Light Condition Influences Rutin and Polyphenol Contents in Asparagus Spears in the Mother-fern Culture System during the Summer–Autumn Harvest

We investigated the variation in rutin and total polyphenol contents in asparagus spears under different conditions of mother-fern culture. Rutin and polyphenol contents were highest at early spring harvest at all test sites. The contents gradually decreased during the harvest season. In particular, a considerable decrease was observed after the growth of mother ferns. The absorption of the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical was weaker in summer. Rutin content in the spears of plants grown outdoors was generally higher than in these grown in a greenhouse, except at early spring harvest. Rutin contents decreased in the spears of plants grown under a shading net or an ultraviolet (UV) radiation-filtering film. These results suggest that cultivation conditions, especially light conditions, have a great influence on rutin and polyphenol contents. We evaluated two new methods to increase rutin content by improving light conditions. Light conditions in asparagus rows were improved by using light-reflecting sheets and net screens; however, a significant increase in rutin content was observed only when net screening was used, implying that this method is better than light reflecting sheets to increase the rutin content.