Dormancy and sprouting in onion bulbs. II. Changes in sulphur metabolism

SummaryExperiments were designed to relate features of S metabolism to the dormant and/or sprouting states of onion bulbs (Allium cepa L.) during storage at 18°C. For this purpose, onion bulbs were dissected at 2-week or 4-week intervals into sprout leaves, stem plate, inner scale, and fleshy leaf to measure total sulphur content, sulphate content, γglutamyl transpeptidase (GGT) activity and enzymatically-formed pyruvate (EPY). The determination of initial sprout growth was based on measurements of sprout leaf elongation. Two long-day cultivars (‘Golden Bear F1’ and ‘Copra F1’), with a clear period of dormancy, were chosen for these experiments. Changes in total sulphur content showed no clear relationship to either the dormant or the sprouting phase, whereas the sulphate content of each bulb part studied tended to be lowest at the transition from dormancy to sprouting. EPY, as an equivalent to flavour precursor content and pungency, increased in the inner scale of both cultivars throughout dormancy, and initial sprout growth. In contrast, EPY was lowest in the sprout leaves and stem plate at the end of dormancy in ‘Copra F1’, but subsequently increased during initial sprout growth. GGT activity started to increase in the fleshy leaf and in the inner scale at the start of sprouting in both cultivars. In general, changes in S metabolism at the transition from the dormant to the sprouting state suggest sprouting-enhanced sulphur metabolism, depending on the cultivar. In addition, the results indicated remobilisation and reallocation of sulphur compounds within the onion bulb during dormancy and initial sprouting.