-cysteine-sulfoxides (ACSO). During consumption onion vacuoles lyse allowing the enzyme alliinase to hydrolyze the individual ACSOs in the bulb, giving rise to the compounds responsible for onionʼs characteristic fl avor and aroma. As such, onions accumulate large quantities of sulfur (S). Randle (1992) reported bulb S concentrations in excess of 1% on a dry weight basis in plants grown with high S fertility. Much of the S taken up by onions is partitioned into organosulfur compounds as part of the fl avor biosynthetic pathway. Randle et al. (1995) reported that up to 95% of the organically bound S in onion bulbs grown with low S fertility could be accounted for in compounds from the fl avor biosynthetic pathway. Sulfur enters the plant actively as sulfate (SO4 2-). Sulfate is transported to the leaves, where it can be stored in the vacuole or be reduced to sulfi de and assimilated into cysteine in the chloroplast (Hell, 1997). Cysteine is quickly incorporated into proteins or used in the synthesis of methionine and glutathione (Leustek et al., 2000). In onion, cysteine and glutathione serve as precursors to several gamma-glutamyl peptides (γGP), which are used in the fl avor biosynthetic pathway (Randle and Lancaster, 2002). Because the ACSOs and their breakdown products are organo- sulfur compounds, S fertility has been thoroughly investigated as a means of altering onion fl avor. Onion pungency increased in response to enhanced S fertility (Freeman and Mossadeghi, 1970; Hamilton et al., 1997; Randle 1992; Randle and Bussard, 1993). Changes in S fertility also resulted in alterations in the concentration and composition of the ACSOs in onion, which ultimately could infl uence fl avor quality in the bulb (Randle et

Sulfur and Nitrogen Availability Interact to Affect the Flavor Biosynthetic Pathway in Onion