Increasing Nitrogen Concentration in Hydroponic Solutions Affects Onion Flavor and Bulb Quality

Abstract

<jats:p>To test the effects of high nitrogen (N) fertilization levels on onion quality and bulb flavor, `Granex 33' onions (<jats:italic>Allium cepa</jats:italic> L.) were greenhouse grown in hydroponic solution culture with increasing N concentrations. Nitrogen was adjusted in the solutions with NH<jats:sub>4</jats:sub>NO<jats:sub>3</jats:sub> and increased incrementally from 0.22 g·L<jats:sup>-1</jats:sup> to 0.97 g·L<jats:sup>-1</jats:sup> over five treatments. Plants were harvested at maturity and subjected to quality, flavor, and mineral analysis. As solution N increased, bulb fresh weight and bulb firmness decreased linearly. Gross flavor intensity, as measured by enzymatically developed pyruvic acid (EPY) increased linearly for N concentrations between 0.22 and 0.78 g·L<jats:sup>-1</jats:sup>, but EPY was reduced slightly in bulbs grown at the highest N level (0.97 g·L<jats:sup>-1</jats:sup>). Soluble solids content was unaffected by solution N concentration. Solution N had an affect on flavor quality. Methyl cysteine sulfoxide, which gives rise to cabbage (<jats:italic>Brassica</jats:italic> L. sp.) and fresh onion flavors upon eating, generally increased in concentration as solution N increased. 1-Propenyl cysteine sulfoxide, which imparts heat, mouth burn, pungency, and raw onion flavors increased between the two lowest N concentrations, and then decreased as solution N increased. Propyl cysteine sulfoxide, which imparts fresh onion and sulfur flavors upon eating, generally increased with increasing solution N concentration. Several minerals were also affected by solution N concentration. Total bulb N and NO<jats:sub>3</jats:sub><jats:sup>-</jats:sup> increased linearly while B, Ca, and Mg decreased linearly. Total bulb S and K increased and then decreased quadratically in response to increasing solution N. Nitrogen fertility can have a pronounced affect on onion flavor and as a consequence, needs to be considered when growing onions for specific flavor quality and nutritional attributes.</jats:p>

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

“…Total ACSO concentration has previously been reported to be influenced by very high N fertility levels. 15 N fertility also influenced the concentration of MCSO in the bulb (P ≤ 0.001; Fig 4(A)). Average bulb concentrations of MCSO ranged from 0.48 to 4.32 mg g −1 FW.…”

“…These values are similar to those reported for 'Granex 33' onions grown with high N and similar S fertility levels. 15 Because SO 4 2− is generally stored in the vacuole and is unable to contribute immediately to the production of flavour compounds in onion, it is important to determine the amount of total S that is contributed by SO 4 2− . By knowing total S and SO 4 2− -S, the amount of S that is bound organically (organic-S) in the bulb can be estimated.…”

“…Similarly, the previous study using high solution N levels resulted in high MCSO synthesis and accumulation. 15 Ultimately, changes in these precursor ratios will result in bulb flavour changes, as individual ACSOs impart different flavour sensations. 27 Historically, 1-PRENCSO was reported to be the main ACSO in onion.…”

“…6 -8 However, although N has been well characterized regarding its effects on growth and yield in onion, 9 -14 much less is known about its influence on flavour. 15 Increases in N fertility have been reported to influence sweet corn flavour by increasing concentrations of dimethyl sulphide. 16 In onion, N-form was reported to influence overall flavour intensity.…”

Sulfur and nitrogen fertility affects flavour of field-grown onions

“…Glasshouse studies have demonstrated that sulfate supply is the major environmental factor determining pungency (Randle, 1992;Randle and Bussard, 1993) and therefore mild onion production is conducted on light soils where S fertility can be readily controlled. In addition, hydroponic studies have also shown that nitrogen supply can affect flavour precursor levels (Coolong and Randle, 2003;Randle, 2000). However, surprisingly few field-based studies have demonstrated effects of S (or N) nutrition on pungency (Bakr and Gawish, 1998;Kumar and Sahay, 1954).…”