Effect of Temperature and Storage Time on Fructosyltransferase Activities (1-FFT and 6G-FFT) in Onion Bulb Tissues

About 80% of onion bulb dry matter is non-structural carbohydrates (Darbyshire & Henry, 1981). These carbohydrates are predominantly glucose, fructose, sucrose and low-molecular-weight fructans, while starch and raffinose are absent (Darbyshire & Henry, 1981; Benkeblia et al., 2002). The metabolism of sugars is closely linked to the dormancy and sprouting state (Kato, 1966), and the most important biochemical change occurring during post-harvest life of bulbs, as with other vegetables, is the content variation in the carbohydrate constituents. Sprouting is characterized by a catabolism of the fructans reserve. However, an activity of fructan:fructan fructosyltransferase (1-FFT, EC 2. 4. 1. 100) involved in fructan biosynthesis is also observed simultaneously. Nevertheless, and because the vacuole is also the site of fructan synthesis, it has been suggested that net fructan accumulation only occurs when rate of synthesis exceeds the rate of degradation (Wagner et al., 1986). On the basis of studies of inulin formation in Jerusalem artichoke (Helianthus tuberosus ), the chain elongation of fructans is mediated by 1-FFT using 1-kestose or higher b2, 1-linked fructans as fructosyl donors and I-kestose or higher b-2, 1-linked fructans as fructosyl acceptors to build gradually longer fructans. Neokestose is a specific trisaccharide found in Liliaceae such as onion bulb and asparagus root (Chatterton et al., 1990; Shiomi et al., 1976). Neokestose is thought to be formed by another fructosyltransferase: fructan:fructan 6-fructosyltransferase (6G-FFT) activity using preferentially 1-kestose as fructosyl donor and sucrose as acceptor (Shiomi, 1981). 1-FFT and 6G-FFT activities during growing of onion bulbs were not extensively investigated except in the studies of Darbyshire & Henry (1979), Henry & Darbyshire (1980) and Shiomi (1989), while surprisingly no investigation was carried out on these activities during post-harvest life of onion bulbs. The purpose of this study was to investigate the activities of the 1-FFT and particularly 6G-FFT in onion bulbs, including the effect of temperature and dormancy state.