Resource allocation and the origin of flavour precursors in onion bulbs

Abstract

SummaryThe effect of day-length on the synthesis and distribution of flavour precursors in onion (Allium cepa L. cv. Renate F1) was studied by growing onions under different photoperiod conditions: SD (short-days), LD (long-days; obtained by extending the SD treatment with incandescent light), or NC (natural long-day conditions). Plants showed a strong bulbing response to the photoperiod regime. Plants in NC and LD formed bulbs, while those under SD did not form bulbs and continued to produce foliage leaves. It was possible to distinguish between the effects of daylength on bulb initiation and development, and light integral on yield. During plant growth, the pattern of accumulation of S-alk(en)yl-L-cysteine sulphoxide (ACSO) flavour precursors differed depending on the growth conditions. Under NC and LD, an increase in ACSO concentrations in the leaves in early development was followed by a decline in the leaves and a simultaneous increase in the bulbs after plants started bulbing. There was little evidence of interconversion between ACSOs during their translocation from leaves to bulbs, suggesting that ACSOs are transported directly. Plants continued to synthesise and accumulate flavour precursors in their leaves in SD, resulting in higher overall levels than in LD. Measurements of dry weight, sulphur and nitrogen contents, showed a redistribution from leaves to bulbs during bulb formation in LD and NC. The results confirmed that the leaf is the major site of precursor synthesis, and strongly support the proposal that the flavour precursors in bulb scales originated in the leaves, and were redistributed in the plant as part of the reallocation of resources that takes place during bulb formation.