Sulfur and nitrogen fertility affects flavour of field-grown onions |
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Authors: | John McCallum Noel Porter Bruce Searle Martin Shaw Bodhi Bettjeman Michael McManus |
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Institution: | (1) Crop & Food Research, Private Bag 4704, Christchurch, New Zealand;(2) Crop & Food Research, Box 85, Hastings, New Zealand;(3) Institute of Molecular Biosciences, Massey University, Private Bag 11222, Palmerston North, New Zealand |
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Abstract: | Although glasshouse studies have conclusively demonstrated that S nutrition can affect onion (Allium cepaL.) 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. |
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Keywords: | gas chromatography lachrymatory factor nitrogen onions pungency sulfur |
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