Effect of Supplemental Light on the Quality of Green Asparagus Spears in Winter ‘Fusekomi’ Forcing Culture

Asparagus (Asparagus officinalis L.) is known to be a rich of some functional compounds beneficial for human health. In recent years, consumer’s interests on functional food such as flavonoids are increasing (Maeda et al., 2010). Green asparagus contains certain amount of rutin (Chin et al., 2002, Maeda et al., 2005; Sun et al., 2007; Maeda et al., 2010; 2012; Motoki et al., 2012). Rutin is one of the major flavonoids that have been reported to show biological and pharmacological activities such as anti-inflammatory, antitumor, and anti-bacterial/viral properties along with potent radical-scavenging activity, as well as protective effects in protecting against capillary fragility and arteriosclerotic vascular changes (Griffith et al., 1944; Hellerstein et al., 1951; Calabro et al., 2005; Guo et al., 2007). Asparagus is widely grown in Japan. The conventional spring harvest and mother-fern culture is the most popular method of harvesting asparagus. Therefore, during winter season, domestic production of asparagus almost drops to zero; the price tends to be high substantially; but asparagus are imported from other country to respond the market needs. Winter ‘fusekomi’ forcing culture system has been conducted in Japan to make asparagus production possible during this season (Koizumi et al., 2003) by heating one to two years old rootstocks in a ‘fusekomi’ forcing system. The preparation of this method is usually started in January; the seeds are sown and grown in the greenhouse. Then in the early May, seedlings are transplanted into an open field. After the yellowing of the fern (October-November), the rootstocks are dug up from the field and densely planted in the ‘fusekomi’ forcing system. In the ‘fusekomi’ forcing system, heating wires were set under the bed, and the rootstocks were covered by the soil and warmed by the heating wires. Finally asparagus could be produced even in the winter, but spears are mostly growing under small amount of light because of much snow and low sunlight in the production area. These conditions cause some problems; namely, the color of the spear is pale, and rutin content is relatively low as compared to those harvested in spring. According to the previous study, reported by Maeda et al., 2010, in the mother fern culture, improvement of the light condition by net screen method led to both the increase of rutin content of spears and the improvement of spear color. On the other hand, ‘fusekomi’ forcing cultivation is conducted in a relatively small space with dense planting (planting density is more than twenty times, comparing to the open field cultivation). Therefore, this method makes ‘environment control’ easier such as light condition. The objective of this study was to obtain basic knowledge to improve the quality (color and rutin) of asparagus