Sulfur and nitrogen fertility affects flavour of field-grown onions

Abstract

Although glasshouse studies have conclusively demonstrated that S nutrition can affect onion (Allium cepa L.) pungency this has been rarely observed in field-based studies due to difficulties in controlling S nutrition and lack of efficient methods for measurement of flavour bioactives. We have developed a rapid automated method for determination of onion lachrymatory factor ((Z, E)-thiopropanal-S-oxide; LF) based on juice extraction into dichloromethane and gas chromatography (GC) analysis with flame photometric detection. We evaluated this in a field trial of a mild (cv. 'Encore') and a pungent (cv. 'Kojak') onion cultivar grown on a low S soil with and without S addition, under high or low N treatments. No treatments significantly affected bulb fresh weight but S fertilisation significantly increased bulb total S, sulfate, pungency, LF and flavour precursor levels in both varieties. Analysis of bulb flavour precursors by HPLC confirmed that juice LF levels paralleled levels of the flavour precursor S-1-propenyl cysteine sulfoxide. The pungent cultivar also exhibited significant N main effects on bulb LF, total S and sulfate. We also assayed the key S-assimilatory enzyme, APS reductase (APR) in leaves before and during bulbing. Specific activities in the range of 1 to 11 nmol mg −1 ·min −1 were observed in youngest leaves, but only the milder cultivar exhibited significant stage × N × S effects. These findings suggest that sulfur metabolism of mild and pungent onions respond differently to N fertility, and that GC of LF is practical for field-based studies of onion flavour.Abbreviations: ACSO -alkyl cysteine sulfoxide; APR -adenosine 5 phosphosulfate reductase (EC 1.8.99.2); EPY -enzymatically produced pyruvic acid; FID -flame ionisation detector; FPD -flame photometric detector; GC-MS -gas chromatography-mass spectroscopy; HPLC -high performance liquid chromatography; LFlachrymatory factor (Z, E)-thiopropanal-S-oxide; MCSO -S-methyl-L-cysteine sulfoxide; PrCSO -S-1-propenyl cysteine sulfoxide.

Interactions among genotype, environment and agronomic practices on production and quality of storage onion (Allium cepa L.) - A review

“…Moreover, sulphur content in the soil has a great influence on the accumulation of ACSOs, directly affecting flavour and pungency of onion (Randle 1992a,b;Hamilton et al 1997;McCallum et al 2005;Guo et al 2007). Notably, the concentration and lachrymatory factor of thiosulfonates linearly increases as the S content in the soil increases (Randle 1992a;Randle, Bussard 1993;Randle et al 1994), but pungency is not affected by high sulphur content in the soil (Hamilton et al 1997;Lee et al 2009b) and it is not correlated with S level from 16 to 97 p.m. (Yoo et al 2006).…”

Genetic mapping of a major gene affecting onion bulb fructan content

“…For quantitative HPLC analysis of glucose, fructose, sucrose and DP 3-4 fructo-oligosaccharides, ground, freeze-dried tissue (20 mg) was extracted in 1 ml of 80% v/v ethanol by agitation for 15 min at room temperature, centrifuged (13,000·g for 5 min) and filtered through a 0.2 lm Minisart filter. For preliminary HPLC analyses of the C·P12 population, juice extracts were prepared from bulb tissue as described by McCallum et al (2005), diluted by addition of four volumes of ethanol and clarified as above. Carbohydrates in these extracts were fractionated by HPLC on a 220·4.6 mm Applied Biosystems (Foster City, CA, USA) Brownlee amino column and 15·3.2 mm NewGuard amino guard column, on a Waters (Milford, MA, USA) liquid chromatograph (model 626 pump and controller, model 717-plus autosampler).…”

Pungency of Spring Onion as Affected by Inoculation with Arbuscular Mycorrhizal Fungi and Sulfur Supply

“…The composition and total concentrations of ACSOs determine the flavor profile of the plants, differ among Allium species and cultivars (Yoo and Pike, 1998), and vary with the external environment (Randle and Lancaster, 2002). As the direct measurement of the flavor precursors or the active flavor compounds is laborious, technically demanding and costly for large scale routine agronomic or breeding studies (McCallum et al, 2005), enzymatically generated pyruvic acid (EPY) has been used to determine the gross pungency of Allium plants due to the simplicity of the measurement. Early studies showed that the flavor intensity of onion plants correlated well with the levels of EPY in the juice (Schwimmer and Weston, 1961).…”

“…Studies have shown that water supply (Freeman and Mossedeghi, 1973), temperature (Randle et al, 1993), harvest time (Bloem et al, 2004), S supply (Randle, 1992;Randle et al, 1999), the amount (Coolong et al, 2004) and the forms of nitrogen (N) supplied (Gamiely et al, 1991), and N and S interactions (Coolong and Randle, 2003;Bloem et al, 2004;McCallum et al, 2005) can affect plant growth, maturity, yield, flavor attributes, and bulb firmness. However, S supply in the substrate is considered to be the primary factor because S is directly involved in the synthesis of the ACSOs and is a major constitutent of the flavor compounds.…”